Arena Pharmaceuticals (ARNA) 10-K2 Mar 152014 FY Annual reportFinancial data

Company Profile

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
In Billions, except Share data, unless otherwise specified	Dec. 31, 2014	Feb. 24, 2015	Jun. 30, 2014
Document Information [Line Items]
Document Type	10-K
Amendment Flag	FALSE
Document Period End Date	31-Dec-14
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Trading Symbol	ARNA
Entity Registrant Name	ARENA PHARMACEUTICALS INC
Entity Central Index Key	1080709
Current Fiscal Year End Date	-19
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		241,463,035
Entity Public Float			$1.30

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Current assets:
Cash and cash equivalents	$163,209	$221,878
Accounts receivable	3,712	10,602
Inventory	10,831	12,759
Prepaid expenses and other current assets	4,144	3,571
Total current assets	181,896	248,810
Land, property and equipment, net	82,919	77,388
Intangibles, net	8,482	10,182
Other non-current assets	3,088	3,427
Total assets	276,385	339,807
Current liabilities:
Accounts payable and other accrued liabilities	10,209	10,205
Accrued clinical and preclinical study fees	7,027	1,317
Payable to Siegfried for acquisition of land and building	8,217	0
Current portion of deferred revenues	15,238	37,861
Derivative liabilities	474	0
Current portion of lease financing obligations	2,492	2,056
Total current liabilities	67,362	70,744
Deferred rent	369	247
Deferred revenues, less current portion	93,064	101,329
Derivative liabilities	0	4,892
Lease financing obligations, less current portion	68,245	70,738
Commitments and contingencies
Stockholders' equity:
Preferred stock, $.0001 par value: 7,500,000 shares authorized and 0 shares issued and outstanding at December 31, 2014, and 2013	0	0
Common stock, $.0001 par value: 367,500,000 shares authorized at December 31, 2014, and 2013; 220,321,645 shares issued and outstanding at December 31, 2014; 218,816,242 shares issued and outstanding at December 31, 2013	22	22
Additional paid-in capital	1,312,656	1,293,840
Accumulated other comprehensive income	2,908	5,728
Accumulated deficit	-1,268,241	-1,207,733
Total stockholders' equity	47,345	91,857
Total liabilities and stockholders' equity	276,385	339,807
Eisai
Current liabilities:
Payable to Eisai	23,705	19,305
Current portion of deferred revenues	14,622	37,301
Deferred revenues, less current portion	$86,933	$95,102

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Preferred stock, par value	$0.00	$0.00
Preferred stock, shares authorized	7,500,000	7,500,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$0.00	$0.00
Common stock, shares authorized	367,500,000	367,500,000
Common stock, shares issued	220,321,645	218,816,242
Common stock, shares outstanding	220,321,645	218,816,242

Consolidated_Statements_of_Ope

Consolidated Statements of Operations and Comprehensive Loss (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenues:
Toll manufacturing	$1,497	$2,690	$3,817
Total revenues	36,970	81,394	27,587
Operating Costs and Expenses:
Cost of product sales	6,369	1,803	0
Cost of toll manufacturing	1,390	4,377	3,671
Research and development	100,347	66,468	54,112
General and administrative	34,137	31,681	26,226
Amortization of intangibles	0	0	691
Total operating costs and expenses	142,243	104,329	84,700
Loss from operations	-105,273	-22,935	-57,113
Interest and Other Income (Expense):
Interest income	83	89	119
Interest expense	-6,915	-7,091	-9,120
Gain (loss) from valuation of derivative liabilities	4,418	10,150	-13,425
Gain on sale of available-for-sale securities	49,553	0	0
Loss on extinguishment of debt	0	0	-6,338
Other	-2,374	352	400
Total interest and other income (expense), net	44,765	3,500	-28,364
Net loss	-60,508	-19,435	-85,477
Deemed dividend related to beneficial conversion feature of convertible preferred stock	0	0	-2,824
Net loss allocable to common stockholders	-60,508	-19,435	-88,301
Net loss per share allocable to common stockholders:
Basic	($0.28)	($0.09)	($0.45)
Diluted	($0.28)	($0.09)	($0.45)
Shares used in calculating net loss per share allocable to common stockholders:
Basic	219,733,539	218,104,323	196,523,708
Diluted	219,733,539	218,104,323	196,523,708
Comprehensive Loss:
Net loss	-60,508	-19,435	-85,477
Foreign currency translation gain (loss)	-2,820	239	746
Comprehensive loss	-63,328	-19,196	-84,731
Eisai
Revenues:
Net product sales	15,983	5,702	0
Other collaborative revenue	18,611	72,416	23,617
Total revenues	34,594	78,118	23,617
Other
Revenues:
Other collaborative revenue	$879	$586	$153

Consolidated_Statements_of_Sto

Consolidated Statements of Stockholders' Equity (USD $)	Total	Expensed	Capitalized Cost	Deerfield	Deerfield	Deerfield	Issuance Of Common Stock Under Equity Line Of Credit	Convertible Preferred Stock	Convertible Preferred Stock	Common Stock	Common Stock	Common Stock	Common Stock	Common Stock	Additional Paid-In Capital	Additional Paid-In Capital	Additional Paid-In Capital	Additional Paid-In Capital	Additional Paid-In Capital	Additional Paid-In Capital	Additional Paid-In Capital	Treasury Stock	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
In Thousands, except Share data	USD ($)	USD ($)	USD ($)	USD ($)	Series D Preferred Stock	Warrants	USD ($)	USD ($)	Deerfield	USD ($)	Deerfield	Deerfield	Deerfield	Issuance Of Common Stock Under Equity Line Of Credit	USD ($)	Expensed	Capitalized Cost	Deerfield	Deerfield	Deerfield	Issuance Of Common Stock Under Equity Line Of Credit	USD ($)	USD ($)	USD ($)
					USD ($)	USD ($)			Series D Preferred Stock		USD ($)	Series D Preferred Stock	Warrants	USD ($)		USD ($)	USD ($)	USD ($)	Series D Preferred Stock	Warrants	USD ($)
												USD ($)	USD ($)						USD ($)	USD ($)
Beginning Balance at Dec. 31, 2011	$10,562							$0		$15					$1,108,625							($23,070)	$4,743	($1,079,751)
Beginning Balance (in shares) at Dec. 31, 2011										146,092,819
Issuance of common stock upon exercise of options (in shares)										1,071,661
Issuance of common stock upon exercise of options	4,657														4,657
Issuance of common stock under employee stock purchase plan (in shares)	341,108									341,108
Issuance of common stock under employee stock purchase plan	470														470
Issuance of common stock (in shares)									9,953	12,650,000	9,953,250		23,000,000	14,414,370
Issuance of common stock	65,700			14,561		39,202	24,727			1	1		3	1	65,699			14,560		39,199	24,726
Share-based compensation expense		5,072	54													5,072	54
Issuance of preferred stock to Deerfield					14,561														14,561
Exchange of Deerfield warrants	3,803														3,803
Issuance of common stock upon conversion (in shares)									-9,953			9,953,250
Issuance of common stock upon conversion					1							1
Beneficial conversion feature of preferred stock	2,824														2,824
Deemed dividend related to beneficial conversion feature of preferred stock	-2,824														-2,824
Retirement of treasury stock	-23,070																					23,070		-23,070
Translation gain (loss)	746																						746
Net loss	-85,477																							-85,477
Ending Balance at Dec. 31, 2012	98,639							0		22					1,281,426							0	5,489	-1,188,298
Ending Balance (in shares) at Dec. 31, 2012										217,476,458
Issuance of common stock upon exercise of options (in shares)										954,174
Issuance of common stock upon exercise of options	2,375														2,375
Issuance of common stock under employee stock purchase plan (in shares)	334,360									334,360
Issuance of common stock under employee stock purchase plan	852														852
Issuance of common stock upon vesting of restricted stock unit awards										41,250
Issuance of common stock (in shares)										10,000
Issuance of common stock	88														88
Share-based compensation expense		9,024	75													9,024	75
Deemed dividend related to beneficial conversion feature of preferred stock	0
Retirement of treasury stock	0
Translation gain (loss)	239																						239
Net loss	-19,435																							-19,435
Ending Balance at Dec. 31, 2013	91,857							0		22					1,293,840							0	5,728	-1,207,733
Ending Balance (in shares) at Dec. 31, 2013										218,816,242
Issuance of common stock upon exercise of options (in shares)										1,115,068
Issuance of common stock upon exercise of options	4,078														4,078
Issuance of common stock under employee stock purchase plan (in shares)	304,085									304,085
Issuance of common stock under employee stock purchase plan	1,148														1,148
Issuance of common stock upon vesting of restricted stock unit awards										86,250
Share-based compensation expense		13,509	81													13,509	81
Deemed dividend related to beneficial conversion feature of preferred stock	0
Retirement of treasury stock	0
Translation gain (loss)	-2,820																						-2,820
Net loss	-60,508																							-60,508
Ending Balance at Dec. 31, 2014	$47,345							$0		$22					$1,312,656							$0	$2,908	($1,268,241)
Ending Balance (in shares) at Dec. 31, 2014										220,321,645

Consolidated_Statements_of_Sto1

Consolidated Statements of Stockholders' Equity (Parenthetical) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2012
Costs of public offering	$3,875

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating Activities
Net loss	($60,508)	($19,435)	($85,477)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Depreciation and amortization	8,655	7,733	9,055
Amortization of intangibles	506	469	691
Share-based compensation	13,509	9,024	5,072
(Gain) loss from valuation of derivative liabilities	-4,418	-10,150	13,425
Gain on sale of available-for-sale securities	-49,553	0	0
Amortization of prepaid financing costs	136	136	292
Loss on extinguishment of debt	0	0	6,338
(Gain) loss on disposal or sale of equipment	172	49	-31
Changes in assets and liabilities:
Accounts receivable	6,407	-4,473	-5,260
Inventory	870	-6,065	-5,875
Prepaid expenses and other assets	-772	-65	-1,524
Payables and accrued liabilities	13,240	19,572	276
Deferred revenues	-29,764	75,880	17,849
Deferred rent	122	125	-103
Net cash provided by (used in) operating activities	-101,398	72,800	-44,047
Investing Activities
Proceeds from sale of available-for-sale securities	49,553	0	0
Purchases of land, property and equipment	-8,905	-9,164	-1,777
Proceeds from sale of equipment	47	60	31
Other non-current assets	209	439	-425
Net cash provided by (used in) investing activities	40,904	-8,665	-2,171
Financing Activities
Principal payments on lease financing obligations	-2,057	-1,664	-1,313
Proceeds from issuance of common stock	5,225	3,315	151,218
Proceeds from issuance of preferred stock	0	0	16,462
Net cash provided by financing activities	3,168	1,651	144,106
Effect of exchange rate changes on cash	-1,343	1	571
Net increase (decrease) in cash and cash equivalents	-58,669	65,787	98,459
Cash and cash equivalents at beginning of year	221,878	156,091	57,632
Cash and cash equivalents at end of year	163,209	221,878	156,091
Supplemental Disclosure Of Cash Flow Information:
Interest paid	6,778	6,954	7,670
Supplemental Disclosure Of Non-Cash Investing and Financing Information:
Conversion of preferred stock into common stock	0	0	14,561
Deemed dividend related to beneficial conversion feature of convertible preferred stock	0	0	2,824
Retirement of treasury stock	0	0	23,070
Purchases of land, property,equipment and building	250	72	110
Deerfield
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Accretion of note payable	0	0	1,225
Financing Activities
Payments on note payable	0	0	-22,261
Siegfried
Supplemental Disclosure Of Non-Cash Investing and Financing Information:
Purchases of land, property,equipment and building	$8,217	$0	$0

The_Company_and_Summary_of_Sig

The Company and Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2014

The Company and Summary of Significant Accounting Policies

(1) The Company and Summary of Significant Accounting Policies

The Company

Arena Pharmaceuticals, Inc., or Arena, was incorporated on April 14, 1997, and commenced operations in July 1997. We are a biopharmaceutical company focused on discovering, developing and commercializing novel drugs that target G protein-coupled receptors, or GPCRs, to address unmet medical needs. We operate in one business segment. Our US operations are located in San Diego, California, and our operations outside of the United States, including our commercial manufacturing facility, are located in Zofingen, Switzerland.

Our internally discovered drug, lorcaserin, has been approved for marketing in the United States for weight management in adults who are overweight with a comorbidity or obese, and is being marketed under the brand name BELVIQ® (which is pronounced as “BEL-VEEK”). In June 2013, BELVIQ was made available to patients by prescription in the United States by our collaborator, Eisai. In addition, in February 2015, BELVIQ was approved for marketing in South Korea for weight management in adults who are overweight with a comorbidity or obese (see Note 18). BELVIQ is our first and only drug approved for marketing by any regulatory agency.

Our wholly owned subsidiary, Arena Pharmaceuticals GmbH, or Arena GmbH, granted Eisai Inc. and Eisai Inc.’s parent company, Eisai Co., Ltd. (collectively with Eisai Inc., Eisai) exclusive commercialization rights to market lorcaserin in all of the countries in the world, except for South Korea, Taiwan, Australia, New Zealand and Israel. Arena GmbH also granted exclusive commercialization rights to market lorcaserin for weight loss or weight management to Ildong Pharmaceutical Co., Ltd., or Ildong, for South Korea; to CY Biotech Company Limited, or CYB, for Taiwan; and to Teva Pharmaceuticals Ltd.’s Israeli subsidiary, Abic Marketing Limited, or Teva, for Israel.

The marketing of BELVIQ is subject to applicable regulatory approval. BELVIQ has not been approved for marketing outside of the United States or South Korea (see Note 18).

With our collaborators or independently, we intend to continue to explore lorcaserin’s therapeutic potential for additional indications, using new formulations and in combination with other drugs. We also intend to continue our research and development efforts to advance our earlier-stage drug candidates and to discover and advance additional compounds.

Lorcaserin and our earlier-stage drug candidates and compounds have resulted from our GPCR-focused drug discovery and development approach, specialized expertise and technologies.

Basis of Presentation

The accompanying consolidated financial statements have been prepared in accordance with US generally accepted accounting principles, or GAAP, and reflect all of our activities, including those of our wholly owned subsidiaries. All material intercompany accounts and transactions have been eliminated in consolidation.

New Accounting Guidance

In May 2014, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, No. 2014-09, “Revenue from Contracts with Customers.” ASU No. 2014-09 outlines a comprehensive revenue recognition model and supersedes most current revenue recognition guidance. ASU No. 2014-09 is effective for annual reporting periods, and interim periods within those periods, beginning after December 15, 2016. ASU No. 2014-09 allows for two methods of adoption: (a) “full retrospective” adoption, meaning the standard is applied to all periods presented, or (b) “modified retrospective” adoption, meaning the cumulative effect of applying ASU No. 2014-09 is recognized as an adjustment to the fiscal 2017 opening retained earnings balance. We have not yet selected an adoption method as we are currently evaluating the impact of ASU No. 2014-09 on our consolidated financial statements.

In August 2014, FASB issued ASU No. 2014-15, “Presentation of Financial Statements – Going Concern: Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern.” Under GAAP, continuation of a reporting entity as a going concern is presumed as the basis for preparing financial statements unless and until the entity’s liquidation becomes imminent. Preparation of financial statements under this presumption is commonly referred to as the going concern basis of accounting. If and when an entity’s liquidation becomes imminent, financial statements should be prepared under the liquidation basis of accounting. Even when an entity’s liquidation is not imminent, there may be conditions or events that raise substantial doubt about the entity’s ability to continue as a going concern. In those situations, financial statements should continue to be prepared under the going concern basis of accounting, but the amendments in ASU No. 2014-15 should be followed to determine whether to disclose information about the relevant conditions and events. The amendments in ASU No. 2014-15 are effective for the annual reporting period ending after December 15, 2016, and for annual and interim periods thereafter. We do not expect the adoption of ASU No. 2014-15 to have a material impact on our consolidated financial statements.

Use of Estimates

The preparation of financial statements in conformity with GAAP requires our management to make estimates and assumptions that affect the reported amounts (including assets, liabilities, revenues and expenses) and related disclosures. The amounts reported could differ under different estimates and assumptions.

Reclassifications

Certain prior year amounts have been reclassified to conform to the current year presentation.

Cash and Cash Equivalents

Cash and cash equivalents consist of cash and highly liquid investments with remaining maturities of three months or less when purchased.

Inventory

Inventory is stated at the lower of cost or market. We determine cost, which includes amounts related to materials, labor and overhead, using a first-in, first-out basis. We evaluate our inventory each period to identify potential obsolete, excess or otherwise non-saleable items. If non-saleable items are observed and there are no alternate uses for the inventory, we will record a write-down to net realizable value in the period that the decline in value is first recognized.

Concentrations of Risk and Geographical Data

Financial instruments, which potentially subject us to concentrations of credit risk, consist primarily of cash and cash equivalents. We limit our exposure to credit loss by holding our cash primarily in US dollars or, from time to time, placing our cash and investments in US government, agency or government-sponsored enterprise obligations and in corporate debt instruments that are rated investment grade, in accordance with an investment policy approved by our Board of Directors.

Through December 31, 2014, Eisai was our only significant customer for BELVIQ. Eisai and Ildong are the exclusive distributors of BELVIQ in the United States and South Korea, respectively, which are the only jurisdictions for which BELVIQ has received regulatory approval for marketing. We also produce drug products for Siegfried AG, or Siegfried, under a toll manufacturing agreement, and all of our toll manufacturing revenues are attributable to Siegfried.

Percentages of our total revenues are as follows:

Year Ended December 31,

2014

2013

2012

Eisai marketing and supply agreement (See Note 13)

93.60%

96.00%

85.60%

Toll manufacturing agreement with Siegfried

4.00%

3.30%

13.80%

Other collaborative agreements

2.40%

0.70%

0.60%

Total percentage of revenues

100.0%

Percentages of our total accounts receivable are as follows:

At December 31,

2014

2013

2012

Eisai marketing and supply agreement (See Note 13)

93.10%

94.50%

2.00%

Ildong marketing and supply agreement (See Note 13)

0.40%

1.00%

85.50%

Toll manufacturing agreement with Siegfried

0.00%

4.30%

12.30%

Other collaborative agreements

6.50%

0.20%

Total percentage of accounts receivable

100.0%

We purchase raw materials, starting materials, intermediates, API, excipients and other materials from commercial sources. To decrease the risk of an interruption to our supply, when we believe it is reasonable for us to do so, we source these materials from multiple suppliers so that, in general, the loss of any one source of supply would not have a material adverse effect on commercial production, project timelines or inventory of supplies for our studies or clinical trials. However, currently we have only one or a limited number of suppliers for some of these materials for BELVIQ and for other of our programs. The loss of a primary source of supply would potentially delay our production of BELVIQ or our development projects and potentially those of current or future collaborators. We intend to maintain a safety stock of certain of these materials to help avoid delays in production, but we do not know whether such stock will be sufficient. Our facility in Zofingen, Switzerland is the only manufacturer of finished drug product for BELVIQ. We intend to have a second source of supply for finished drug product of BELVIQ, but we believe that it could take longer than one year to secure another source.

Long-lived assets located in the United States and Switzerland were $49.0 million and $42.4 million, respectively, at December 31, 2014. Long-lived assets located in the United States and Switzerland were $50.6 million and $37.0 million, respectively, at December 31, 2013.

Property and Equipment

Property and equipment are stated at cost and depreciated over the estimated useful lives of the assets (generally 3 to 15 years) using the straight-line method. Buildings are stated at cost and depreciated over an estimated useful life of approximately 20 years using the straight-line method. Leasehold improvements are stated at cost and amortized over the shorter of the estimated useful lives of the assets or the lease term using the straight-line method. Capital improvements are stated at cost and amortized over the estimated useful lives of the underlying assets using the straight-line method.

Intangibles

Intangible assets consist of our manufacturing facility production licenses we acquired from Siegfried in January 2008 and are amortized using the straight-line method over their estimated useful life of 20 years.

Long-lived Assets

If indicators of impairment exist, we assess the recoverability of the affected long-lived assets by determining whether the carrying value of such assets can be recovered through undiscounted cash flows. If impairment is indicated, we measure the impairment loss by comparing the fair value of the asset, estimated using discounted cash flows expected to be generated from the asset, to the carrying value.

Deferred Rent

For financial reporting purposes, rent expense is recognized on a straight-line basis over the term of the lease. The difference between rent expense and amounts paid under lease agreements is recorded as deferred rent in the liability section of our consolidated balance sheets.

Derivative Liabilities

We account for warrants and other derivative financial instruments as either equity or liabilities based upon the characteristics and provisions of each instrument. Warrants classified as equity are recorded as additional paid-in capital on our consolidated balance sheets and no further adjustments to their valuation are made. Warrants classified as derivative liabilities and other derivative financial instruments that require separate accounting as liabilities are recorded on our consolidated balance sheets at their fair value on the date of issuance and are revalued on each balance sheet date until such instruments are exercised or expire, with changes in the fair value between reporting periods recorded as other income or expense. We estimate the fair value of warrants classified as derivative liabilities using the Black-Scholes option pricing model.

Foreign Currency

The functional currency of our wholly owned subsidiary in Switzerland, Arena GmbH, is the Swiss franc. Accordingly, all assets and liabilities of this subsidiary are translated to US dollars based on the applicable exchange rate on the balance sheet date. Revenue and expense components are translated to US dollars at weighted-average exchange rates in effect during the period. Gains and losses resulting from foreign currency translation are reported as a separate component of accumulated other comprehensive income or loss in the stockholders’ equity section of our consolidated balance sheets.

Foreign currency transaction gains and losses, which are primarily the result of remeasuring US dollar-denominated receivables and payables at Arena GmbH, are recorded in the interest and other income (expense) section of our consolidated statement of operations and comprehensive loss. For the year ended December 31, 2014, we recognized foreign currency transaction losses, net of $2.2 million. For the years ended December 31, 2013, and 2012, we recognized foreign currency transaction gains, net of $0.3 million and $0.2 million, respectively.

Share-based Compensation

Our share-based awards are measured at fair value and recognized over the requisite service or performance period. The fair value of each stock option is estimated on the date of grant using the Black-Scholes option pricing model, based on the market price of the underlying common stock, expected life, expected stock price volatility and expected risk-free interest rate. Expected volatility is computed using a combination of historical volatility for a period equal to the expected term and implied volatilities from traded options to buy our common stock, with historical volatility being weighted at 75%. The expected life of options is determined based on historical experience of similar awards, giving consideration to the contractual terms of the share-based awards, vesting schedules and post-vesting terminations. The risk-free interest rates are based on the US Treasury yield curve, with a remaining term approximately equal to the expected term used in the option pricing model. The fair value of each restricted stock unit award is estimated based on the market price of the underlying common stock on the date of the grant. The fair value of restricted stock unit awards that include market-based performance conditions is estimated on the date of grant using a Monte Carlo simulation model, based on the market price of the underlying common stock, expected performance measurement period, expected stock price volatility and expected risk-free interest rate. We estimate forfeitures at the time of grant and revise our estimate in subsequent periods if actual forfeitures differ from those estimates.

Revenue Recognition

Our revenues to date have been generated primarily through collaborative agreements and, to a lesser extent, a toll manufacturing agreement. Our collaborative agreements may contain multiple elements including commercialization rights, services (joint steering committee and research and development services) and manufactured products. Consideration we receive under these arrangements may include upfront payments, research and development funding, cost reimbursements, milestone payments and payments for net product sales. We recognize revenue when (i) persuasive evidence of an arrangement exists, (ii) delivery has occurred and title has passed, (iii) the price is fixed or determinable and (iv) collectability is reasonably assured. Any advance payments we receive in excess of amounts earned are classified as deferred revenues. We defer recognition of revenue at the time we sell BELVIQ to our collaborators because we presently do not have the ability to estimate product that may be returned to us. Instead, we recognize revenues from net product sales when our collaborators ship BELVIQ to their distributors. See Note 13.

We evaluate deliverables in a multiple-element arrangement to determine whether each deliverable represents a separate unit of accounting. A deliverable constitutes a separate unit of accounting when it has standalone value to the customer. If the delivered element does not have standalone value without one of the undelivered elements in the arrangement, we combine such elements and account for them as a single unit of accounting. We allocate the consideration to each unit of accounting at the inception of the arrangement based on the relative selling price.

Non-refundable upfront payments received under our collaborative agreements for commercialization rights have been deferred as such rights have not been deemed to have standalone value without the ongoing services required under the agreement. Such amounts are recognized as revenue on a straight-line basis over the period in which we expect to perform the services. Amounts we receive as reimbursement for our research and development expenditures are recognized as revenue as the services are performed.

Under the milestone method, we recognize revenue that is contingent upon the achievement of a substantive milestone in its entirety in the period in which the milestone is achieved. A milestone is an event (i) that can be achieved in whole or in part on either our performance or on the occurrence of a specific outcome resulting from our performance, (ii) for which there is substantive uncertainty at the date the arrangement is entered into that the event will be achieved and (iii) that would result in additional payments being due us. A milestone payment is considered substantive when the consideration payable to us for each milestone (a) is consistent with our performance necessary to achieve the milestone or the increase in value to the collaboration resulting from our performance, (b) relates solely to our past performance and (c) is reasonable relative to all of the other deliverables and payments under the arrangement. In making this assessment, we consider all facts and circumstances relevant to the arrangement, including factors such as the scientific, regulatory, commercial and other risks that must be overcome to achieve the respective milestone, the level of effort and investment required to achieve the respective milestone and whether any portion of the milestone consideration is related to future performance or deliverables. Other contingent-based payments received are recognized when earned.

We manufacture drug products under a toll manufacturing agreement for a single customer, Siegfried. Upon Siegfried’s acceptance of drug products manufactured by us, we recognize toll manufacturing revenues.

Research and Development Expenses

Research and development expenses, which consist primarily of salaries and other personnel costs, clinical trial costs and preclinical study fees, manufacturing costs for non-commercial products, and the development of earlier-stage programs and technologies, are expensed as incurred when these expenditures have no alternative future uses.

We accrue clinical trial expenses based on work performed. In determining the amount to accrue, we rely on estimates of total costs incurred based on enrollment, the completion of trials and other events. We follow this method because we believe reasonably dependable estimates of the costs applicable to various stages of a clinical trial can be made. However, the actual costs and timing of clinical trials are highly uncertain, subject to risks and may change depending on a number of factors. Differences between the actual clinical trial costs and the estimated clinical trial costs that we have accrued in any prior period are recognized in the subsequent period in which the actual costs become known. Historically, these differences have not been material; however, material differences could occur in the future. Payments made to reimburse collaborators for our share of their research and development activities are recorded as research and development expenses, and are recognized as the work is performed.

Comprehensive Loss

Comprehensive loss is defined as the change in equity during a period from transactions and other events and circumstances from non-owner sources. We report components of comprehensive loss in the period in which they are recognized. For the years ended December 31, 2014, 2013, and 2012, comprehensive loss consisted of net loss and foreign currency translation gains and losses.

Net Loss Per Share

We calculate basic and diluted net loss per share allocable to common stockholders using the weighted-average number of shares of common stock outstanding during the period.

Since we are in a net loss position, in addition to excluding potentially dilutive out-of-the money securities, we have excluded from our calculation of diluted net loss per share all potentially dilutive in-the-money (i) stock options, (ii) restricted stock unit awards, or RSUs, (iii) Total Stockholder Return, or TSR, performance restricted stock unit, or PRSU, awards, (iv) unvested restricted stock in our deferred compensation plan and (v) warrants, and our diluted net loss per share is the same as our basic net loss per share. The table below presents the weighted-average number of potentially dilutive securities that were excluded from our calculation of diluted net loss per share allocable to common stockholders for the years ended December 31, 2014, 2013, and 2012, in thousands.

December 31,

2014

2013

2012

Stock options

15,530

14,435

13,110

Warrants

370

776

607

RSUs and unvested restricted stock

476

306

270

Total

16,376

15,517

13,987

Because the market condition for the PRSUs was not satisfied at December 31, 2014 and 2013, such securities are excluded from the table above.

In January 2015, we issued 21,000,000 shares of our common stock (see Note 18). These shares are not included in the weighted-average number of shares common stock outstanding during the year ended December 31, 2014, but are expected to be included in future reporting periods.

Income Taxes

We use the asset and liability method of accounting for income taxes. Deferred tax assets and liabilities are recognized for the estimated future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Our deferred tax assets and liabilities are determined using the enacted tax rates expected to be in effect for the years in which those tax assets are expected to be realized.

The realization of our deferred tax assets is dependent upon our ability to generate sufficient future taxable income. We establish a valuation allowance when it is more-likely-than-not the future realization of all or some of the deferred tax assets will not be achieved. The evaluation of the need for a valuation allowance is performed on a jurisdiction-by-jurisdiction basis, and includes a review of all available evidence, both positive and negative.

The impact of an uncertain income tax position is recognized at the largest amount that is more-likely-than-not to be sustained upon audit by the relevant taxing authority. An uncertain income tax position will not be recognized if it has less than a 50% likelihood of being sustained.

Fair_Value_Disclosures

Fair Value Disclosures	12 Months Ended
Fair Value Disclosures	Dec. 31, 2014
Fair Value Disclosures	(2) Fair Value Disclosures

	We measure our financial assets and liabilities at fair value, which is defined as the exit price, or the amount that would be received from selling an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date.



	We use the following three-level valuation hierarchy that maximizes the use of observable inputs and minimizes the use of unobservable inputs to value our financial assets and liabilities:

	Level 1— Observable inputs such as unadjusted quoted prices in active markets for identical instruments.

	Level 2— Quoted prices for similar instruments in active markets or inputs that are observable for the asset or liability, either directly or indirectly.

	Level 3— Significant unobservable inputs based on our assumptions.

	The following tables present our valuation hierarchy for our financial assets and liabilities that are measured at fair value on a recurring basis at December 31 2014, and 2013, in thousands:



			Fair Value Measurements at December 31, 2014
			Balance		Quoted Prices in		Significant Other		Significant
					Active Markets		Observable		Unobservable Inputs
					(Level 1)		Inputs		(Level 3)
							(Level 2)
	Assets:
	Money market funds1		$	143,913	$	143,913	$	0	$	0
	Liabilities:
	Warrant derivative liabilities		$	474	$	0	$	474	$	0



			Fair Value Measurements at December 31, 2013
			Balance		Quoted Prices in		Significant Other		Significant
					Active Markets		Observable		Unobservable Inputs
					(Level 1)		Inputs		(Level 3)
							(Level 2)
	Assets:
	Money market funds1		$	208,833	$	208,833	$	0	$	0
	Liabilities:
	Warrant derivative liabilities		$	4,892	$	0	$	4,892	$	0


	(1)	Included in cash and cash equivalents on our consolidated balance sheets.

Shortterm_Investments_Availabl

Short-term Investments, Available-for-Sale	12 Months Ended
Short-term Investments, Available-for-Sale	Dec. 31, 2014
Short-term Investments, Available-for-Sale	(3) Short-term Investments, Available-for-Sale
	We held an investment in TaiGen Biotechnology Co., Ltd., or TaiGen, that from December 31, 2011, to January 17, 2014, had a cost basis of zero due to prior impairment charges. On January 17, 2014, TaiGen completed an initial public offering and its common stock began to trade on the GreTai Securities Listed Market, under the name “TaiGen Biopharmaceuticals Holding Limited.” Such market is deemed to be comparable to a US over-the-counter market such that the fair value of our former investment in TaiGen, which previously had been accounted for as a cost method investment with a cost basis of zero, became readily determinable. Accordingly, on January 17, 2014, we recorded our former investment in TaiGen of 29.6 million shares based on its fair value of approximately $49.1 million. We began recording our former investment in TaiGen at fair value based on the trading price of TaiGen’s common stock, and the remaining former investment was revalued on each balance sheet date.

	Gains and losses on the sale of available-for-sale securities are determined using the specific-identification method. During the year ended December 31, 2014, we sold all of our shares of TaiGen and recorded a realized gain of $49.6 million.

Inventory

Inventory	12 Months Ended
Inventory	Dec. 31, 2014
Inventory	(4) Inventory

	Inventory consisted of the following, in thousands:



		December 31,
		2014		2013
	Raw materials	$	1,167	$	657
	Work in process		3,520		4,104
	Finished goods at Arena GmbH		3,681		0
	Finished goods at Eisai		2,463		7,998

	Total inventory	$	10,831	$	12,759

Land_Property_and_Equipment

Land, Property and Equipment	12 Months Ended
Land, Property and Equipment	Dec. 31, 2014
Land, Property and Equipment	(5) Land, Property and Equipment

	Land, property and equipment consisted of the following, in thousands:



		December 31,
		2014			2013
	Land	$	11,339		$	10,854
	Building and capital improvements		74,629			67,747
	Leasehold improvements		17,984			17,854
	Machinery and equipment		53,247			55,143
	Computers and software		15,363			11,568
	Furniture and office equipment		2,376			2,207

			174,938			165,373
	Less accumulated depreciation and amortization		(92,019	)		(87,985	)

	Land, property and equipment, net	$	82,919		$	77,388

Intangibles

Intangibles	12 Months Ended
Intangibles	Dec. 31, 2014
Intangibles	Intangibles

	Intangibles consisted of the following, in thousands:



		December 31,
		2014			2013
	Acquired manufacturing production licenses – gross	$	13,049		$	14,545
	Acquired manufacturing production licenses – accumulated amortization		(4,567	)		(4,363	)

	Intangibles, net	$	8,482		$	10,182


	We capitalize into inventory amortization expense related to the manufacturing of BELVIQ. Such amortization will subsequently be recognized as cost of product sales when the related inventory is sold. Using the exchange rate in effect on December 31, 2014, we expect to record amortization of $0.7 million per year through 2027 for our manufacturing facility production licenses.

Accounts_Payable_and_Other_Acc

Accounts Payable and Other Accrued Liabilities	12 Months Ended
Accounts Payable and Other Accrued Liabilities	Dec. 31, 2014
Accounts Payable and Other Accrued Liabilities	(7) Accounts Payable and Other Accrued Liabilities

	Accounts payable and other accrued liabilities consisted of the following, in thousands:



		December 31,
		2014		2013
	Accounts payable	$	2,844	$	3,721
	Accrued compensation		4,792		4,205
	Other accrued liabilities		2,573		2,279

	Total accounts payable and other accrued liabilities	$	10,209	$	10,205

Agreements_with_Siegfried

Agreements with Siegfried	12 Months Ended
Agreements with Siegfried	Dec. 31, 2014
Agreements with Siegfried	(8) Agreements with Siegfried

	In January 2008, we acquired from Siegfried certain drug product facility assets, including manufacturing facility production licenses, fixtures, equipment, other personal property and real estate assets in Zofingen, Switzerland, under an asset purchase agreement. These assets are being used to manufacture and package lorcaserin as well as certain drug products for Siegfried. From time to time, we may also use this facility to manufacture and package tablets and capsules for other of our programs or for other entities. The purchase price under the asset purchase agreement consisted of cash consideration of CHF 31.8 million and common stock valued at $8.0 million.

	In connection with this transaction, we also entered into a long-term supply agreement for the active pharmaceutical ingredient of lorcaserin, a toll manufacturing agreement and a technical services agreement with Siegfried. For the years ended December 31, 2014, 2013, and 2012, we recognized expenses of $2.5 million, $2.8 million and $2.6 million, respectively, for services incurred under the technical services agreement. The technical services agreement provides us with administrative and other services to operate the facility.

	The real estate assets we acquired in January 2008 pursuant to the asset purchase agreement consisted of approximately 67,000 square feet of space in a building that consists of approximately 134,000 square feet of space along with an option to purchase the remaining Siegfried-occupied portion of the building along with the underlying land at a price of CHF 15.0 million, plus an inflation adjustment. Siegfried also had the option to sell us such remaining portion of the building with the underlying land at a price of CHF 8.0 million, plus an inflation adjustment. In July 2014, Siegfried provided us notice of its exercise of the option to sell us the remaining Siegfried-occupied portion of the building with the underlying land. In December 2014, we took title of the remaining portion of the building with the underlying land with the purchase price of CHF 8.2 million to be paid in July 2015. Accordingly, we have recorded this amount in land, property and equipment, net and as a payable to Siegfried on our consolidated balance sheet at December 31, 2014. In connection with the exercise of the option, we entered into an agreement to lease this newly acquired building space back to Siegfried through December 31, 2016, for an annual base rent amount of CHF 0.4 million. Siegfried has the right to partially or fully terminate this lease with six months’ notice, provided that Siegfried cannot terminate any portion of the lease prior to December 31, 2015. Siegfried has an annual option to extend the lease for an additional year with the last extension term ending on December 31, 2019. At any time during the extension terms, we have the right to partially or fully terminate this lease with six months’ notice, but with a termination date no earlier than December 31, 2017.

Transactions_with_Deerfield

Transactions with Deerfield	12 Months Ended
Transactions with Deerfield	Dec. 31, 2014
Transactions with Deerfield	(9) Transactions with Deerfield

	In July 2009, pursuant to a Facility Agreement we entered into in June 2009, or the Facility Agreement, with Deerfield Private Design Fund, L.P., Deerfield Private Design International, L.P., Deerfield Partners, L.P., Deerfield International Limited, Deerfield Special Situations Fund, L.P., and Deerfield Special Situations Fund International Limited, or collectively Deerfield, Deerfield provided us with a $100.0 million secured loan. We received net proceeds of $95.6 million from this loan and had the right, at any time, to prepay any or all of the outstanding principal at par. In connection with the funding of this loan, we issued Deerfield warrants to purchase an aggregate of 28,000,000 shares of our common stock, which were exercisable until June 17, 2013, at an exercise price of $5.42 per share. As described below, the Deerfield loan has been repaid in full and none of Deerfield’s former warrants remain outstanding.

	As of the July 2009 funding of the loan, we separately valued the following four components under the Facility Agreement: (i) the formerly outstanding $100.0 million loan was valued at $47.9 million on a relative fair value basis and recorded as a liability, (ii) the formerly outstanding warrants to purchase 28,000,000 shares of our common stock were valued at $39.1 million on a relative fair value basis and recorded as additional paid-in capital, (iii) Deerfield’s former right to loan us up to an additional $20.0 million under the Facility Agreement, which we refer to as the Deerfield Additional Loan Election, was valued at $9.5 million and classified as a liability and (iv) Deerfield’s former ability to accelerate principal payments under the loan under certain circumstances was valued at $0.5 million and classified as a liability.



	As part of our various transactions with Deerfield subsequent to the funding of the loan, we amended the terms of the Facility Agreement, repaid portions of the loan and exchanged all of the original warrants for a lesser number of warrants at lower exercise prices. We exchanged certain of the warrants as part of equity financings with Deerfield in June 2010, March 2011, and January 2012. Other than the exercise period, the exercise price and certain provisions related to cashless exercise and early termination of the warrants, all of the warrants issued in exchange contained substantially the same terms as the original warrants. In May 2012, we repaid the remaining portion of our note payable to Deerfield.

	In addition to various transactions with Deerfield that included stock purchases and warrant exchanges, the following Deerfield transactions occurred in the year ended December 31, 2012, as follows:


		•		In January 2012, Deerfield purchased 9,953,250 shares of our common stock at $1.65775 per share and approximately 9,953 shares of our Series D Convertible Preferred Stock, or Series D Preferred, at $1,657.75 per share. In February 2012, Deerfield converted all of the Series D Preferred into a total of 9,953,250 shares of common stock. The fair value of the common stock into which the Series D Preferred was convertible on the date of issuance of the Series D Preferred exceeded the proceeds allocated to the Series D Preferred on a relative fair value basis by $2.8 million, resulting in a beneficial conversion feature that we recognized as a decrease to additional paid-in capital and a deemed dividend to the Series D Preferred stockholders in 2012. Net proceeds to us from this transaction, after prepayment of $5.0 million of the then-outstanding principal balance on the loan, were $27.9 million. In conjunction with this transaction, we issued Deerfield warrants to purchase 8,631,410 shares of our common stock at an exercise price of $1.745 per share in exchange for the cancellation of outstanding warrants to purchase 11,800,000 shares of our common stock at an exercise price of $5.42 per share and outstanding warrants to purchase 1,831,410 shares of our common stock at an exercise price of $3.45 per share. On a relative fair value basis, we determined that the incremental value of these new warrants was $3.8 million, which was recorded as a component of the stock issuance and warrant exchange. With respect to the $5.0 million prepayment, we retired a proportional share of the debt discount and issuance costs directly related to the repaid debt and recognized a non-cash loss on extinguishment of debt of $1.7 million in 2012.


		•		In April and May 2012, Deerfield exercised certain of its warrants to purchase a total of 4,000,000 shares of our common stock, and elected to pay the exercise price by canceling $6.7 million of the then-outstanding principal balance on its loan. In May 2012, we prepaid the remaining outstanding principal balance and unpaid interest on the Deerfield loan, and the Facility Agreement was terminated. In connection with these transactions, we retired the related debt discount and issuance costs and recognized a non-cash loss on extinguishment of debt of $4.7 million, which, along with the January 2012 amount above, totaled $6.4 million in 2012.


		•		From June to August 2012, we received net proceeds totaling $32.5 million from the cash exercise of Deerfield’s remaining warrants to purchase a total of 19,000,000 shares of our common stock.



	The following table summarizes the principal repayments made on the Deerfield loan from its inception through the date it was repaid in full, in thousands:



			Loan Principal
	Original loan principal		$	100,000
	July 2009 repayment			(10,000	)
	August 2010 repayment			(30,000	)
	January 2011 repayment			(20,000	)
	March 2011 repayment			(17,739	)
	January 2012 repayment			(5,000	)
	April and May 2012 cancellations as part of warrant exercises			(6,720	)
	May 2012 repayment			(10,541	)

	Outstanding principal balance at December 31, 2012		$	0


	Total interest expense of $1.9 million, including accretion of the debt discount attributable to the warrants and the other derivative financial instruments and amortization of capitalized issuance costs, was recognized in connection with this loan for the year ended December 31, 2012.

Derivative_Liabilities

Derivative Liabilities	12 Months Ended
Derivative Liabilities	Dec. 31, 2014
Derivative Liabilities	(10) Derivative Liabilities

	In June 2006 and August 2008, we issued seven-year warrants, which we refer to as the Series B Warrants, to purchase 829,856 and 1,106,344 shares of our common stock, respectively, at an exercise price of $15.49 and $7.71 per share, respectively. As a result of the warrants’ anti-dilution provision and certain subsequent equity issuances at prices below the adjustment price of $6.72 defined in the Series B Warrants, the number of shares issuable upon exercise of the warrants increased and the exercise price decreased. In June 2013, a portion of the June 2006 Series B Warrant was exercised to purchase 10,000 shares of our common stock, resulting in net proceeds to us of $0.1 million, and the remaining portion of the June 2006 Series B Warrant to purchase 1,457,405 shares of common stock expired pursuant to its terms in June 2013. At December 31, 2014, the number of shares issuable upon exercise of the outstanding August 2008 Series B Warrant was 1,965,418 at an exercise price of $4.34 per share. At December 31, 2014, the outstanding August 2008 Series B Warrant was valued at $0.5 million and recorded as a current derivative liability on our consolidated balance sheet. At December 31, 2013, the outstanding August 2008 Series B Warrant was valued at $4.9 million and recorded as a long-term derivative liability on our consolidated balance sheet.

	Our outstanding warrant is revalued on each balance sheet date, with changes in the fair value between reporting periods recorded in the interest and other income (expense) section of our consolidated statements of operations and comprehensive loss.

	We recognized the following gain (loss) from valuation of derivative liabilities for the years ended December 31, 2014, 2013, and 2012, in thousands:



		December 31,
		2014		2013		2012
	Warrants	$	4,418	$	10,150	$	(13,480	)
	Former Deerfield acceleration right		0		0		55

	Total gain (loss) from valuation of derivative liabilities	$	4,418	$	10,150	$	(13,425	)


	The Deerfield acceleration right, which we separately valued at $0.5 million as of the July 2009 issuance date and previously recorded as a derivative liability, related to a formerly outstanding right to require us to accelerate principal payments under our formerly outstanding loan from certain Deerfield entities. Until this right was terminated in connection with the repayment of the Deerfield loan in May 2012 (see Note 9), such right was revalued on each balance sheet date, with changes in the fair value between reporting periods recorded as other income or expense.

Commitments

Commitments	12 Months Ended
Commitments	Dec. 31, 2014
Commitments	(11) Commitments

	We occupy four US properties under sale and leaseback agreements that allow us the option to repurchase these properties at various dates between 2017 and 2027 and, in some cases, include renewal options. The terms of these leases stipulate annual increases in monthly rental payments of 2.5%. We accounted for our sale and leaseback transactions using the required financing method because our options to repurchase these properties in the future are considered continued involvement. Under the financing method, the book value of the properties and related accumulated depreciation remain on our balance sheet and no sale is recognized. Instead, the sales price of the properties is recorded as a financing obligation, and a portion of each lease payment is recorded as interest expense. We recorded interest expense of $6.9 million, $7.1 million and $7.2 million for the years ended December 31, 2014, 2013, and 2012, respectively, related to these leases. We expect interest expense related to our facilities to total $50.4 million from December 31, 2014, through the remaining terms of the leases. At December 31, 2014, the total financing obligation for these facilities was $70.7 million. The aggregate residual value of the facilities at the end of the lease terms is $10.0 million.

	We lease an additional US property under an operating lease, which expires in May 2027, contains a purchase option and stipulates annual increases in monthly rental payments of 2.5%. We also lease space in various facilities in Zofingen, Switzerland that can be terminated with 12 months written notice under an agreement that expires in 2032. We also lease a separate office space in Zofingen under an operating lease which expires in August 2020.

	In accordance with the lease terms for certain of our US properties, we are required to maintain deposits for the benefit of the landlord throughout the term of the leases. A total of $1.4 million was recorded in other non-current assets on our consolidated balance sheets at December 31, 2014, and 2013, respectively, related to such leases.

	We recognize rent expense on a straight-line basis over the term of each lease. Rent expense of $1.1 million, $1.1 million and $1.7 million was recognized for the years ended December 31, 2014, 2013, and 2012, respectively.

	Annual future obligations at December 31, 2014, are as follows, in thousands:




	Year ending December 31,	Financing			Operating
	Year ending December 31,	Obligations			Leases
	2015	$	8,292		$	969
	2016		9,262			1,126
	2017		9,494			1,147
	2018		9,731			1,170
	2019		8,053			1,192
	Thereafter		66,328			7,827

	Total minimum lease payments		111,160		$	13,431

	Less amounts representing interest		(50,413	)
	Add amounts representing residual value		9,990

	Lease financing obligations		70,737
	Less current portion		(2,492	)

		$	68,245

Stockholders_Equity

Stockholders' Equity	12 Months Ended
Stockholders' Equity	Dec. 31, 2014
Stockholders' Equity	(12) Stockholders’ Equity

	Equity Compensation Plans.

	On June 10, 2013, our stockholders approved our 2013 Long-Term Incentive Plan, or 2013 LTIP. Upon such approval, our 2012 Long-Term Incentive Plan, or 2012 LTIP, was terminated. However, notwithstanding such termination or the previous termination of our 2009 Long-Term Incentive Plan, 2006 Long-Term Incentive Plan, as amended, 2002 Equity Compensation Plan, Amended and Restated 2000 Equity Compensation Plan, and Amended and Restated 1998 Equity Compensation Plan (together with the 2012 LTIP, the “Prior Plans”), all outstanding awards under the Prior Plans will continue to be governed under the terms of the Prior Plans. The number of shares of common stock authorized for issuance under the 2013 LTIP may be increased by the number of shares subject to any stock awards under the Prior Plans that are forfeited, expire or otherwise terminate without the issuance of such shares and would otherwise be returned to the share reserve under the Prior Plans but for their termination and as otherwise provided in the 2013 LTIP.

	The 2013 LTIP provides for the grant of a total of 30 million shares of our common stock (subject to adjustment for certain corporate events), as (i) decreased for grants made under the Prior Plans between December 31, 2012, and the approval of the 2013 LTIP and (ii) increased by the number of shares subject to any stock awards under the Prior Plans that, between December 31, 2012, and the approval of the 2013 LTIP, are forfeited, expire or settled for cash and as otherwise provided in the 2013 LTIP.

	Shares under the 2013 LTIP may be granted as incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards and performance awards. Subject to certain limited exceptions, stock options and stock appreciation rights granted under the 2013 LTIP reduce the available number of shares by one share for every share issued while awards other than stock options and stock appreciation rights granted under the 2013 LTIP reduce the available number of shares by 1.25 shares for every share issued. In addition, shares that are released from awards granted under the Prior Plans or the 2013 LTIP because the awards expire, are forfeited or are settled for cash will increase the number of shares available under the 2013 LTIP by one share for each share released from a stock option or stock appreciation right and by 1.25 shares for each share released from awards other than stock options and stock appreciation rights.

	Stock options granted under the 2013 LTIP generally vest 25% a year for four years and are exercisable for up to seven years from the date of grant. The recipient of a restricted stock award has all rights of a stockholder at the date of grant, subject to certain restrictions on transferability and a risk of forfeiture. Restricted stock unit awards generally vest over one or four years from the date of grant. The minimum performance period under a performance award is 12 months. Neither the exercise price of an option nor the grant price of a stock appreciation right may be less than 100% of the fair market value of the common stock on the date such equity award is granted, except in specified situations. The 2013 LTIP prohibits option and stock appreciation right repricings (other than to reflect stock splits, spin-offs or certain other corporate events) without stockholder approval.

	In 2003, we set up a deferred compensation plan for our executive officers, whereby executive officers elected to contribute their shares of restricted stock into the plan. There were 79,169 shares of restricted stock in the plan at December 31, 2014, 2013, and 2012.

	The following table summarizes our stock option activity under the Prior Plans and the 2013 LTIP, or collectively, our Equity Compensation Plans, for the year ended December 31, 2014, in thousands (except per share data):



			Options				Weighted-			Weighted-Average		Aggregate
							Average			Remaining		Intrinsic
							Exercise Price			Contractual		Value
										Term (in years)
	Outstanding at December 31, 2013			14,681			$	4.99
	Granted			2,618			$	6.21
	Exercised			(1,115	)		$	3.66
	Forfeited/cancelled/expired			(353	)		$	6.62

	Outstanding at December 31, 2014			15,831			$	5.25			5.48	$	10,962

	Vested and expected to vest at December 31, 2014			15,407			$	5.23			5.46	$	10,889

	Vested and exercisable at December 31, 2014			8,717			$	5.56			4.77	$	6,140




	The aggregate intrinsic value in the above table is calculated as the difference between the closing price of our common stock at December 31, 2014, of $3.47 per share and the exercise price of stock options that had strike prices below the closing price. The intrinsic value of all stock options exercised during the years ended December 31, 2014, 2013, and 2012, was $2.7 million, $4.6 million and $5.4 million, respectively. During the year ended December 31, 2014, cash of $4.1 million was received from stock option exercises and cash of $1.1 million was received from stock purchases under the employee stock purchase plans. There is no tax impact related to share-based compensation or stock option exercises because we are in a net operating loss position with a full valuation allowance.

	In June 2014, we granted to our non-employee directors 177,688 RSUs that vest in equal monthly installments over one year from the date of grant, and will convert to the underlying common shares at the earliest of (i) the three-year anniversary of the grant date, (ii) the director’s separation from service or (iii) a change in control of Arena. In December 2014, we granted to our executive officers 150,000 RSUs that vest 25% per year over four years from the date of grant. The following table summarizes activity with respect to our time-based RSUs under our Equity Compensation Plans for the year ended December 31, 2014, in thousands (except per share data):



			RSUs				Weighted-Average			Aggregate
							Grant-Date Fair			Intrinsic
							Value			Value
	Unvested at January 1, 2014			369			$	7.23
	Granted			328			$	5.23
	Vested			(241	)		$	7.35
	Forfeited/cancelled			0

	Unvested at December 31, 2014			456			$	5.72

	Outstanding at December 31, 2014			676			$	6.42		$	2,347


	The total fair value of RSUs vested during the years ended December 31, 2014 and 2013, was $1.8 million and $0.9 million, respectively, and there were no RSUs vested during the year ended December 31, 2012. The weighted-average estimated grant-date fair value of RSUs granted during the years ended December 31, 2014, 2013, and 2012 was $5.23, $6.91 and $8.87, respectively.

	In March 2014 and March 2013, we granted our executive officers PRSU awards. The PRSUs may be earned and converted into outstanding shares of our common stock based on the TSR of our common stock relative to the TSR over a three-year performance period beginning March 1 of the year granted of the NASDAQ Biotech Index. In the aggregate, the target number of shares of common stock that may be earned under the PRSUs granted in March 2014 and March 2013 is 695,000 and 780,000, respectively; however, the actual number of shares that may be earned ranges from 0% to 200% of such amounts. In addition, there is a cap on the number of shares that can be earned under the PRSUs equal to six times the grant-date fair value of each award, and funding is capped at 100% if the absolute 3-year TSR is negative even if performance is above the median. As these awards contain a market condition, we used a Monte Carlo simulation model to estimate the grant-date fair value, which totaled $5.0 million and $5.9 million for the March 2014 and March 2013 grants, respectively, and which is being recognized over the performance period. The aggregate intrinsic value of the outstanding PRSUs at December 31, 2014, was $5.1 million. All of the PRSUs were outstanding and unvested at December 31, 2014.

	Employee Stock Purchase Plans.

	In June 2012, our stockholders approved our 2009 Employee Stock Purchase Plan, as amended, or 2009 ESPP. Under the 2009 ESPP substantially all employees can choose to have up to 15% of their annual compensation withheld to purchase up to 625 shares of common stock per purchase period, subject to certain limitations. The shares of common stock may be purchased over an offering period with a maximum duration of 24 months and at a price of not less than 85% of the lesser of the fair market value of the common stock on (i) the first trading day of the applicable offering period or (ii) the last trading day of the applicable three-month purchase period. Under applicable accounting guidance, the 2009 ESPP is considered a compensatory plan. At December 31, 2014, a total of 596,574 shares of common stock were available for issuance under the 2009 ESPP.



During the years ended December 31, 2014, 2013, and 2012, 304,085, 334,360 and 341,108 shares, respectively, were purchased under the 2009 ESPP.

Share-based Compensation.

We estimate the grant-date fair value of all of our share-based awards in determining our share-based compensation expense. Our share-based awards include (i) stock options, (ii) options to purchase stock granted under our employee stock purchase plan, (iii) RSUs, and (iv) PRSU awards.

The table below sets forth the weighted-average assumptions and estimated fair value of stock options we granted under our Equity Compensation Plans during the years ended December 31, 2014, 2013, and 2012:



		December 31,
		2014				2013			2012
Risk-free interest rate			1.80%				1.30%			1.40%
Dividend yield			0%				0%			0%
Expected volatility			81%				80%			90%
Expected life (years)			6.17				6.24			6.05
Weighted-average estimated fair value per share of stock options granted		$	4.37			$	5.25		$	2.27

The table below sets forth the assumptions and estimated fair value of the options to purchase stock granted under our employee stock purchase plan for multiple offering periods during the years ended December 31, 2014, 2013, and 2012:



		December 31,
		2014		2013		2012
Risk-free interest rate		0.0% - 0.6%		0.0% - 0.5%		0.0% - 0.7%
Dividend yield		0%		0%		0%
Expected volatility		53% - 81%		79% - 105%		85% - 106%
Expected life (years)		0.25 - 2.0		0.25 - 2.0		0.25 - 2.0
Range of fair value per share of options granted under employee stock purchase plan		$1.37 to $4.22		$0.90 to $5.44		$0.56 to $5.44

The table below sets forth the assumptions and estimated fair value of PRSU awards granted during the years ended December 31, 2014, and 2013:



		December 31,
		2014				2013
Risk-free interest rate			0.70%				0.40%
Dividend yield			0%				0%
Expected volatility			78%				89%
Performance period (years)			2.99				2.99
Estimated fair value per share of PRSUs granted		$	7.16			$	7.50



We recognized share-based compensation expense as follows, in thousands, except per share data:



		December 31,
		2014				2013			2012
Cost of product sales		$	0			$	17		$	0
Research and development			7,118				4,318			1,822
General and administrative			6,391				4,689			3,250

Total share-based compensation expense and impact on net loss allocable to common stockholders		$	13,509			$	9,024		$	5,072

Impact on net loss per share allocable to common stockholders, basic and diluted		$	0.06			$	0.04		$	0.03

Share-based compensation capitalized into inventory		$	81			$	75		$	54


In June 2012, we began to capitalize into inventory share-based compensation related to awards granted to employees involved with the manufacturing of BELVIQ. Such compensation will subsequently be recognized as cost of product sales when the related inventory is sold.

The table below sets forth our total unrecognized estimated compensation expense at December 31, 2014, by type of award and the weighted-average remaining requisite service period over which such expense is expected to be recognized:



		December 31, 2014
		Unrecognized				Remaining
		Expense (in				Weighted-Average
		thousands)				Recognition
						Period (in years)
Unvested stock options		$	18,653				2.18
RSUs			2,523				2.62
PRSUs			5,876				1.63

Common Stock Reserved for Future Issuance.

The following shares of our common stock are reserved for future issuance at December 31, 2014, in thousands:



Outstanding warrants			1,965
Equity Compensation Plans			41,786
2009 ESPP			597
Deferred compensation plan			79

Total			44,427

Collaborations

12 Months Ended

Dec. 31, 2014

Collaborations

(13) Collaborations

Eisai.

In November 2013, Arena GmbH and Eisai entered into the Second Amended and Restated Marketing and Supply Agreement, or Eisai Agreement. The Eisai Agreement amended and restated the previous agreement and expanded Eisai’s exclusive commercialization rights for lorcaserin to all of the countries in the world, except for South Korea, Taiwan, Australia, New Zealand and Israel. Lorcaserin is approved in the United States for chronic weight management in adults who are overweight with a comorbidity or obese (marketed under the brand name BELVIQ), and was made available to patients by prescription in the United States by Eisai in June 2013. In addition to providing commercialization rights, which are subject to applicable regulatory approval, we provide Eisai with services related to development and regulatory activities, and manufacture and sell lorcaserin to Eisai. Under the Eisai Agreement, we have received an upfront payment and payments from sales of lorcaserin, and are entitled to receive payments from future sales of lorcaserin, milestone payments based on the achievement of regulatory filings and approvals, one-time purchase price adjustment payments and other payments.

Prior to entering into the Eisai Agreement, Arena GmbH and Eisai Inc. entered into the original marketing and supply agreement in July 2010, under which we granted Eisai Inc. exclusive commercialization rights for lorcaserin solely in the United States and its territories and possessions. In May 2012, Arena GmbH and Eisai Inc. amended and restated such agreement by entering into the first amended agreement, which expanded Eisai Inc.’s exclusive commercialization rights to include most of North and South America.

The following table summarizes the revenues we recognized under our collaboration with Eisai for the periods presented, in thousands:

December 31,

From Inception

Through

December 31,

2014

2013

2012

2014

Net product sales

15,983

5,702

21,685

Amortization of upfront payments

7,630

4,035

3,503

20,526

Reimbursement of development expenses

10,037

2,020

15,420

Milestone payments

500

66,000

20,000

86,500

Reimbursement of patent and trademark expenses

444

361

892

Subtotal other Eisai collaborative revenue

18,611

72,416

23,617

123,338

Total

34,594

78,118

23,617

145,023

The following table summarizes the deferred revenues under our collaboration with Eisai at December 31, 2014, and 2013, in thousands:

December 31,

2014

2013

Upfront payments

94,474

102,104

Net product sales

7,081

30,299

Total deferred revenues attributable to Eisai

101,555

132,403

Less current portion

(14,622

)

(37,301

)

Deferred revenues attributable to Eisai, less current portion

86,933

95,102

Upfront and milestone payments.

In connection with entering into the Eisai Agreement, we received from Eisai an upfront payment of $60.0 million. This payment is in addition to the $50.0 million and $5.0 million in upfront payments we received from Eisai in connection with entering into the original agreement and the first amended agreement, respectively. Revenues from these upfront payments were deferred, as we determined that the exclusive rights did not have standalone value without our ongoing development and regulatory activities. Accordingly, these payments are recognized ratably as revenue over the periods in which we expect the services to be rendered, which are approximately 15 years for the Eisai Agreement and first amended agreement and 16 years for the original agreement.

In addition to the upfront payments, we have received from Eisai a total of $86.5 million in milestones payments, comprised of (i) $65.0 million in 2013 earned upon the final scheduling designation for BELVIQ by the US Drug Enforcement Administration, or DEA, (ii) $20.0 million earned in 2012 for the inclusion in the approved prescribing information of the FDA of the efficacy and safety data from the Phase 3 BLOOM-DM (Behavioral modification and Lorcaserin for Overweight and Obesity Management in Diabetes Mellitus) clinical trial in patients with type 2 diabetes, (iii) $0.5 million earned in 2014 upon Eisai filing for regulatory approval of lorcaserin for weight management in Brazil, (iv) $0.5 million earned in 2013 upon Eisai filing for regulatory approval of lorcaserin for weight management in Mexico and (v) $0.5 million earned in 2013 upon Eisai filing for regulatory approval of lorcaserin for weight management in Canada.

Under the Eisai Agreement, we are eligible to receive up to an aggregate of $176.0 million in additional regulatory and development milestone payments.

Product purchase price and purchase price adjustment payments.

We manufacture lorcaserin at our facility in Switzerland, and sell lorcaserin to Eisai for Eisai’s commercialization in the United States and, subject to applicable regulatory approval, in the other territories under the Eisai Agreement (other than Europe, China and Japan) for a purchase price starting at 31.5% and 30.75%, respectively (and starting at 27.5% in Europe, China and Japan), of Eisai’s aggregate annual net product sales (which are the gross invoiced sales less certain deductions described in the Eisai Agreement), or the Product Purchase Price, in the respective territory. The Product Purchase Price will increase on a tiered basis in the United States and the other territories (other than Europe, China and Japan) to as high as 36.5% and 35.75%, respectively, on the portion of Eisai’s annual aggregate net product sales exceeding $750.0 million in all territories other than Europe, China and Japan. The Product Purchase Price will increase to 35% in Europe, China and Japan on the portion of Eisai’s annual aggregate net product sales exceeding $500.0 million in such territories. The Product Purchase Price is subject to reduction (for sales in a particular country), including in the event of generic competition in the applicable country. The revenue we recognize for BELVIQ product revenue related to redemption of vouchers and product samples is based on our cost of goods sold.

In addition to payments for purchases of lorcaserin, we are eligible to receive up to an aggregate of $1.56 billion in one-time purchase price adjustment payments and other payments. These payments include up to an aggregate of $1.19 billion that are based on Eisai’s annual net product sales of lorcaserin in all of the territories under the Eisai Agreement on an aggregate basis, with the first and last amounts payable with annual net product sales of $250.0 million and $2.5 billion, respectively. Of these payments, Eisai will pay us a total of $330.0 million for annual net product sales of up to $1.0 billion. The $1.56 billion also includes $370.0 million in one-time purchase price adjustment payments we are eligible to receive based on annual net product sales in the non-US territories, comprised of $185.0 million based on Eisai’s annual net product sales in the non-US territories in North and South America and $185.0 million based on Eisai’s annual net product sales in the territories outside of North and South America. The first and last amounts are payable upon first achievement of annual net product sales of $100.0 million and $1.0 billion, respectively, with respect to each of the following areas: (i) the non-US territories in North and South America and (ii) the territories outside of North and South America. In addition, we are also eligible to receive certain payments by Eisai if certain annual minimum sales requirements in Mexico, Canada and Brazil are not met during the first ten years after initial commercial sale in such territories.

The amount that Eisai pays us for lorcaserin product supply is based on Eisai’s estimated price at the time the order is shipped, which is Eisai’s estimate of the Product Purchase Price, and is subject to change on April 1 and October 1 of each year. Eisai’s estimate of the Product Purchase Price was changed as of October 1, 2013, and there was no further change as of April 1, 2014, and October 1, 2014. At the end of Eisai’s fiscal year (March 31), the estimated price paid to us for product that Eisai sold to their distributors is compared to the Product Purchase Price of such product, and the difference is either refunded back to Eisai (for overpayments) or paid to us (for underpayments). On a monthly basis, Eisai provides us the total amount of net product sales for the month, details of the total deductions from gross to net product sales and the sales in units. We recognize our revenues monthly based on our percentage of Eisai’s monthly net product sales figures. When the revenues we recognize differ from the estimated price that Eisai paid us for such product, the difference is reclassified from deferred revenues to a receivable or payable account, as appropriate. We also adjust the deferred revenues balance for the product supply held at Eisai based on the most current net product sales figures provided to us, with the difference reclassified from deferred revenues to a receivable or payable account.

We recognized total revenues from BELVIQ net product sales of $16.0 million for the year ended December 31, 2014, of which $14.2 million related to sales at the Product Purchase Price, $1.3 million related to redemptions of vouchers and $0.5 million related to product sampling. The Product Purchase Price for the product Eisai has sold to date was lower than the initial estimated price that Eisai paid us for such product, primarily because the price that Eisai paid us did not include deductions for the use of vouchers and savings cards or for certain items related to product launch. In September 2014, Eisai determined to include product sampling as part of its commercialization efforts and they allocated certain bottles of BELVIQ for the initial product sampling. Eisai initiated product sampling in October 2014. Under the Eisai Agreement, Eisai pays us our cost of goods for these product samples. The allocation of BELVIQ bottles for product sampling reduced our deferred revenues and increased our payable to Eisai by $6.0 million in September 2014. In January 2015, Eisai announced the launch of a new savings card which will enable eligible patients without commercial coverage for BELVIQ to pay no more than $75 for each monthly prescription while those patients with commercial coverage for BELVIQ will be able to use the card to obtain additional savings if their copay is greater than $50 per monthly prescription. The new savings card is subject to certain restrictions, including that patients who are eligible for state or federal healthcare programs are excluded. The launch of the new savings card increased the estimated deductions from Eisai’s gross invoiced sales price in December 2014 which (i) reduced our revenues for the month of December 2014 and (ii) reduced our deferred revenues and increased our payable to Eisai by $1.8 million at December 31, 2014.

These excess payments, which total the $23.7 million classified as Payable to Eisai on our consolidated balance sheet at December 31, 2014, are primarily related to the above deductions, product sampling and the launch of the new savings card in January 2015. On a quarterly basis, subsequent to the end of each calendar quarter, we refund to Eisai the portion of these excess payments related to product sampling for product shipped to physicians during the quarter. On an annual basis, subsequent to the end of Eisai’s fiscal year, we refund to Eisai the portion of these excess payments related to product sold by Eisai to their distributors through March 31.

Development payments.

In connection with the US approval of BELVIQ, the FDA is requiring (i) an evaluation as part of the cardiovascular outcomes trial, or CVOT, of the effect of long-term treatment with BELVIQ on the incidence of major adverse cardiovascular events, or MACE, in overweight and obese patients with cardiovascular disease or multiple cardiovascular risk factors and (ii) the conduct of postmarketing studies to assess the safety and efficacy of BELVIQ for weight management in obese pediatric patients. In addition to the FDA-required studies, we and Eisai are prioritizing the development areas of smoking cessation, a once-daily formulation, co-administration with phentermine, as well as exploring, including as part of the CVOT, BELVIQ’s effect on conversion to type 2 diabetes and improvements in cardiovascular outcomes.

The chart below summarizes the general agreement regarding cost sharing between Eisai and us for significant development activities under the Eisai Agreement. In addition, Eisai or we may from time to time conduct approved development of lorcaserin at such party’s own expense. For example, Eisai was responsible for the expenses of the pilot study of 12-week duration to preliminarily assess lorcaserin and phentermine when co-administered. For the years ended December 31, 2014, 2013, and 2012, we recognized expenses of $35.3 million, $11.7 million and $8.6 million, respectively, for non-commercial manufacturing and other development costs related to lorcaserin.

Eisai Second Amended and Restated Marketing and Supply Agreement: Cost Sharing for Development

United States

Rest of

Remaining Territories

North and South America

BELVIQ

Not Applicable

General

Up to total of $100.0 million -

- Pre-approval*

Eisai: 90%; Arena: 10%

Eisai: 50%; Arena: 50%

Certain stability work Eisai: 50%; Arena: 50%

Thereafter, Eisai: 100%

BELVIQ

General - Eisai: 90%; Arena 10%

General

Up to total of $50.0 million -

- Post-approval*

Eisai: 90%; Arena: 10%

Eisai: 50%; Arena: 50%

Non-FDA required portion of CVOT

Up to $80.0 million -

Certain stability work

Thereafter, Eisai: 90%;

Eisai: 50%; Arena: 50%

Arena: 10%

Thereafter, Eisai: 100%

Certain pediatric studies

Eisai: 50%; Arena: 50%

Lorcaserin

Up to a total of $250.0 million (as reduced by up to $80.0 million for non-FDA required portion of CVOT) - Eisai: 50%; Arena: 50%

products other than

BELVIQ

- Pre-approval

Lorcaserin

Up to a total of $100.0 million in the aggregate across all additional products -

products other than

Eisai: 50%; Arena: 50%

BELVIQ

- Post-approval

Thereafter, Eisai: 90%; Arena: 10%

Development required by a regulatory authority, with the exception of the non-FDA required portion of the CVOT.

Certain other terms.

Eisai and we have agreed to limitations on the ability to commercialize outside of the Eisai Agreement any weight management product or addiction disorder product in the territories under the agreement. The agreement includes a stand-still provision limiting Eisai’s ability to acquire our securities and assets.

Eisai may terminate the Eisai Agreement with respect to any country in the territory following the later of the expiration of all issued lorcaserin patents in such country and 12 years after the first commercial sale of the first lorcaserin product in such country. Arena GmbH and Eisai each has the right to terminate the Eisai Agreement early in certain circumstances in its entirety or with respect to the applicable country or product, including (a) if the other party is in material breach, (b) for commercialization concerns, and (c) for certain intellectual property infringement. Eisai also has the right to terminate the Eisai Agreement early in its entirety or with respect to each country in certain circumstances, including (i) termination in a country if sales of generic equivalents of a lorcaserin product in such country exceed sales of the lorcaserin product in that country (based on volume), and (ii) if Eisai is acquired by a company that has a product that competes with a lorcaserin product. In addition, Arena GmbH can terminate the Eisai Agreement early in its entirety or with respect to each country in the non-US territories in North and South America in certain circumstances, including termination in each country if Eisai does not satisfy certain regulatory filing and commercialization diligence requirements in such country.

Eisai will indemnify us for losses resulting from certain third-party claims, including for (a) Eisai’s negligence, willful misconduct or violation of law, but excluding product liability claims, (b) Eisai’s breach of the Eisai Agreement or related agreements, but excluding product liability claims, (c) certain uses or misuses of a lorcaserin product, (d) certain governmental investigations of Eisai related to a lorcaserin product, and (e) infringement relating to Eisai’s use of certain trademarks, tag lines and logos related to a lorcaserin product. Arena GmbH will indemnify Eisai for losses resulting from certain third-party claims, including for (i) Arena GmbH’s negligence, willful misconduct, failure to comply with law, breach of any agreement with a third party with respect to product development prior to the effective date of the original agreement with Eisai, but excluding product liability claims, (ii) Arena GmbH’s negligence or willful misconduct with respect to certain uses or misuses of a lorcaserin product outside of the agreement, (iii) certain uses or misuses of a lorcaserin product after the term of the agreement, in any territory no longer under the agreement or with respect to any product after the termination of the agreement with respect to such product, (iv) Arena GmbH’s negligence, willful misconduct or violation of law, but excluding product liability claims, (v) Arena GmbH’s breach of the Eisai Agreement or related agreements, but excluding product liability claims, (vi) certain infringement of intellectual rights of a third party, and (vii) infringement relating to Eisai’s use of certain trademarks related to a lorcaserin product. We are unable to predict the maximum potential amount of any indemnification claims. At December 31, 2014, we have not incurred any losses under these indemnification provisions.

Arena GmbH and Eisai will, in general, share equally in losses resulting from third-party product liability claims, except where one party’s acts or omissions did not contribute to the events or circumstances leading to such product liability claim and the other party’s actual willful misconduct, violation of law or breach of its obligations under the Eisai Agreement or certain other agreements between Arena GmbH and Eisai were the sole and direct cause of the product liability claim. We are unable to predict the range of loss from future product liability claims.

Recall.

In December 2014, Eisai and we discovered that a small number of bottles of BELVIQ in a limited number of lots had a missing or incomplete label. This labeling issue related to the packaging of BELVIQ and not the tablets. As a precautionary measure, Eisai initiated a recall from wholesalers of the involved lots and restocked this inventory in December 2014 without any anticipated supply interruption at the retail level. Eisai considered this a class III recall, which includes product recalled because of a defect that is unlikely to cause patient harm, but causes the product to be non-compliant with marketing authorizations or specifications. In December 2014, we recorded an expense of $1.1 million, of which $0.4 million represents the cost of the recalled bottles and $0.7 million represents the estimated amount we expect to reimburse Eisai for the costs of the recall efforts, within cost of products sold.

Other Collaborations.

In addition to the Eisai Agreement, Arena GmbH entered into the Marketing and Supply Agreement, or Ildong BELVIQ Agreement, with Ildong for South Korea in November 2012, into the Marketing and Supply Agreement, or CYB Agreement, with CYB for Taiwan in July 2013 and into the Marketing and Supply Agreement, or Teva Agreement, with Teva for Israel in July 2014. These agreements provide such collaborators with rights to lorcaserin for weight loss or weight management in obese and overweight patients, subject to applicable regulatory approval, as well as the possibility of us granting them rights to additional lorcaserin products or indications.

Under the Ildong BELVIQ Agreement, we (i) received from Ildong an upfront payment of $5.0 million, less withholding taxes, in January 2013 (ii) earned a $3.0 million milestone, which we expect to receive, less withholding taxes, in March 2015, upon the February 2015 approval of BELVIQ for marketing for weight management in adults who are overweight with a combordity or obese by the Ministry of Food and Drug Safety, or MFDS (see Note 18) and (iii) will receive payments from sales of BELVIQ and purchase price adjustment payments based on Ildong’s annual net product sales (which are the gross invoiced sales less certain deductions described in the Ildong BELVIQ Agreement). We recorded the upfront payment as deferred revenue and are recognizing it as revenue ratably over approximately 14 years, which is the period in which we expect to provide services under the arrangement. At December 31, 2014, our consolidated balance sheet included $0.4 million and $3.9 million for the current and non-current portion, respectively, of the deferred revenue attributable to such upfront payment. For the years ended December 31, 2014, 2013, and 2012 we recognized revenues of $0.4 million, $0.5 million and $0.1 million, respectively, under the Ildong BELVIQ Agreement.

Under the CYB Agreement, we received from CYB an upfront payment of $2.0 million, less withholding taxes. We recorded this upfront payment as deferred revenue and are recognizing it as revenue ratably over approximately 14 years, which is the period in which we expect to provide services under the arrangement. At December 31, 2014, our consolidated balance sheet included $0.2 million and $1.8 million for the current and non-current portion, respectively, of the deferred revenue attributable to such upfront payment. For the years ended December 31, 2014, and 2013, we recognized revenues of $0.2 million and $0.1 million, respectively, under the CYB Agreement. Subject to regulatory approval of lorcaserin by the Taiwan Food and Drug Administration, or TFDA, we will receive payments from sales of lorcaserin, and are eligible to receive purchase price adjustment payments based on CYB’s annual net product sales (which are the gross invoiced sales less certain deductions described in the CYB Agreement), as well as a milestone payment upon approval of the first additional indication for lorcaserin by the TFDA.

Under the TEVA Agreement, we received from Teva an upfront payment of $500,000 and a milestone payment of $250,000 earned upon its application for regulatory approval of lorcaserin in Israel. We recorded the upfront payment as deferred revenue and are recognizing it as revenue ratably over approximately nine years, which is the period in which we expect to provide services under the arrangement. At December 31, 2014, our consolidated balance sheet included $0.1 million and $0.4 million for the current and non-current portion, respectively, of the deferred revenue attributable to the upfront payment. For the year ended December 31, 2014, we recognized revenues of $0.3 million under the Teva Agreement. Subject to regulatory approval of lorcaserin by the Israeli Ministry of Public Health, or MOH, we will receive payments from sales of lorcaserin, and are eligible to receive purchase price adjustment payments based on Teva’s annual net product sales (which are the gross invoiced sales less certain deductions described in the Teva Agreement), as well as additional milestone payments upon receiving marketing authorizations by the MOH.

Arena GmbH also entered into the Co-Development and License Agreement with Ildong for temanogrel, our internally discovered candidate intended for the treatment of thrombotic diseases. Under such agreement, we granted Ildong exclusive rights to commercialize temanogrel in South Korea for myocardial infarction, acute coronary syndrome, stroke, peripheral artery disease, and other cardiovascular diseases, subject to further development and regulatory approval of temanogrel. Initially, Ildong will be responsible for funding and conducting, under the direction of a joint steering committee, the next two planned clinical trials in this program: an additional Phase 1 trial in healthy volunteers to investigate the safety of co-administration with clopidogrel and aspirin and a Phase 2a proof-of-concept trial in patients. To date, we have not recognized any revenue under this agreement. We will maintain ownership of temanogrel outside of South Korea, and have the rights to use data generated by Ildong for the development and potential commercialization of temanogrel outside of South Korea by us or other Arena licensees. In addition, Ildong has agreed to pay us a $2.0 million development milestone if the planned additional Phase 1 and Phase 2a clinical trials conducted by Ildong support continued development and we or another Arena licensee initiates a Phase 2b clinical trial of temanogrel. We are also eligible to receive a royalty on net product sales of temanogrel in South Korea, and Ildong is eligible to receive a share of future payments received by us related to licensing transactions and sales of temanogrel in other territories.

Employee_Benefit_Plans

Employee Benefit Plans	12 Months Ended
Employee Benefit Plans	Dec. 31, 2014
Employee Benefit Plans	(14) Employee Benefit Plans

	401(k) Plan.

	All of our US employees are eligible to participate in our defined contribution retirement plan that complies with Section 401(k) of the Internal Revenue Code. We match 100% of each participant’s voluntary contributions, subject to a maximum of 6% of the participant’s eligible compensation. Our matching portion, which totaled $1.6 million, $1.5 million and $1.1 million for the years ended December 31, 2014, 2013, and 2012, respectively, vests over a five-year period from the date of hire.



	Pension Plan.

	Arena GmbH contributes to a multiemployer defined benefit pension plan, established under an affiliated group of employers, for the purpose of providing mandatory occupational pension benefits for its employees. The risks of participating in a multiemployer plan are different from a single-employer plan in that (i) assets contributed to the multiemployer plan by one employer may be used to provide benefits to employees of other participating employers, (ii) if a participating employer stops contributing to the plan, the unfunded obligations of the plan may be borne by the remaining participating employers, (iii) if Arena GmbH elects to stop participating in the multiemployer plan, Arena GmbH may be required to pay the plan an amount based on the underfunded status of the plan, referred to as a withdrawal liability, and (iv) Arena GmbH has no involvement in the management of the multiemployer plan’s investments. We currently have no intention of withdrawing from the multiemployer plan.

	Our contributions to the multiemployer plan were $0.7 million, $0.7 million and $0.6 million for the years ended December 31, 2014, 2013, and 2012, respectively.

Income_Taxes

Income Taxes	12 Months Ended
Income Taxes	Dec. 31, 2014
Income Taxes	(15) Income Taxes

	Our loss before benefit for income taxes is summarized by region as follows, in thousands:



			December 31,
			2014				2013				2012
	United States		$	(16,607	)		$	11,573			$	(62,674	)
	Foreign			(43,901	)			(31,008	)			(22,803	)

	Total loss before income taxes		$	(60,508	)		$	(19,435	)		$	(85,477	)


	We have not recorded a benefit for income taxes for the years ended December 31, 2014, 2013, and 2012 because we have a full valuation allowance.

	Our effective income tax rate differs from the statutory Federal rate of 34% for the years ended December 31, 2014, 2013, and 2012, due to the following, in thousands:



			December 31,
			2014				2013				2012
	Benefit for income taxes at statutory Federal rate		$	(20,573	)		$	(6,608	)		$	(29,062	)
	State income tax, net of Federal benefit and valuation allowance			0				0				0
	Permanent differences and other			2,318				2,122				(2,770	)
	Gain (loss) from valuation of derivative liabilities			(1,507	)			(3,922	)			5,244
	Foreign losses at lower effective rates			13,318				9,527				6,744
	Research and development and Orphan Drug credits			(2,992	)			(2,594	)			(1,005	)
	Adjustment to research and development credits and net operating losses, or NOLs			0				(59,790	)			4,831
	Change in Federal and foreign valuation allowance			9,436				61,265				16,018

	Benefit for income taxes		$	0			$	0			$	0




	The components of our net deferred tax assets are as follows, in thousands:



			December 31,
			2014				2013
	Deferred tax assets:
	Foreign NOL carryforwards		$	9,518			$	9,678
	Federal and California NOL carryforwards			216,906				231,450
	Federal and California research and development credit carryforwards			44,022				40,948
	Deferred revenues			36,448				21,109
	Depreciation			3,714				4,245
	Share-based compensation expense			8,549				7,223
	Other, net			4,011				5,921

	Total deferred tax assets			323,168				320,574
	Deferred tax liabilities			(767	)			(1,853	)

	Net deferred tax assets			322,401				318,721
	Valuation allowance			(322,401	)			(318,721	)

	Net deferred tax liabilities		$	0			$	0


	A valuation allowance is recorded against all of our deferred tax assets, as realization of such assets is not more-likely-than-not. The realization of our deferred tax assets is dependent upon future taxable income. Our ability to generate taxable income is analyzed regularly on a jurisdiction-by-jurisdiction basis. At such time as it is more-likely-than-not that we will generate taxable income in a jurisdiction, we will reduce or remove the valuation allowance. The valuation allowance increased by $3.7 million from December 31, 2013, to December 31, 2014.

	At December 31, 2014, we had Federal NOL carryforwards of $548.0 million that will begin to expire in 2022 unless previously utilized. At the same date, we had California NOL carryforwards of $584.8 million, which will begin expiring in 2015 and foreign NOL carryforwards of $115.9 million, which will begin to expire in 2015. At December 31, 2014, approximately $8.9 million of the Federal and California NOL carryforwards related to stock option exercise windfalls, which will result in an increase to additional paid-in capital, or APIC, and a decrease in income taxes payable at the time such carryforwards are utilized. At December 31, 2014, we also had Federal and California research and development tax credit carryforwards, net of reserves, of $29.1 million and $22.2 million, respectively. At December 31, 2014, we had a Federal Orphan Drug Credit carryforward of $0.2 million. Federal credit carryforwards will begin to expire in 2026 unless previously utilized. The California research and development credit carries forward indefinitely.

Sections 382 and 383 of the Internal Revenue Code, or IRC, limit the utilization of tax attribute carryforwards that arise prior to certain cumulative changes in a corporation’s ownership. We have completed an IRC Section 382/383 analysis through 2014 and identified ownership changes that limit our utilization of tax attribute carryforwards. We reduced deferred tax assets associated with such tax attribute carryforwards to remove deferred tax assets that will expire prior to utilization.

In accordance with authoritative guidance, the impact of an uncertain income tax position on the income tax return must be recognized at the largest amount that is more-likely-than-not to be sustained upon audit by the relevant taxing authority. An uncertain income tax position will not be recognized if it has less than a 50% likelihood of being sustained.



A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows, in thousands:



		December 31,
		2014				2013				2012
Gross unrecognized tax benefits at the beginning of the year		$	4,629			$	0			$	0
Additions from tax positions taken in the current year			585				541				0
Additions from tax positions taken in prior years			0				4,088				0
Reductions from tax positions taken in prior years			0				0				0
Tax settlements			0				0				0

Gross unrecognized tax benefits at end of the year		$	5,214			$	4,629			$	0


Of our total unrecognized tax benefits at December 31, 2014, $3.9 million will impact our effective tax rate in the event the valuation allowance is removed. We do not anticipate that there will be a substantial change in unrecognized tax benefits within the next 12 months.

Our practice is to recognize interest and/or penalties related to income tax matters in income tax expense. Because we have incurred net losses since our inception, we did not have any accrued interest or penalties included in our consolidated balance sheets at December 31, 2014, or 2013, and did not recognize any interest and/or penalties in our consolidated statements of operations and comprehensive loss for the years ended December 31, 2014, 2013, or 2012.

We have elected the “with and without method – direct effects only”, prescribed in accordance with authoritative guidance, with respect to recognition of stock option windfall tax benefits within APIC and will utilize general NOLs to offset taxable income before utilization of NOLs attributable to windfall tax benefits.

We are subject to income taxation in the United States at the Federal and state levels. All tax years are subject to examination by US and California tax authorities due to the carryforward of unutilized NOLs and tax credits. We are also subject to foreign income taxes in the countries in which we operate. To our knowledge, we are not currently under examination by any taxing authorities.

At December 31, 2014, no foreign subsidiaries have accumulated earnings and, as such, there are no unrepatriated earnings.

Our Swiss subsidiary, Arena GmbH, has been granted a conditional incentive tax holiday by the Canton of Aargau for its operations in Switzerland. Without a tax holiday or other tax incentives, the standard effective tax rate of a company located in Aargau is approximately 19%. As a result of the tax holiday and other tax incentives, we expect the effective tax rate for Arena GmbH to be approximately half of such rate. The tax holiday came into effect on January 1, 2013, and will continue for a period of up to 10 years, not to extend beyond December 31, 2022. As a result of foreign losses and a full valuation allowance, no net tax benefit was derived for the years ended December 31, 2014, and 2013, as a result of the tax holiday.

Legal_Proceedings

Legal Proceedings	12 Months Ended
Legal Proceedings	Dec. 31, 2014
Legal Proceedings	(16) Legal Proceedings

	Beginning on September 20, 2010, a number of complaints were filed in the US District Court for the Southern District of California against us and certain of our current and former employees and directors on behalf of certain purchasers of our common stock. The complaints were brought as purported stockholder class actions, and, in general, include allegations that we and certain of our current and former employees and directors violated federal securities laws by making materially false and misleading statements regarding our BELVIQ program, thereby artificially inflating the price of our common stock. The plaintiffs sought unspecified monetary damages and other relief. On August 8, 2011, the Court consolidated the actions and appointed a lead plaintiff and lead counsel. On November 1, 2011, the lead plaintiff filed a consolidated amended complaint. On March 28, 2013, the Court dismissed the consolidated amended complaint without prejudice. On May 13, 2013, the lead plaintiff filed a second consolidated amended complaint. On November 5, 2013, the Court dismissed the second consolidated amended complaint without prejudice as to all parties except for Robert E. Hoffman, who was dismissed from the action with prejudice. On November 27, 2013, the lead plaintiff filed a motion for leave to amend the second consolidated amended complaint. On March 20, 2014, the Court denied plaintiff’s motion and dismissed the second consolidated amended complaint with prejudice. On April 18, 2014, the lead plaintiff filed a notice of appeal, and on August 27, 2014, the lead plaintiff filed his appellate brief in the US Court of Appeals for the Ninth Circuit. On October 24, 2014, we filed our answering brief in response to the lead plaintiff’s appeal. On December 5, 2014, the lead plaintiff filed his reply brief. Due to the stage of these proceedings, we are not able to predict or reasonably estimate the ultimate outcome or possible losses relating to these claims.

Quarterly_Financial_Data_Unaud

Quarterly Financial Data (Unaudited)	12 Months Ended
Quarterly Financial Data (Unaudited)	Dec. 31, 2014
Quarterly Financial Data (Unaudited)	(17) Quarterly Financial Data (Unaudited)

	The following tables present quarterly data for the years ended December 31, 2014, and 2013, in thousands, except per share data:



	2014	Quarter ended			Quarter ended			Quarter ended		Quarter ended			Year ended
	2014	December 31			September 30			June 30		March 31			December 31
	Revenues	$	9,191		$	8,164		$	12,801	$	6,814		$	36,970
	Operating costs and expenses	$	39,351		$	34,373		$	38,167	$	30,352		$	142,243
	Net income (loss)	$	(32,061	)	$	(10,672	)	$	7,480	$	(25,255	)	$	(60,508	)
	Net income (loss) per share, basic and diluted	$	(0.15	)	$	(0.05	)	$	0.03	$	(0.12	)	$	(0.28	)



	2013	Quarter ended			Quarter ended			Quarter ended		Quarter ended			Year ended
	2013	December 31			September 30			June 30		March 31			December 31
	Revenues	$	6,516		$	3,578		$	68,927	$	2,373		$	81,394
	Operating costs and expenses	$	28,487		$	23,444		$	29,021	$	23,377		$	104,329
	Net income (loss)	$	(23,459	)	$	(17,200	)	$	40,100	$	(18,876	)	$	(19,435	)
	Net income (loss) per share, basic and diluted	$	(0.11	)	$	(0.08	)	$	0.18	$	(0.09	)	$	(0.09	)

Subsequent_Events

Subsequent Events	12 Months Ended
Subsequent Events	Dec. 31, 2014
Subsequent Events	(18) Subsequent Events

	In January 2015, we sold 21,000,000 shares of our common stock, par value $0.0001 per share, at a price of $4.8139 per share to the underwriters. We received approximately $100.7 million in net proceeds from this offering after deducting offering expenses.

	In February 2015, BELVIQ was approved by the MFDS for marketing for weight management in adults who are overweight with a comorbidity or obese in South Korea. Pursuant to this approval and the Ildong BELVIQ Agreement, we earned a $3.0 million milestone that we expect to receive, less withholding taxes, in March 2015.

The_Company_and_Summary_of_Sig1

The Company and Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2014

Basis of Presentation

New Accounting Guidance

Use of Estimates

Reclassifications

Certain prior year amounts have been reclassified to conform to the current year presentation.

Cash and Cash Equivalents

Cash and cash equivalents consist of cash and highly liquid investments with remaining maturities of three months or less when purchased.

Inventory

Concentrations of Risk and Geographical Data

Percentages of our total revenues are as follows:

Year Ended December 31,

2014

2013

2012

Eisai marketing and supply agreement (See Note 13)

93.60%

96.00%

85.60%

Toll manufacturing agreement with Siegfried

4.00%

3.30%

13.80%

Other collaborative agreements

2.40%

0.70%

0.60%

Total percentage of revenues

100.0%

Percentages of our total accounts receivable are as follows:

At December 31,

2014

2013

2012

Eisai marketing and supply agreement (See Note 13)

93.10%

94.50%

2.00%

Ildong marketing and supply agreement (See Note 13)

0.40%

1.00%

85.50%

Toll manufacturing agreement with Siegfried

0.00%

4.30%

12.30%

Other collaborative agreements

6.50%

0.20%

Total percentage of accounts receivable

100.0%

Property and Equipment

Intangibles

Long-lived Assets

Deferred Rent

Derivative Liabilities

Foreign Currency

Share-based Compensation

Revenue Recognition

Research and Development Costs

Research and Development Expenses

Comprehensive Loss

Net Loss Per Share

We calculate basic and diluted net loss per share allocable to common stockholders using the weighted-average number of shares of common stock outstanding during the period.

December 31,

2014

2013

2012

Stock options

15,530

14,435

13,110

Warrants

370

776

607

RSUs and unvested restricted stock

476

306

270

Total

16,376

15,517

13,987

Because the market condition for the PRSUs was not satisfied at December 31, 2014 and 2013, such securities are excluded from the table above.

Income Taxes

Fair Value Measurements

We measure our financial assets and liabilities at fair value, which is defined as the exit price, or the amount that would be received from selling an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date.

We use the following three-level valuation hierarchy that maximizes the use of observable inputs and minimizes the use of unobservable inputs to value our financial assets and liabilities:

Level 1— Observable inputs such as unadjusted quoted prices in active markets for identical instruments.

Level 2— Quoted prices for similar instruments in active markets or inputs that are observable for the asset or liability, either directly or indirectly.

Level 3— Significant unobservable inputs based on our assumptions.

Investments

We held an investment in TaiGen Biotechnology Co., Ltd., or TaiGen, that from December 31, 2011, to January 17, 2014, had a cost basis of zero due to prior impairment charges. On January 17, 2014, TaiGen completed an initial public offering and its common stock began to trade on the GreTai Securities Listed Market, under the name “TaiGen Biopharmaceuticals Holding Limited.” Such market is deemed to be comparable to a US over-the-counter market such that the fair value of our former investment in TaiGen, which previously had been accounted for as a cost method investment with a cost basis of zero, became readily determinable. Accordingly, on January 17, 2014, we recorded our former investment in TaiGen of 29.6 million shares based on its fair value of approximately $49.1 million. We began recording our former investment in TaiGen at fair value based on the trading price of TaiGen’s common stock, and the remaining former investment was revalued on each balance sheet date.

Gains and losses on the sale of available-for-sale securities are determined using the specific-identification method. During the year ended December 31, 2014, we sold all of our shares of TaiGen and recorded a realized gain of $49.6 million.

Other Intangible Assets

We capitalize into inventory amortization expense related to the manufacturing of BELVIQ. Such amortization will subsequently be recognized as cost of product sales when the related inventory is sold.

Sale and leaseback agreements and operating leases

We occupy four US properties under sale and leaseback agreements that allow us the option to repurchase these properties at various dates between 2017 and 2027 and, in some cases, include renewal options. The terms of these leases stipulate annual increases in monthly rental payments of 2.5%. We accounted for our sale and leaseback transactions using the required financing method because our options to repurchase these properties in the future are considered continued involvement. Under the financing method, the book value of the properties and related accumulated depreciation remain on our balance sheet and no sale is recognized. Instead, the sales price of the properties is recorded as a financing obligation, and a portion of each lease payment is recorded as interest expense. We recorded interest expense of $6.9 million, $7.1 million and $7.2 million for the years ended December 31, 2014, 2013, and 2012, respectively, related to these leases. We expect interest expense related to our facilities to total $50.4 million from December 31, 2014, through the remaining terms of the leases. At December 31, 2014, the total financing obligation for these facilities was $70.7 million. The aggregate residual value of the facilities at the end of the lease terms is $10.0 million.

We lease an additional US property under an operating lease, which expires in May 2027, contains a purchase option and stipulates annual increases in monthly rental payments of 2.5%. We also lease space in various facilities in Zofingen, Switzerland that can be terminated with 12 months written notice under an agreement that expires in 2032. We also lease a separate office space in Zofingen under an operating lease which expires in August 2020.

In accordance with the lease terms for certain of our US properties, we are required to maintain deposits for the benefit of the landlord throughout the term of the leases. A total of $1.4 million was recorded in other non-current assets on our consolidated balance sheets at December 31, 2014, and 2013, respectively, related to such leases.

We recognize rent expense on a straight-line basis over the term of each lease. Rent expense of $1.1 million, $1.1 million and $1.7 million was recognized for the years ended December 31, 2014, 2013, and 2012, respectively.

The_Company_and_Summary_of_Sig2

The Company and Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2014
Potentially Dilutive Securities Excluded from Calculation of Diluted Net Loss Per Share	The table below presents the weighted-average number of potentially dilutive securities that were excluded from our calculation of diluted net loss per share allocable to common stockholders for the years ended December 31, 2014, 2013, and 2012, in thousands.



		December 31,
		2014		2013		2012
	Stock options		15,530		14,435		13,110
	Warrants		370		776		607
	RSUs and unvested restricted stock		476		306		270

	Total		16,376		15,517		13,987

Sales Revenue, Net
Percentages of Total Revenues and Accounts Receivable	Percentages of our total revenues are as follows:



		Year Ended December 31,
		2014		2013		2012
	Eisai marketing and supply agreement (See Note 13)		93.60%		96.00%		85.60%
	Toll manufacturing agreement with Siegfried		4.00%		3.30%		13.80%
	Other collaborative agreements		2.40%		0.70%		0.60%

	Total percentage of revenues		100.0%		100.0%		100.0%

Accounts Receivable
Percentages of Total Revenues and Accounts Receivable	Percentages of our total accounts receivable are as follows:



		At December 31,
		2014		2013		2012
	Eisai marketing and supply agreement (See Note 13)		93.10%		94.50%		2.00%
	Ildong marketing and supply agreement (See Note 13)		0.40%		1.00%		85.50%
	Toll manufacturing agreement with Siegfried		0.00%		4.30%		12.30%
	Other collaborative agreements		6.50%		0.20%		0.20%

	Total percentage of accounts receivable		100.0%		100.0%		100.0%

Fair_Value_Disclosures_Tables

Fair Value Disclosures (Tables)	12 Months Ended
Fair Value Disclosures (Tables)	Dec. 31, 2014
Financial Assets and Liabilities Measured at Fair Value on Recurring Basis	The following tables present our valuation hierarchy for our financial assets and liabilities that are measured at fair value on a recurring basis at December 31 2014, and 2013, in thousands:



			Fair Value Measurements at December 31, 2014
			Balance		Quoted Prices in		Significant Other		Significant
					Active Markets		Observable		Unobservable Inputs
					(Level 1)		Inputs		(Level 3)
							(Level 2)
	Assets:
	Money market funds1		$	143,913	$	143,913	$	0	$	0
	Liabilities:
	Warrant derivative liabilities		$	474	$	0	$	474	$	0



			Fair Value Measurements at December 31, 2013
			Balance		Quoted Prices in		Significant Other		Significant
					Active Markets		Observable		Unobservable Inputs
					(Level 1)		Inputs		(Level 3)
							(Level 2)
	Assets:
	Money market funds1		$	208,833	$	208,833	$	0	$	0
	Liabilities:
	Warrant derivative liabilities		$	4,892	$	0	$	4,892	$	0


	(1)	Included in cash and cash equivalents on our consolidated balance sheets.

Inventory_Tables

Inventory (Tables)	12 Months Ended
Inventory (Tables)	Dec. 31, 2014
Components of Inventory	Inventory consisted of the following, in thousands:



		December 31,
		2014		2013
	Raw materials	$	1,167	$	657
	Work in process		3,520		4,104
	Finished goods at Arena GmbH		3,681		0
	Finished goods at Eisai		2,463		7,998

	Total inventory	$	10,831	$	12,759

Land_Property_and_Equipment_Ta

Land, Property and Equipment (Tables)	12 Months Ended
Land, Property and Equipment (Tables)	Dec. 31, 2014
Land, Property and Equipment	Land, property and equipment consisted of the following, in thousands:



		December 31,
		2014			2013
	Land	$	11,339		$	10,854
	Building and capital improvements		74,629			67,747
	Leasehold improvements		17,984			17,854
	Machinery and equipment		53,247			55,143
	Computers and software		15,363			11,568
	Furniture and office equipment		2,376			2,207

			174,938			165,373
	Less accumulated depreciation and amortization		(92,019	)		(87,985	)

	Land, property and equipment, net	$	82,919		$	77,388

Intangibles_Tables

Intangibles (Tables)	12 Months Ended
Intangibles (Tables)	Dec. 31, 2014
Intangibles	Intangibles consisted of the following, in thousands:



		December 31,
		2014			2013
	Acquired manufacturing production licenses – gross	$	13,049		$	14,545
	Acquired manufacturing production licenses – accumulated amortization		(4,567	)		(4,363	)

	Intangibles, net	$	8,482		$	10,182

Accounts_Payable_and_Other_Acc1

Accounts Payable and Other Accrued Liabilities (Tables)	12 Months Ended
Accounts Payable and Other Accrued Liabilities (Tables)	Dec. 31, 2014
Accounts Payable and Other Accrued Liabilities	Accounts payable and other accrued liabilities consisted of the following, in thousands:



		December 31,
		2014		2013
	Accounts payable	$	2,844	$	3,721
	Accrued compensation		4,792		4,205
	Other accrued liabilities		2,573		2,279

	Total accounts payable and other accrued liabilities	$	10,209	$	10,205

Transactions_with_Deerfield_Ta

Transactions with Deerfield (Tables)	12 Months Ended
Transactions with Deerfield (Tables)	Dec. 31, 2014
Summary of Principal Repayments on Loan	The following table summarizes the principal repayments made on the Deerfield loan from its inception through the date it was repaid in full, in thousands:



		Loan Principal
	Original loan principal	$	100,000
	July 2009 repayment		(10,000	)
	August 2010 repayment		(30,000	)
	January 2011 repayment		(20,000	)
	March 2011 repayment		(17,739	)
	January 2012 repayment		(5,000	)
	April and May 2012 cancellations as part of warrant exercises		(6,720	)
	May 2012 repayment		(10,541	)

	Outstanding principal balance at December 31, 2012	$	0

Derivative_Liabilities_Tables

Derivative Liabilities (Tables)	12 Months Ended
Derivative Liabilities (Tables)	Dec. 31, 2014
Gain (Loss) from Change in Fair Value of Derivative Liabilities	We recognized the following gain (loss) from valuation of derivative liabilities for the years ended December 31, 2014, 2013, and 2012, in thousands:



		December 31,
		2014		2013		2012
	Warrants	$	4,418	$	10,150	$	(13,480	)
	Former Deerfield acceleration right		0		0		55

	Total gain (loss) from valuation of derivative liabilities	$	4,418	$	10,150	$	(13,425	)

Commitments_Tables

Commitments (Tables)	12 Months Ended
Commitments (Tables)	Dec. 31, 2014
Annual Future obligations	Annual future obligations at December 31, 2014, are as follows, in thousands:



	Year ending December 31,	Financing			Operating
	Year ending December 31,	Obligations			Leases
	2015	$	8,292		$	969
	2016		9,262			1,126
	2017		9,494			1,147
	2018		9,731			1,170
	2019		8,053			1,192
	Thereafter		66,328			7,827

	Total minimum lease payments		111,160		$	13,431

	Less amounts representing interest		(50,413	)
	Add amounts representing residual value		9,990

	Lease financing obligations		70,737
	Less current portion		(2,492	)

		$	68,245

Stockholders_Equity_Tables

Stockholders' Equity (Tables)	12 Months Ended
Stockholders' Equity (Tables)	Dec. 31, 2014
Summary of Stock Option Activity	The following table summarizes our stock option activity under the Prior Plans and the 2013 LTIP, or collectively, our Equity Compensation Plans, for the year ended December 31, 2014, in thousands (except per share data):



		Options			Weighted-		Weighted-Average		Aggregate
					Average		Remaining		Intrinsic
					Exercise Price		Contractual		Value
							Term (in years)
	Outstanding at December 31, 2013		14,681		$	4.99
	Granted		2,618		$	6.21
	Exercised		(1,115	)	$	3.66
	Forfeited/cancelled/expired		(353	)	$	6.62

	Outstanding at December 31, 2014		15,831		$	5.25		5.48	$	10,962

	Vested and expected to vest at December 31, 2014		15,407		$	5.23		5.46	$	10,889

	Vested and exercisable at December 31, 2014		8,717		$	5.56		4.77	$	6,140

Summary of Restricted Stock Activity	The following table summarizes activity with respect to our time-based RSUs under our Equity Compensation Plans for the year ended December 31, 2014, in thousands (except per share data):



		RSUs			Weighted-Average		Aggregate
					Grant-Date Fair		Intrinsic
					Value		Value
	Unvested at January 1, 2014		369		$	7.23
	Granted		328		$	5.23
	Vested		(241	)	$	7.35
	Forfeited/cancelled		0

	Unvested at December 31, 2014		456		$	5.72

	Outstanding at December 31, 2014		676		$	6.42	$	2,347

Assumptions and Estimated Fair Value of Options	The table below sets forth the weighted-average assumptions and estimated fair value of stock options we granted under our Equity Compensation Plans during the years ended December 31, 2014, 2013, and 2012:



		December 31,
		2014			2013		2012
	Risk-free interest rate		1.80%			1.30%		1.40%
	Dividend yield		0%			0%		0%
	Expected volatility		81%			80%		90%
	Expected life (years)		6.17			6.24		6.05
	Weighted-average estimated fair value per share of stock options granted	$	4.37		$	5.25	$	2.27
Assumptions and Estimated Fair Value of Options to Purchase Stock granted Under Employee Stock Purchase Plan	The table below sets forth the assumptions and estimated fair value of the options to purchase stock granted under our employee stock purchase plan for multiple offering periods during the years ended December 31, 2014, 2013, and 2012:



		December 31,
		2014		2013	2012
	Risk-free interest rate	0.0% - 0.6%		0.0% - 0.5%	0.0% - 0.7%
	Dividend yield	0%		0%	0%
	Expected volatility	53% - 81%		79% - 105%	85% - 106%
	Expected life (years)	0.25 - 2.0		0.25 - 2.0	0.25 - 2.0
	Range of fair value per share of options granted under employee stock purchase plan	$1.37 to $4.22		$0.90 to $5.44	$0.56 to $5.44
Assumptions and Estimated Fair Value of Performance Restricted Stock Unit Awards	The table below sets forth the assumptions and estimated fair value of PRSU awards granted during the years ended December 31, 2014, and 2013:



		December 31,
		2014			2013
	Risk-free interest rate		0.70%			0.40%
	Dividend yield		0%			0%
	Expected volatility		78%			89%
	Performance period (years)		2.99			2.99
	Estimated fair value per share of PRSUs granted	$	7.16		$	7.50
Share Based Compensation Expense	We recognized share-based compensation expense as follows, in thousands, except per share data:



		December 31,
		2014			2013		2012
	Cost of product sales	$	0		$	17	$	0
	Research and development		7,118			4,318		1,822
	General and administrative		6,391			4,689		3,250

	Total share-based compensation expense and impact on net loss allocable to common stockholders	$	13,509		$	9,024	$	5,072

	Impact on net loss per share allocable to common stockholders, basic and diluted	$	0.06		$	0.04	$	0.03

	Share-based compensation capitalized into inventory	$	81		$	75	$	54

Total Unrecognized Estimated Compensation Expense	The table below sets forth our total unrecognized estimated compensation expense at December 31, 2014, by type of award and the weighted-average remaining requisite service period over which such expense is expected to be recognized:



		December 31, 2014
		Unrecognized			Remaining
		Expense (in			Weighted-Average
		thousands)			Recognition
					Period (in years)
	Unvested stock options	$	18,653			2.18
	RSUs		2,523			2.62
	PRSUs		5,876			1.63
Shares of Common Stock Reserved for Future Issuance	The following shares of our common stock are reserved for future issuance at December 31, 2014, in thousands:



	Outstanding warrants		1,965
	Equity Compensation Plans		41,786
	2009 ESPP		597
	Deferred compensation plan		79

	Total		44,427

Collaborations_Tables

Collaborations (Tables)	12 Months Ended
Collaborations (Tables)	Dec. 31, 2014
Revenues Recognized under Eisai Agreement	The following table summarizes the revenues we recognized under our collaboration with Eisai for the periods presented, in thousands:



			December 31,								From Inception
											Through
											December 31,
			2014			2013			2012		2014
	Net product sales		$	15,983		$	5,702		$	0	$	21,685

	Amortization of upfront payments			7,630			4,035			3,503		20,526
	Reimbursement of development expenses			10,037			2,020			27		15,420
	Milestone payments			500			66,000			20,000		86,500
	Reimbursement of patent and trademark expenses			444			361			87		892

	Subtotal other Eisai collaborative revenue			18,611			72,416			23,617		123,338

	Total		$	34,594		$	78,118		$	23,617	$	145,023

Deferred Revenues Attributable to Eisai	The following table summarizes the deferred revenues under our collaboration with Eisai at December 31, 2014, and 2013, in thousands:



			December 31,
			2014			2013
	Upfront payments		$	94,474		$	102,104
	Net product sales			7,081			30,299

	Total deferred revenues attributable to Eisai			101,555			132,403
	Less current portion			(14,622	)		(37,301	)

	Deferred revenues attributable to Eisai, less current portion		$	86,933		$	95,102

Summary of Cost Sharing Allocation	The chart below summarizes the general agreement regarding cost sharing between Eisai and us for significant development activities under the Eisai Agreement. In addition, Eisai or we may from time to time conduct approved development of lorcaserin at such party’s own expense. For example, Eisai was responsible for the expenses of the pilot study of 12-week duration to preliminarily assess lorcaserin and phentermine when co-administered. For the years ended December 31, 2014, 2013, and 2012, we recognized expenses of $35.3 million, $11.7 million and $8.6 million, respectively, for non-commercial manufacturing and other development costs related to lorcaserin.

	Eisai Second Amended and Restated Marketing and Supply Agreement: Cost Sharing for Development



			United States		Rest of	Remaining Territories

					North and South America
	BELVIQ		Not Applicable		General	Up to total of $100.0 million -

	- Pre-approval*				Eisai: 90%; Arena: 10%	Eisai: 50%; Arena: 50%



					Certain stability work Eisai: 50%; Arena: 50%	Thereafter, Eisai: 100%
	BELVIQ		General - Eisai: 90%; Arena 10%		General	Up to total of $50.0 million -

	- Post-approval*				Eisai: 90%; Arena: 10%	Eisai: 50%; Arena: 50%

			Non-FDA required portion of CVOT

			Up to $80.0 million -		Certain stability work	Thereafter, Eisai: 90%;

			Eisai: 50%; Arena: 50%		Eisai: 50%; Arena: 50%	Arena: 10%

			Thereafter, Eisai: 100%

			Certain pediatric studies

			Eisai: 50%; Arena: 50%
	Lorcaserin		Up to a total of $250.0 million (as reduced by up to $80.0 million for non-FDA required portion of CVOT) - Eisai: 50%; Arena: 50%

	products other than

	BELVIQ

	- Pre-approval
	Lorcaserin		Up to a total of $100.0 million in the aggregate across all additional products -

	products other than		Eisai: 50%; Arena: 50%

	BELVIQ

	- Post-approval		Thereafter, Eisai: 90%; Arena: 10%


	*	Development required by a regulatory authority, with the exception of the non-FDA required portion of the CVOT.

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
Income Taxes (Tables)	Dec. 31, 2014
Summary of Loss Before Provision (Benefit) for Income Taxes by Region	Our loss before benefit for income taxes is summarized by region as follows, in thousands:



		December 31,
		2014			2013			2012
	United States	$	(16,607	)	$	11,573		$	(62,674	)
	Foreign		(43,901	)		(31,008	)		(22,803	)

	Total loss before income taxes	$	(60,508	)	$	(19,435	)	$	(85,477	)

Reconciliation of Provision (Benefit) for Income Taxes at Statutory Federal Rate to Provision (Benefit) for Income Taxes	Our effective income tax rate differs from the statutory Federal rate of 34% for the years ended December 31, 2014, 2013, and 2012, due to the following, in thousands:



		December 31,
		2014			2013			2012
	Benefit for income taxes at statutory Federal rate	$	(20,573	)	$	(6,608	)	$	(29,062	)
	State income tax, net of Federal benefit and valuation allowance		0			0			0
	Permanent differences and other		2,318			2,122			(2,770	)
	Gain (loss) from valuation of derivative liabilities		(1,507	)		(3,922	)		5,244
	Foreign losses at lower effective rates		13,318			9,527			6,744
	Research and development and Orphan Drug credits		(2,992	)		(2,594	)		(1,005	)
	Adjustment to research and development credits and net operating losses, or NOLs		0			(59,790	)		4,831
	Change in Federal and foreign valuation allowance		9,436			61,265			16,018

	Benefit for income taxes	$	0		$	0		$	0

Components of Net Deferred Tax Assets	The components of our net deferred tax assets are as follows, in thousands:



		December 31,
		2014			2013
	Deferred tax assets:
	Foreign NOL carryforwards	$	9,518		$	9,678
	Federal and California NOL carryforwards		216,906			231,450
	Federal and California research and development credit carryforwards		44,022			40,948
	Deferred revenues		36,448			21,109
	Depreciation		3,714			4,245
	Share-based compensation expense		8,549			7,223
	Other, net		4,011			5,921

	Total deferred tax assets		323,168			320,574
	Deferred tax liabilities		(767	)		(1,853	)

	Net deferred tax assets		322,401			318,721
	Valuation allowance		(322,401	)		(318,721	)

	Net deferred tax liabilities	$	0		$	0

Reconciliation of Unrecognized Tax Benefits	A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows, in thousands:



		December 31,
		2014			2013			2012
	Gross unrecognized tax benefits at the beginning of the year	$	4,629		$	0		$	0
	Additions from tax positions taken in the current year		585			541			0
	Additions from tax positions taken in prior years		0			4,088			0
	Reductions from tax positions taken in prior years		0			0			0
	Tax settlements		0			0			0

	Gross unrecognized tax benefits at end of the year	$	5,214		$	4,629		$	0

Quarterly_Financial_Data_Unaud1

Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
Quarterly Financial Data (Unaudited) (Tables)	Dec. 31, 2014
Quarterly Data	The following tables present quarterly data for the years ended December 31, 2014, and 2013, in thousands, except per share data:



	2014	Quarter ended			Quarter ended			Quarter ended		Quarter ended			Year ended
	2014	December 31			September 30			June 30		March 31			December 31
	Revenues	$	9,191		$	8,164		$	12,801	$	6,814		$	36,970
	Operating costs and expenses	$	39,351		$	34,373		$	38,167	$	30,352		$	142,243
	Net income (loss)	$	(32,061	)	$	(10,672	)	$	7,480	$	(25,255	)	$	(60,508	)
	Net income (loss) per share, basic and diluted	$	(0.15	)	$	(0.05	)	$	0.03	$	(0.12	)	$	(0.28	)



	2013	Quarter ended			Quarter ended			Quarter ended		Quarter ended			Year ended
	2013	December 31			September 30			June 30		March 31			December 31
	Revenues	$	6,516		$	3,578		$	68,927	$	2,373		$	81,394
	Operating costs and expenses	$	28,487		$	23,444		$	29,021	$	23,377		$	104,329
	Net income (loss)	$	(23,459	)	$	(17,200	)	$	40,100	$	(18,876	)	$	(19,435	)
	Net income (loss) per share, basic and diluted	$	(0.11	)	$	(0.08	)	$	0.18	$	(0.09	)	$	(0.09	)

Company_and_Summary_of_Signifi

Company and Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	12 Months Ended			1 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 31, 2015
	Segment	Dec. 31, 2013	Dec. 31, 2012	Jan. 31, 2015
Schedule Of Significant Accounting Policies [Line Items]
Number of business segments	1
Intangible assets, estimated useful life	20 years
Foreign currency transaction gain (loss)	$2.20	$0.30	$0.20
Weighted average volatility	75.00%
Subsequent Event
Schedule Of Significant Accounting Policies [Line Items]
Issuance of common stock				21,000,000
UNITED STATES
Schedule Of Significant Accounting Policies [Line Items]
Long-lived assets	49	50.6
SWITZERLAND
Schedule Of Significant Accounting Policies [Line Items]
Long-lived assets	$42.40	$37
Building
Schedule Of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life	20 years
Leasehold improvements
Schedule Of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful lives	Over the shorter of the estimated useful lives of the assets or the lease term
Capital Improvements
Schedule Of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful lives	Over the estimated useful lives of the underlying assets
Minimum
Schedule Of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life	3 years
Maximum
Schedule Of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life	15 years

Percentages_of_Total_Revenues_

Percentages of Total Revenues (Detail) (Sales Revenue, Net)	12 Months Ended
Percentages of Total Revenues (Detail) (Sales Revenue, Net)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Concentration Risk [Line Items]
Percentage of revenues	100.00%	100.00%	100.00%
Eisai marketing and supply agreement
Concentration Risk [Line Items]
Percentage of revenues	93.60%	96.00%	85.60%
Toll manufacturing services agreement with Siegfried
Concentration Risk [Line Items]
Percentage of revenues	4.00%	3.30%	13.80%
Other collaborative agreements
Concentration Risk [Line Items]
Percentage of revenues	2.40%	0.70%	0.60%

Percentages_of_Total_Accounts_

Percentages of Total Accounts Receivable (Detail) (Accounts Receivable)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Concentration Risk [Line Items]
Percentage of accounts receivable	100.00%	100.00%	100.00%
Eisai marketing and supply agreement
Concentration Risk [Line Items]
Percentage of accounts receivable	93.10%	94.50%	2.00%
Ildong marketing and supply agreement
Concentration Risk [Line Items]
Percentage of accounts receivable	0.40%	1.00%	85.50%
Toll manufacturing services agreement with Siegfried
Concentration Risk [Line Items]
Percentage of accounts receivable	0.00%	4.30%	12.30%
Other collaborative agreements
Concentration Risk [Line Items]
Percentage of accounts receivable	6.50%	0.20%	0.20%

Potentially_Dilutive_Securitie

Potentially Dilutive Securities Excluded From Calculation of Diluted Net Loss Per Share (Detail)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities excluded from calculation of diluted net loss per share	16,376	15,517	13,987
Stock Options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities excluded from calculation of diluted net loss per share	15,530	14,435	13,110
Warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities excluded from calculation of diluted net loss per share	370	776	607
RSUs and Unvested Restricted Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities excluded from calculation of diluted net loss per share	476	306	270

Financial_Assets_and_Liabiliti

Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (Fair Value Measurements, Recurring, USD $)	Dec. 31, 2014		Dec. 31, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Assets:
Money market funds	$143,913	[1]	$208,833	[1]
Liabilities:
Warrant derivative liabilities	474		4,892
Quoted Prices in Active Markets (Level 1)
Assets:
Money market funds	143,913	[1]	208,833	[1]
Liabilities:
Warrant derivative liabilities	0		0
Significant Other Observable Inputs (Level 2)
Assets:
Money market funds	0	[1]	0	[1]
Liabilities:
Warrant derivative liabilities	474		4,892
Significant Unobservable Inputs (Level 3)
Assets:
Money market funds	0	[1]	0	[1]
Liabilities:
Warrant derivative liabilities	$0		$0

[1]	Included in cash and cash equivalents on our consolidated balance sheets.

Shortterm_Investments_Availabl1

Short-term Investments Available-for-Sale - Additional Information (Detail) (TaiGen Biotechnology Co., Ltd., USD $)	12 Months Ended
Share data in Millions, unless otherwise specified	Dec. 31, 2014	Jan. 17, 2014	Dec. 31, 2011
TaiGen Biotechnology Co., Ltd.
Investment Holdings [Line Items]
Equity investment			$0
Investment, shares		29.6
Investment, fair value		49,100,000
Realized gains from sale of shares	$49,600,000

Components_of_Inventory_Detail

Components of Inventory (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Inventory [Line Items]
Raw materials	$1,167	$657
Work in process	3,520	4,104
Total inventory	10,831	12,759
Arena GmbH
Inventory [Line Items]
Finished goods	3,681	0
Eisai
Inventory [Line Items]
Finished goods	$2,463	$7,998

Land_Property_and_Equipment_De

Land, Property and Equipment (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Land, property and equipment, gross	$174,938	$165,373
Less accumulated depreciation and amortization	-92,019	-87,985
Land, property and equipment, net	82,919	77,388
Land
Property, Plant and Equipment [Line Items]
Land, property and equipment, gross	11,339	10,854
Building and capital improvements
Property, Plant and Equipment [Line Items]
Land, property and equipment, gross	74,629	67,747
Leasehold improvements
Property, Plant and Equipment [Line Items]
Land, property and equipment, gross	17,984	17,854
Machinery and equipment
Property, Plant and Equipment [Line Items]
Land, property and equipment, gross	53,247	55,143
Computers and software
Property, Plant and Equipment [Line Items]
Land, property and equipment, gross	15,363	11,568
Furniture and Fixtures
Property, Plant and Equipment [Line Items]
Land, property and equipment, gross	$2,376	$2,207

Intangibles_Detail

Intangibles (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Acquired Finite-Lived Intangible Assets [Line Items]
Acquired manufacturing production licenses - gross	$13,049	$14,545
Acquired manufacturing production licenses - accumulated amortization	-4,567	-4,363
Intangibles, net	$8,482	$10,182

Intangibles_Additional_Informa

Intangibles - Additional Information (Detail) (USD $)	Dec. 31, 2014
In Millions, unless otherwise specified	Dec. 31, 2014
Finite-Lived Intangible Assets [Line Items]
Expected amortization expense, next 12 months	$0.70
Expected amortization expense, 2016	0.7
Expected amortization expense, 2017	0.7
Expected amortization expense, 2018	0.7
Expected amortization expense, 2019	0.7
Expected amortization expense each year, 2020 through 2027	$0.70

Accounts_Payable_and_Other_Acc2

Accounts Payable and Other Accrued Liabilities (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Accounts Payable and Accrued Liabilities, Current [Abstract]
Accounts payable	$2,844	$3,721
Accrued compensation	4,792	4,205
Other accrued liabilities	2,573	2,279
Total accounts payable and other accrued liabilities	$10,209	$10,205

Agreements_with_Siegfried_Addi

Agreements with Siegfried - Additional Information (Detail)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Jan. 31, 2008	Jan. 31, 2008
	USD ($)	USD ($)	USD ($)	CHF	USD ($)	CHF
					sqft
Transactions with Third Party [Line Items]
Asset purchase agreement, purchase price in cash						31.8
Asset purchase agreement, purchase price in value of common shares					8
Expenses incurred under technical services agreement	2.5	2.8	2.6
Asset purchase agreement, area of building acquired					67,000	67,000
Asset purchase agreement, total area of building					134,000	134,000
Company's optional purchase price of remaining portion of the building						15
Seller's optional selling price of remaining portion of the building						8
Real estate property purchase price				8.2
Cash payment date	2015-07
Lease agreement, expiration date	31-Dec-16
Extended lease expiration date	31-Dec-19
Annual base rent amount				0.4
Early termination date lessee option	31-Dec-15
Early termination date lessor option	31-Dec-17

Transactions_with_Deerfield_Ad

Transactions with Deerfield - Additional Information (Detail) (USD $)

12 Months Ended

1 Months Ended

2 Months Ended

3 Months Ended

1 Months Ended

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Jan. 31, 2012

31-May-12

Aug. 31, 2012

Feb. 29, 2012

31-May-12

Mar. 31, 2011

Jan. 31, 2011

Aug. 31, 2010

Jul. 31, 2009

Aug. 31, 2008

Jun. 30, 2006

Debt Instrument [Line Items]

Issuance of warrants to purchase stock, number of shares

1,106,344

829,856

Exercise Price

$4.34

Deemed dividend

$2,824,000

Non-cash loss extinguishment on debt

-6,338,000

Net proceeds from exercise of warrant to purchase stock

100,000

Total interest expense

1,900,000

Secured Debt

Debt Instrument [Line Items]

Fair value amount recorded as liability

47,900,000

Fair value of warrants

39,100,000

Deerfield Additional Loan Election

Debt Instrument [Line Items]

Additional loan under Facility Agreement

20,000,000

Fair value of embedded liabilities

9,500,000

Deerfield Acceleration Right

Debt Instrument [Line Items]

Fair value of embedded liabilities

500,000

Common Stock

Debt Instrument [Line Items]

Stock issued

10,000

12,650,000

Deerfield

Debt Instrument [Line Items]

Issuance of warrants to purchase stock, number of shares

8,631,410

Exercise Price

$1.75

Stock issued

4,000,000

19,000,000

Net proceeds after prepayment of loan

27,900,000

Value increment associated with warrant issuance

3,800,000

Non-cash loss extinguishment on debt

-1,700,000

Net proceeds from exercise of warrant to purchase stock

32,500,000

Deerfield | Warrant 1

Debt Instrument [Line Items]

Warrants cancelled under exchange agreement

11,800,000

Deerfield | Warrant 2

Debt Instrument [Line Items]

Warrants cancelled under exchange agreement

1,831,410

Deerfield | Series D Convertible Preferred Stock

Debt Instrument [Line Items]

Stock issued

9,953

Sale of stock, price per share

$1,657.75

Deemed dividend

2,800,000

Deerfield | Cancelled Warrant | Warrant 1

Debt Instrument [Line Items]

Exercise Price

$5.42

Deerfield | Cancelled Warrant | Warrant 2

Debt Instrument [Line Items]

Exercise Price

$3.45

Deerfield | Common Stock

Debt Instrument [Line Items]

Stock issued

9,953,250

Sale of stock, price per share

$1.66

Number of shares converted

9,953,250

Deerfield | Facility Agreement

Debt Instrument [Line Items]

Entry date into agreement

1-Jul-09

Secured loan

100,000,000

Net proceeds from secured loan

95,600,000

Issuance of warrants to purchase stock, number of shares

28,000,000

Expiration Date

17-Jun-13

Exercise Price

$5.42

Periodic repayments of loan

5,000,000

10,541,000

17,739,000

20,000,000

30,000,000

10,000,000

Non-cash loss extinguishment on debt

-6,400,000

-4,700,000

Cancellation of Outstanding Principal Balance Loan

-6,720,000

Summary_of_Principal_Repayment

Summary of Principal Repayments on Loan (Detail) (Deerfield, USD $)	1 Months Ended						2 Months Ended
In Thousands, unless otherwise specified	31-May-12	Jan. 31, 2012	Mar. 31, 2011	Jan. 31, 2011	Aug. 31, 2010	Jul. 31, 2009	31-May-12	Dec. 31, 2012
Debt Instrument [Line Items]
Outstanding principal balance at December 31, 2012								$0
Facility Agreement
Debt Instrument [Line Items]
Cancellations as part of warrant exercises							-6,720
Original loan principal						100,000
Periodic repayments of loan	($10,541)	($5,000)	($17,739)	($20,000)	($30,000)	($10,000)

Derivative_Liabilities_Additio

Derivative Liabilities - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2014	Aug. 31, 2008	Jun. 30, 2006	Jul. 31, 2009
Derivative [Line Items]
Issuance of warrant to purchase stock, number of shares			1,106,344	829,856
Original exercise price of warrant			$7.71	$15.49
Revised exercise price of warrant		$4.34
Warrant outstanding		1,965,418
Adjustment price of warrant in anti-dilution provision		$6.72
Derivative liabilities, current	$0	$474,000
Derivative liabilities, non-current	4,892,000	0
Number of shares of common stock into which expired warrant could have been converted	1,457,405
Net proceeds from exercise of warrant to purchase stock	100,000
Deerfield Acceleration Right
Derivative [Line Items]
Fair value of embedded liabilities					$500,000

Gain_Loss_from_Change_in_Fair_

Gain (Loss) from Change in Fair Value of Derivative Liabilities (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivative [Line Items]
Total gain (loss) from valuation of derivative liabilities	$4,418	$10,150	($13,425)
Warrants
Derivative [Line Items]
Total gain (loss) from valuation of derivative liabilities	4,418	10,150	-13,480
Deerfield Acceleration Right
Derivative [Line Items]
Total gain (loss) from valuation of derivative liabilities	$0	$0	$55

Commitments_Additional_Informa

Commitments - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
	Property	Dec. 31, 2013	Dec. 31, 2012
Commitments [Line Items]
Number of properties under sale and leaseback agreements	4
Sale and leaseback transaction, interest expense	$6,900,000	$7,100,000	$7,200,000
Sale and leaseback transaction, expected interest expense through lease terms	50,413,000
Sale and leaseback transaction, total financing obligation	70,737,000
Residual value of facilities at end of lease term	9,990,000
Deposits related to leases	1,400,000	1,400,000
Rent expense	$1,100,000	$1,100,000	$1,700,000
Minimum
Commitments [Line Items]
Sale and leaseback transaction optional repurchase date, year	2017
Maximum
Commitments [Line Items]
Sale and leaseback transaction optional repurchase date, year	2027
Properties under sale and leaseback agreements
Commitments [Line Items]
Percentage of annual increase in monthly rental payments	2.50%
Properties under operating lease \| SWITZERLAND
Commitments [Line Items]
Notice period for Operating lease termination	12 months
Operating lease agreement expiration year	2032
Operating lease agreement expiration date	2020-08
Properties under operating lease \| UNITED STATES
Commitments [Line Items]
Percentage of annual increase in monthly rental payments	2.50%
Operating lease agreement expiration date	2027-05

Annual_Future_Obligations_Deta

Annual Future Obligations (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Financing Obligations
2015	$8,292
2016	9,262
2017	9,494
2018	9,731
2019	8,053
Thereafter	66,328
Total minimum lease payments	111,160
Less amounts representing interest	-50,413
Add amounts representing residual value	9,990
Lease financing obligations	70,737
Less current portion	-2,492	-2,056
Lease financing obligations, less current portion	68,245	70,738
Operating Leases
2015	969
2016	1,126
2017	1,147
2018	1,170
2019	1,192
Thereafter	7,827
Total minimum lease payments	$13,431

Stockholders_Equity_Additional

Stockholders' Equity - Additional Information (Detail) (USD $)	12 Months Ended			1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Jun. 30, 2014	Mar. 31, 2014	Mar. 31, 2013	Jun. 10, 2013
Stockholders Equity [Line Items]
Common stock closing price	$3.47			3.47
Intrinsic value of stock options exercised	$2,700,000	$4,600,000	$5,400,000
Proceeds from stock option exercises	4,100,000
Common stock issued under employee stock purchase plan	304,085	334,360	341,108
Restricted Stock Units (RSU)
Stockholders Equity [Line Items]
RSUs and restricted stock unvested	456,000	369,000		456,000
Units granted	328,000
Total fair value of RSUs vested	1,800,000	900,000	0
Weighted-average estimated grant-date fair value	$5.23	$6.91	$8.87
Aggregate intrinsic value of units outstanding	2,347,000			2,347,000
Restricted Stock Units (RSU) \| Non Employee Directors
Stockholders Equity [Line Items]
Vesting period					1 year
Units granted					177,688
Conversion to underlying common shares description				Convert to the underlying common shares at the earliest of (i) the three-year anniversary of the grant date, (ii) the director's separation from service or (iii) a change in control of Arena.
Restricted Stock Units (RSU) \| Executive Officers
Stockholders Equity [Line Items]
Vesting percentage				25.00%
Vesting period				4 years
Units granted				150,000
Performance restricted stock units
Stockholders Equity [Line Items]
Vesting period						3 years	3 years
Units granted						695,000	780,000
Weighted-average estimated grant-date fair value	$7.16	$7.50
Multiplier of grant-date fair value for cap on number of PRSUs that may be granted						6
Cap on percentage of funding if the absolute 3-year TSR is negative						100.00%
PRSU grant-date fair value						5,000,000	5,900,000
Aggregate intrinsic value of units outstanding						5,100,000
Performance restricted stock units \| Minimum
Stockholders Equity [Line Items]
Number of shares to be awarded as a percentage of target amounts						0.00%	0.00%
Performance restricted stock units \| Maximum
Stockholders Equity [Line Items]
Number of shares to be awarded as a percentage of target amounts						200.00%	200.00%
Deferred Compensation Plan
Stockholders Equity [Line Items]
RSUs and restricted stock unvested	79,169	79,169	79,169	79,169
Long Term Incentive Plan Twenty Thirteen
Stockholders Equity [Line Items]
Shares authorized								30,000,000
Stock options exercisable period		7 years
Long Term Incentive Plan Twenty Thirteen \| Stock Options And Stock Appreciation Rights
Stockholders Equity [Line Items]
Reduction in number of shares available for grant for every share issued		1
Increase in number of shares available for grant for every share released		1
Long Term Incentive Plan Twenty Thirteen \| Other Than Stock Options And Stock Appreciation Rights
Stockholders Equity [Line Items]
Reduction in number of shares available for grant for every share issued		1.25
Increase in number of shares available for grant for every share released		1.25
Long Term Incentive Plan Twenty Thirteen \| Stock Options
Stockholders Equity [Line Items]
Vesting percentage		25.00%
Vesting period		4 years
Long Term Incentive Plan Twenty Thirteen \| Restricted Stock Units (RSU) \| Minimum
Stockholders Equity [Line Items]
Vesting period		1 year
Long Term Incentive Plan Twenty Thirteen \| Restricted Stock Units (RSU) \| Maximum
Stockholders Equity [Line Items]
Vesting period		4 years
Long Term Incentive Plan Twenty Thirteen \| Performance restricted stock units \| Minimum
Stockholders Equity [Line Items]
Vesting period		12 months
Long Term Incentive Plan Twenty Thirteen \| Stock Options and Stock Appreciation Rights (SARs) \| Minimum
Stockholders Equity [Line Items]
Purchase price of common shares as a percentage of fair market value		100.00%
Employee Stock Purchase Plan
Stockholders Equity [Line Items]
Proceeds from stock issued under employee stock purchase plans	$1,100,000
Maximum Percentage of Annual Compensation contributable to ESPP	15.00%			15.00%
Maximum number of shares allowed to purchase per PURCHASE period	625
Maximum ESPP offering period	24 months
Shares available for issuance	596,574			596,574
Employee Stock Purchase Plan \| Minimum
Stockholders Equity [Line Items]
Purchase price of common shares as a percentage of fair market value	85.00%

Summary_of_Stock_Option_Activi

Summary of Stock Option Activity (Detail) (Stock Options, USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014
Stock Options
Number of stock options
Outstanding at December 31, 2013	14,681,000
Granted	2,618,000
Exercised	-1,115,000
Forfeited/cancelled/expired	-353,000
Outstanding at December 31, 2014	15,831,000
Vested and expected to vest at December 31, 2014	15,407,000
Vested and exercisable at December 31, 2014	8,717,000
Weighted Average Exercise Price
Outstanding at December 31, 2013	$4.99
Granted	$6.21
Exercised	$3.66
Forfeited/cancelled/expired	$6.62
Outstanding at December 31, 2014	$5.25
Vested and expected to vest at December 31, 2014	$5.23
Vested and exercisable at December 31, 2014	$5.56
Weighted Average Remaining Contractual Life (in years)
Outstanding at December 31, 2014	5 years 5 months 23 days
Vested and expected to vest at December 31, 2014	5 years 5 months 16 days
Vested and exercisable at December 31, 2014	4 years 9 months 7 days
Aggregate intrinsic value
Outstanding at December 31, 2014	$10,962
Vested and expected to vest at December 31, 2014	10,889
Vested and exercisable at December 31, 2014	$6,140

Summary_of_Restricted_Stock_Un

Summary of Restricted Stock Units Activity (Detail) (Restricted Stock Units (RSU), USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Restricted Stock Units (RSU)
Restricted Stock Units
Unvested at January 1, 2014	369,000
Granted	328,000
Vested	-241,000
Forfeited/cancelled	0
Unvested at December 31, 2014	456,000	369,000
Outstanding at December 31, 2014	676,000
Weighted-Average Grant-Date Fair Value
Unvested at January 1, 2014	$7.23
Granted	$5.23	$6.91	$8.87
Vested	$7.35
Forfeited/cancelled	$0
Unvested at December 31, 2014	$5.72	$7.23
Outstanding at December 31, 2014	$6.42
Aggregate Intrinsic Value
Outstanding at December 31, 2014	$2,347

WeightedAverage_Assumptions_an

Weighted-Average Assumptions and Estimated Fair Value of Stock Options (Detail) (Stock Options, USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.80%	1.30%	1.40%
Dividend yield	0.00%	0.00%	0.00%
Expected volatility	81.00%	80.00%	90.00%
Expected life (years)	6 years 2 months 1 day	6 years 2 months 27 days	6 years 18 days
Weighted-average estimated fair value per share of stock options granted	$4.37	$5.25	$2.27

Assumptions_and_Estimated_Fair

Assumptions and Estimated Fair Value of Options (Detail) (Employee Stock Purchase Plan, USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate Minimum	0.00%	0.00%	0.00%
Risk-free interest rate Maximum	0.60%	0.50%	0.70%
Dividend yield	0.00%	0.00%	0.00%
Expected volatility Minimum	53.00%	79.00%	85.00%
Expected volatility Maximum	81.00%	105.00%	106.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life (years)	3 months	3 months	3 months
Range of fair value per share of options granted under our employee stock purchase plans	1.37	0.9	0.56
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life (years)	2 years	2 years	2 years
Range of fair value per share of options granted under our employee stock purchase plans	4.22	5.44	5.44

Assumptions_and_Estimated_Fair1

Assumptions and Estimated Fair Value of Performance Restricted Stock Unit Awards (Detail) (Performance restricted stock units, USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Performance restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	0.70%	0.40%
Dividend yield	0.00%	0.00%
Expected volatility	78.00%	89.00%
Performance period (years)	2 years 11 months 27 days	2 years 11 months 27 days
Estimated fair value per share of PRSUs granted	$7.16	$7.50

Share_Based_Compensation_Expen

Share Based Compensation Expense (Detail) (USD $)	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$13,509	$9,024	$5,072
Impact on net loss per share allocable to common stockholders, basic and diluted	$0.06	$0.04	$0.03
Share-based compensation capitalized into inventory	81	75	54
Cost of product sales
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	0	17	0
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	7,118	4,318	1,822
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$6,391	$4,689	$3,250

Total_Unrecognized_Estimated_C

Total Unrecognized Estimated Compensation Expense (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized expense	$18,653
Weighted average recognition period	2 years 2 months 5 days
Restricted Stock Units (RSU)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized expense	2,523
Weighted average recognition period	2 years 7 months 13 days
Performance restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized expense	$5,876
Weighted average recognition period	1 year 7 months 17 days

Common_Stock_Reserved_for_Futu

Common Stock Reserved for Future Issuance (Detail)	Dec. 31, 2014
In Thousands, unless otherwise specified	Dec. 31, 2014
Class of Stock [Line Items]
Common stock shares reserved for future issuance	44,427
Warrants Outstanding
Class of Stock [Line Items]
Common stock shares reserved for future issuance	1,965
Equity Compensation Plans
Class of Stock [Line Items]
Common stock shares reserved for future issuance	41,786
Employee Stock Purchase Plan
Class of Stock [Line Items]
Common stock shares reserved for future issuance	597
Deferred Compensation Plan
Class of Stock [Line Items]
Common stock shares reserved for future issuance	79

Collaborations_Additional_Info

Collaborations - Additional Information (Detail) (USD $)	3 Months Ended								12 Months Ended			54 Months Ended	1 Months Ended
Collaborations - Additional Information (Detail) (USD $)	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Jul. 31, 2013	Nov. 30, 2012	Jul. 31, 2014	Feb. 28, 2015	Sep. 30, 2014	Nov. 30, 2013	Jul. 31, 2010	31-May-12
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reduction in deferred revenues									($29,764,000)	$75,880,000	$17,849,000
Increase in payable to Eisai									13,240,000	19,572,000	276,000
Recall expense									1,100,000
Current portion of deferred revenues	15,238,000				37,861,000				15,238,000	37,861,000		15,238,000
Deferred revenues, less current portion	93,064,000				101,329,000				93,064,000	101,329,000		93,064,000
Recognized revenues	9,191,000	8,164,000	12,801,000	6,814,000	6,516,000	3,578,000	68,927,000	2,373,000	36,970,000	81,394,000	27,587,000
Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Recognized milestone revenue									500,000	66,000,000	20,000,000	86,500,000
Net product sales									15,983,000	5,702,000	0	21,685,000
Payable to Eisai	23,705,000				19,305,000				23,705,000	19,305,000		23,705,000
Expiration period after first commercial sale of BELVIQ										12 years
Current portion of deferred revenues	14,622,000				37,301,000				14,622,000	37,301,000		14,622,000
Deferred revenues, less current portion	86,933,000				95,102,000				86,933,000	95,102,000		86,933,000
Amortization of Upfront Payments									7,630,000	4,035,000	3,503,000	20,526,000
Recognized revenues									34,594,000	78,118,000	23,617,000	145,023,000
lldong Temanogrel
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments on achievement	2,000,000								2,000,000			2,000,000
Lorcaserin
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Expenses incurred related to lorcaserin									35,300,000	11,700,000	8,600,000
lldong BELVIQ
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative agreement upfront payments													5,000,000
Collaborative agreement revenue recognition period														14 years
Current portion of deferred revenues	400,000								400,000			400,000
Deferred revenues, less current portion	3,900,000								3,900,000			3,900,000
Amortization of Upfront Payments									400,000	500,000	100,000
CY Biotech Company Limited
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative agreement upfront payments													2,000,000
Collaborative agreement revenue recognition period													14 years
Current portion of deferred revenues	200,000								200,000			200,000
Deferred revenues, less current portion	1,800,000								1,800,000			1,800,000
Amortization of Upfront Payments									200,000	100,000
TEVA Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative agreement upfront payments															500,000
Collaborative agreement revenue recognition period															9 years
Recognized milestone revenue									250,000
Current portion of deferred revenues	100,000								100,000			100,000
Deferred revenues, less current portion	400,000								400,000			400,000
Recognized revenues									300,000
Recalled Bottles
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Recall expense									400,000
Reimbursement Costs
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Recall expense									700,000
Subsequent Event \| lldong BELVIQ
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Recognized milestone revenue																3,000,000
Product Purchase Price \| Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Net product sales									14,200,000
Voucher Redemption \| Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Net product sales									1,300,000
Product Sampling \| Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Net product sales									500,000
Reduction in deferred revenues																	-6,000,000
Increase in payable to Eisai																	6,000,000
New Savings Card \| Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reduction in deferred revenues									-1,800,000
Increase in payable to Eisai									1,800,000
UNITED STATES \| Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Purchase price range minimum																		31.50%
Purchase price range maximum																		36.50%
Annual net product sales threshold for maximum purchase price																		750,000,000
Non US Territories Other Than Europe, China and Japan \| Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Purchase price range minimum																		30.75%
Purchase price range maximum																		35.75%
Annual net product sales threshold for maximum purchase price																		750,000,000
Europe, China and Japan \| Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Purchase price range minimum																		27.50%
Purchase price range maximum																		35.00%
Annual net product sales threshold for maximum purchase price																		500,000,000
Original Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative agreement initiation date																			2010-07
Collaborative agreement upfront payments																			50,000,000
Collaborative agreement revenue recognition period																			16 years
Eisai First Amended Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative agreement initiation date																				2012-05
Collaborative agreement upfront payments																				5,000,000
Collaborative agreement revenue recognition period																				15 years
Eisai Second Amended Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative agreement initiation date																		2013-11
Collaborative agreement upfront payments																		60,000,000
Collaborative agreement revenue recognition period																		15 years
Recognized milestone revenue												86,500,000
Additional milestone payments on achievement	176,000,000								176,000,000			176,000,000
Aggregate one-time purchase price adjustments	1,560,000,000								1,560,000,000			1,560,000,000
Eisai Second Amended Agreement \| All Territories
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate one-time purchase price adjustments	1,190,000,000								1,190,000,000			1,190,000,000
First annual net sales threshold to earn purchase price adjustments in all territories																		250,000,000
Last annual net sales threshold to earn purchase price adjustments in all territories																		2,500,000,000
Portion of purchase price adjustment payments																		330,000,000
Annual net sales threshold to earn portion of purchase price adjustments in all territories																		1,000,000,000
Eisai Second Amended Agreement \| All Non-US territories
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional one-time purchase price adjustment payments	370,000,000								370,000,000			370,000,000
Eisai Second Amended Agreement \| Non-US Territories in North and South America
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional one-time purchase price adjustment payments	185,000,000								185,000,000			185,000,000
First annual net sales threshold to earn purchase price adjustments outside of the US																		100,000,000
Last annual net sales threshold to earn additional purchase price adjustments outside of the US																		1,000,000,000
Eisai Second Amended Agreement \| Territories Outside of North and South America
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional one-time purchase price adjustment payments	185,000,000								185,000,000			185,000,000
First annual net sales threshold to earn purchase price adjustments outside of the US																		100,000,000
Last annual net sales threshold to earn additional purchase price adjustments outside of the US																		1,000,000,000
US Drug Enforcement \| Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Recognized milestone revenue										65,000,000
US Food and Drug Administration \| Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Recognized milestone revenue											20,000,000
Lorcaserin In Brazil \| Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Recognized milestone revenue									500,000
Lorcaserin In Mexico \| Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Recognized milestone revenue										500,000
Lorcaserin In Canada \| Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Recognized milestone revenue										$500,000

Revenues_Recognized_under_Coll

Revenues Recognized under Collaboration with Eisai (Detail) (USD $)	3 Months Ended								12 Months Ended			54 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total revenues	$9,191,000	$8,164,000	$12,801,000	$6,814,000	$6,516,000	$3,578,000	$68,927,000	$2,373,000	$36,970,000	$81,394,000	$27,587,000
Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Net product sales									15,983,000	5,702,000	0	21,685,000
Amortization of Upfront Payments									7,630,000	4,035,000	3,503,000	20,526,000
Milestone payments									500,000	66,000,000	20,000,000	86,500,000
Subtotal other Eisai collaborative revenue									18,611,000	72,416,000	23,617,000	123,338,000
Total revenues									34,594,000	78,118,000	23,617,000	145,023,000
Eisai \| Research and development
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement revenue									10,037,000	2,020,000	27,000	15,420,000
Eisai \| Patent and trademark expenses
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement revenue									$444,000	$361,000	$87,000	$892,000

Deferred_Revenues_under_Collab

Deferred Revenues under Collaboration with Eisai (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Deferred Revenue Arrangement [Line Items]
Less current portion	($15,238)	($37,861)
Deferred revenues, less current portion	93,064	101,329
Eisai
Deferred Revenue Arrangement [Line Items]
Total deferred revenues	101,555	132,403
Less current portion	-14,622	-37,301
Deferred revenues, less current portion	86,933	95,102
Total deferred revenues	101,555	132,403
Eisai \| Up-front Payment Arrangement
Deferred Revenue Arrangement [Line Items]
Total deferred revenues	94,474	102,104
Total deferred revenues	94,474	102,104
Eisai \| Sales Revenue, Goods, Net
Deferred Revenue Arrangement [Line Items]
Total deferred revenues	7,081	30,299
Total deferred revenues	$7,081	$30,299

Summary_of_Cost_Sharing_Alloca

Summary of Cost Sharing Allocation (Detail) (USD $)	12 Months Ended
Summary of Cost Sharing Allocation (Detail) (USD $)	Dec. 31, 2014
Belviq Product Pre Approval Up To One Hundred Million Dollars \| Territories Outside of North and South America \| Arena GmbH
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate expense cap on equal split of development expenses	$100,000,000	[1]
Portion of expenses	50.00%	[1]
Belviq Product Pre Approval Up To One Hundred Million Dollars \| Territories Outside of North and South America \| Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	50.00%	[1]
Belviq Product Pre Approval More Than One Hundred Million Dollars \| Territories Outside of North and South America \| Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	100.00%	[1]
Belviq Product Post Approval Up To Fifty Million Dollars \| Territories Outside of North and South America \| Arena GmbH
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate expense cap on equal split of development expenses	50,000,000	[1]
Portion of expenses	50.00%	[1]
Belviq Product Post Approval Up To Fifty Million Dollars \| Territories Outside of North and South America \| Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	50.00%	[1]
Belviq Product Post Approval More Than Fifty Million Dollars \| Territories Outside of North and South America \| Arena GmbH
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	10.00%	[1]
Belviq Product Post Approval More Than Fifty Million Dollars \| Territories Outside of North and South America \| Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	90.00%	[1]
Belviq Product Pre Approval \| Non-US Territories in North and South America \| Arena GmbH
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	10.00%	[1]
Belviq Product Pre Approval \| Non-US Territories in North and South America \| Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	90.00%	[1]
Belviq Product Pre Approval Certain Stability Work \| Non-US Territories in North and South America \| Arena GmbH
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	50.00%	[1]
Belviq Product Pre Approval Certain Stability Work \| Non-US Territories in North and South America \| Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	50.00%	[1]
Belviq Product Post Approval \| Non-US Territories in North and South America \| Arena GmbH
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	10.00%	[1]
Belviq Product Post Approval \| Non-US Territories in North and South America \| Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	90.00%	[1]
Belviq Product Post Approval \| UNITED STATES \| Arena GmbH
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	10.00%	[1]
Belviq Product Post Approval \| UNITED STATES \| Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	90.00%	[1]
Belviq Product Post Approval Certain Stability Work \| Non-US Territories in North and South America \| Arena GmbH
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	50.00%	[1]
Belviq Product Post Approval Certain Stability Work \| Non-US Territories in North and South America \| Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	50.00%	[1]
Belviq Product Post Approval Certain Pediatric Studies \| UNITED STATES \| Arena GmbH
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	50.00%	[1]
Belviq Product Post Approval Certain Pediatric Studies \| UNITED STATES \| Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	50.00%	[1]
Belviq Product Post Approval Non Fda Required Portion Of Cardiovascular Outcomes Trial Up To Eighty Million Dollars \| UNITED STATES \| Arena GmbH
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate expense cap on equal split of development expenses	80,000,000	[1]
Portion of expenses	50.00%	[1]
Belviq Product Post Approval Non Fda Required Portion Of Cardiovascular Outcomes Trial Up To Eighty Million Dollars \| UNITED STATES \| Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	50.00%	[1]
Belviq Product Post Approval Non Fda Required Portion Of Cardiovascular Outcomes Trial More Than Eighty Million Dollars \| UNITED STATES \| Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	100.00%	[1]
Lorcaserin Product Other Than Belviq Pre Approval \| Arena GmbH
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	50.00%
Lorcaserin Product Other Than Belviq Pre Approval \| Arena GmbH \| Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate expense cap on equal split of development expenses	250,000,000
Lorcaserin Product Other Than Belviq Pre Approval \| Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	50.00%
Lorcaserin Product Other Than Belviq Post Approval Up To One Hundred Million Dollars \| Arena GmbH
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate expense cap on equal split of development expenses	$100,000,000
Portion of expenses	50.00%
Lorcaserin Product Other Than Belviq Post Approval Up To One Hundred Million Dollars \| Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	50.00%
Lorcaserin Product Other Than Belviq Post Approval More Than One Hundred Million Dollars \| Arena GmbH
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	10.00%
Lorcaserin Product Other Than Belviq Post Approval More Than One Hundred Million Dollars \| Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses	90.00%

[1]	Development required by a regulatory authority, with the exception of the non-FDA required portion of the CVOT.

Employee_Benefit_Plans_Additio

Employee Benefit Plans - Additional Information - 401 (K) Plan (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, percentage employer matching contribution of first 6% contributed	100.00%
Maximum employer matching contribution percentage per employee	6.00%
Defined contribution plan, employer matching contribution vesting period in Years	5 years
Employer matching contributions	$1.60	$1.50	$1.10

Employee_Benefit_Plans_Additio1

Employee Benefit Plans - Additional Information - Pension Plans (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Multiemployer Plans [Line Items]
Multi-employer plan, employer contributions	$0.70	$0.70	$0.60

Summary_of_Loss_Before_Provisi

Summary of Loss Before Provision (Benefit) for Income Taxes by Region (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Loss From Operations Before Provision Benefit For Income Taxes [Line Items]
United States	($16,607)	$11,573	($62,674)
Foreign	-43,901	-31,008	-22,803
Total loss before income taxes	($60,508)	($19,435)	($85,477)

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Detail) (USD $)	12 Months Ended
Income Taxes - Additional Information (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Line Items]
Statutory federal rate	34.00%	34.00%	34.00%
Increase in valuation allowance	$3,700,000
Unrecognized tax benefits, that would impact the effective tax rate	3,900,000
Accrued interest or penalties	0	0
Recognize interest and/or penalties expense	0	0	0
Foreign subsidiaries accumulated earnings	0
Foreign subsidiaries repatriated earnings	0
Maximum
Income Tax Disclosure [Line Items]
Income tax holiday expiration term	10 years
Income tax holiday termination date	31-Dec-22
SWITZERLAND
Income Tax Disclosure [Line Items]
Standard effective tax rate	19.00%
California
Income Tax Disclosure [Line Items]
NOL carryforwards	584,800,000
California \| Stock Option Exercises
Income Tax Disclosure [Line Items]
NOL carryforwards	8,900,000
California \| Research and Development Tax Credit
Income Tax Disclosure [Line Items]
Tax credit carryforwards amount	22,200,000
California \| Minimum
Income Tax Disclosure [Line Items]
NOL carryforwards expiration date, year	2015
Foreign
Income Tax Disclosure [Line Items]
NOL carryforwards	115,900,000
Foreign \| Minimum
Income Tax Disclosure [Line Items]
NOL carryforwards expiration date, year	2015
Federal
Income Tax Disclosure [Line Items]
NOL carryforwards	548,000,000
Federal \| Stock Option Exercises
Income Tax Disclosure [Line Items]
NOL carryforwards	8,900,000
Federal \| Research and Development Tax Credit
Income Tax Disclosure [Line Items]
Tax credit carryforwards amount	29,100,000
Federal \| Minimum
Income Tax Disclosure [Line Items]
NOL carryforwards expiration date, year	2022
Tax credit carryforwards expiration year	2026
Federal \| Minimum \| Orphan Drug Credit
Income Tax Disclosure [Line Items]
Tax credit carryforwards amount	$200,000

Reconciliation_of_Provision_Be

Reconciliation of Provision (Benefit) for Income Taxes at Statutory Federal Rate to Provision (Benefit) for Income Taxes (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reconciliation Of Income Taxes [Line Items]
Benefit for income taxes at statutory Federal rate	($20,573)	($6,608)	($29,062)
State income tax, net of Federal benefit and valuation allowance	0	0	0
Foreign losses at lower effective rates	13,318	9,527	6,744
Research and development and Orphan Drug credits	-2,992	-2,594	-1,005
Change in Federal and foreign valuation allowance	9,436	61,265	16,018
Benefit for income taxes	0	0	0
Permanent differences and other
Reconciliation Of Income Taxes [Line Items]
Other adjustments	2,318	2,122	-2,770
Gain (Loss) from valuation of derivative liabilities
Reconciliation Of Income Taxes [Line Items]
Other adjustments	-1,507	-3,922	5,244
Adjustment to research and development credits and net operating losses, or NOLs
Reconciliation Of Income Taxes [Line Items]
Other adjustments	$0	($59,790)	$4,831

Components_of_Net_Deferred_Tax

Components of Net Deferred Tax Assets (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Deferred tax assets:
Foreign NOL carryforwards	$9,518	$9,678
Federal and California NOL carryforwards	216,906	231,450
Federal and California research and development credit carryforwards	44,022	40,948
Deferred revenues	36,448	21,109
Depreciation	3,714	4,245
Share-based compensation expense	8,549	7,223
Other, net	4,011	5,921
Total deferred tax assets	323,168	320,574
Deferred tax liabilities	-767	-1,853
Net deferred tax assets	322,401	318,721
Valuation allowance	-322,401	-318,721
Net deferred tax liabilities	$0	$0

Reconciliation_of_Unrecognized

Reconciliation of Unrecognized Tax Benefits (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Unrecognized Tax Benefits [Line Items]
Gross unrecognized tax benefits at the beginning of the year	$4,629	$0	$0
Additions from tax positions taken in the current year	585	541	0
Additions from tax positions taken in prior years	0	4,088	0
Reductions from tax positions taken in prior years	0	0	0
Tax settlements	0	0	0
Gross unrecognized tax benefits at end of the year	$5,214	$4,629	$0

Quarterly_Data_Detail

Quarterly Data (Detail) (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Information [Line Items]
Revenues	$9,191	$8,164	$12,801	$6,814	$6,516	$3,578	$68,927	$2,373	$36,970	$81,394	$27,587
Operating costs and expenses	39,351	34,373	38,167	30,352	28,487	23,444	29,021	23,377	142,243	104,329	84,700
Net income (loss)	($32,061)	($10,672)	$7,480	($25,255)	($23,459)	($17,200)	$40,100	($18,876)	($60,508)	($19,435)	($85,477)
Net income (loss) per share, basic and diluted	($0.15)	($0.05)	$0.03	($0.12)	($0.11)	($0.08)	$0.18	($0.09)	($0.28)	($0.09)

Subsequent_Events_Additional_I

Subsequent Events - Additional Information (Detail) (USD $)	12 Months Ended			1 Months Ended
Subsequent Events - Additional Information (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 31, 2015	Feb. 28, 2015
Subsequent Event [Line Items]
Common stock, par value	$0.00	$0.00
Net proceeds from sale of common stock	$5,225,000	$3,315,000	$151,218,000
Subsequent Event
Subsequent Event [Line Items]
Issuance of common stock				21,000,000
Common stock, par value				$0.00
Sale of stock, price per share				$4.81
Net proceeds from sale of common stock				100,700,000
Subsequent Event \| lldong BELVIQ
Subsequent Event [Line Items]
Recognized revenues					$3,000,000