Arena Pharmaceuticals (ARNA) 10-Q 12 Nov 13 2013 Q3 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	9 Months Ended
	Sep. 30, 2013	Nov. 07, 2013
Document Information [Line Items]	'	'
Document Type	'10-Q	'
Amendment Flag	'false	'
Document Period End Date	30-Sep-13	'
Document Fiscal Year Focus	'2013	'
Document Fiscal Period Focus	'Q3	'
Trading Symbol	'ARNA	'
Entity Registrant Name	'ARENA PHARMACEUTICALS INC	'
Entity Central Index Key	'0001080709	'
Current Fiscal Year End Date	'--12-31	'
Entity Filer Category	'Large Accelerated Filer	'
Entity Common Stock, Shares Outstanding	'	218,574,076

Condensed_Consolidated_Balance

Condensed Consolidated Balance Sheets (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Current assets:	'	'
Cash and cash equivalents	$180,703	$156,091	[1]
Accounts receivable	12,379	5,556	[1]
Inventory	10,039	6,058	[1]
Prepaid expenses and other current assets	3,441	3,454	[1]
Total current assets	206,562	171,159	[1]
Land, property and equipment, net	74,484	75,417	[1]
Acquired technology and other intangibles, net	10,212	10,611	[1]
Other non-current assets	3,445	4,019	[1]
Total assets	294,703	261,206	[1]
Current liabilities:	'	'
Accounts payable and other accrued liabilities	24,523	7,123	[1]
Accrued compensation	4,161	3,087	[1]
Current portion of deferred revenues	29,162	15,453	[1]
Current portion of derivative liabilities	0	2,587	[1]
Current portion of lease financing obligations	1,954	1,664	[1]
Total current liabilities	59,800	29,914	[1]
Deferred rent	213	122	[1]
Deferred revenues, less current portion	46,389	47,282	[1]
Derivative liabilities, less current portion	4,941	12,455	[1]
Lease financing obligations, less current portion	71,288	72,794	[1]
Commitments and contingencies	'	'	[1]
Stockholders' equity:	'	'
Common stock	22	22	[1]
Additional paid-in capital	1,290,731	1,281,426	[1]
Accumulated other comprehensive income	5,593	5,489	[1]
Accumulated deficit	-1,184,274	-1,188,298	[1]
Total stockholders' equity	112,072	98,639	[1]
Total liabilities and stockholders' equity	$294,703	$261,206	[1]
[1]	The balance sheet data at December 31, 2012, has been derived from audited financial statements at that date. It does not include, however, all of the information and notes required by US generally accepted accounting principles for complete financial statements.

Condensed_Consolidated_Stateme

Condensed Consolidated Statements of Operations and Comprehensive [Loss] (USD $)	3 Months Ended		9 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Revenues:	'	'	'	'
Net product sales	$2,011	$0	$3,330	$0
Manufacturing services	443	603	2,181	2,924
Total revenues	3,578	1,485	74,878	25,651
Operating Costs and Expenses:	'	'	'	'
Cost of product sales	460	0	1,514	0
Cost of manufacturing services	632	1,396	3,286	2,839
Research and development	14,592	11,619	47,428	40,165
General and administrative	7,760	7,392	23,614	18,963
Amortization of acquired technology and other intangibles	0	168	0	517
Total operating costs and expenses	23,444	20,575	75,842	62,484
Loss from operations	-19,866	-19,090	-964	-36,833
Interest and Other Income (Expense):	'	'	'	'
Interest income	28	41	68	81
Interest expense	-1,768	-1,804	-5,333	-7,324
Gain (Loss) from valuation of derivative liabilities	4,100	5,259	10,101	-13,886
Loss on extinguishment of debt	0	0	0	-6,338
Other	306	73	152	103
Total interest and other income (expense), net	2,666	3,569	4,988	-27,364
Net income (loss)	-17,200	-15,521	4,024	-64,197
Deemed dividend related to beneficial conversion feature of convertible preferred stock	0	0	0	-2,824
Net income (loss) allocable to common stockholders	-17,200	-15,521	4,024	-67,021
Net income (loss) per share allocable to common stockholders:	'	'	'	'
Basic	($0.08)	($0.07)	$0.02	($0.35)
Diluted	($0.08)	($0.07)	$0.02	($0.35)
Shares used in calculating net income (loss) per share allocable to common stockholders:	'	'	'	'
Basic	218,316	213,881	217,923	189,545
Diluted	218,316	213,881	224,354	189,545
Comprehensive Income (Loss):	'	'	'	'
Net income (loss)	-17,200	-15,521	4,024	-64,197
Foreign currency translation gain	1,303	588	104	41
Comprehensive income (loss)	-15,897	-14,933	4,128	-64,156
Eisai BELVIQ Agreement	'	'	'	'
Revenues:	'	'	'	'
Collaborative revenue	973	864	68,933	22,642
Other	'	'	'	'
Revenues:	'	'	'	'
Collaborative revenue	$151	$18	$434	$85

Condensed_Consolidated_Cash_Fl

Condensed Consolidated Cash Flow Statements (USD $)	9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2012
Operating Activities	'		'
Net income (loss)	$4,024		($64,197)
Adjustments to reconcile net income (loss) to net cash used in operating activities:	'		'
Depreciation and amortization	5,763		7,060
Amortization of acquired technology and other intangibles	305		517
Share-based compensation	6,542		3,713
(Gain) Loss from valuation of derivative liabilities	-10,101		13,886
Amortization of prepaid financing costs	102		258
Gain on disposal or sale of equipment	-21		-1
Accretion of note payable to Deerfield	0		1,225
Loss on extinguishment of debt	0		6,338
Changes in assets and liabilities:	'		'
Accounts receivable	-6,330		-438
Inventory	-3,520		-2,777
Prepaid expenses and other assets	37		-2,502
Accounts payable and accrued liabilities	17,823		-328
Deferred revenues	12,565		2,368
Deferred rent	91		-130
Net cash provided by (used in) operating activities	27,280		-35,008
Investing Activities	'		'
Purchases of land, property and equipment	-4,600		-1,035
Proceeds from sale of equipment	60		1
Other non-current assets	467		-320
Net cash used in investing activities	-4,073		-1,354
Financing Activities	'		'
Principal payments on lease financing obligations	-1,216		-956
Principal payments on note payable to Deerfield	0		-22,261
Proceeds from issuance of common stock	2,685		150,657
Proceeds from issuance of preferred stock	0		16,463
Net cash provided by financing activities	1,469		143,903
Effect of exchange rate changes on cash	-64		601
Net increase in cash and cash equivalents	24,612		108,142
Cash and cash equivalents at beginning of period	156,091	[1]	57,632
Cash and cash equivalents at end of period	180,703		165,774
Supplemental Disclosure Of Non-Cash Investing and Financing Information:	'		'
Conversion of preferred stock into common stock	0		14,561
Retirement of treasury stock	0		23,070
Deemed dividend related to beneficial conversion feature of convertible preferred stock	$0		$2,824
[1]	The balance sheet data at December 31, 2012, has been derived from audited financial statements at that date. It does not include, however, all of the information and notes required by US generally accepted accounting principles for complete financial statements.

Basis_of_Presentation_and_BELV

Basis of Presentation and BELVIQ Launch	9 Months Ended
	Sep. 30, 2013
Basis of Presentation and BELVIQ Launch	'
	1. Basis of Presentation and BELVIQ Launch
	Basis of Presentation
	The accompanying unaudited condensed consolidated financial statements of Arena Pharmaceuticals, Inc., which include our wholly owned subsidiaries, should be read in conjunction with the audited consolidated financial statements and notes thereto included in our Annual Report on Form 10-K for the year ended December 31, 2012, as filed with the Securities and Exchange Commission, or SEC, from which we derived our balance sheet as of December 31, 2012. The accompanying financial statements have been prepared in accordance with US generally accepted accounting principles, or GAAP, for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, since they are interim statements, the accompanying financial statements do not include all of the information and notes required by GAAP for complete financial statements. The accompanying financial statements reflect all adjustments, consisting of normal recurring adjustments, that are, in the opinion of our management, necessary to a fair statement of the results for the interim periods presented. Interim results are not necessarily indicative of results for a full year.
	In February 2013, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, No. 2013-02, “Comprehensive Income (Topic 220)—Clarifying Reporting of Amounts Reclassified Out of Accumulated Other Comprehensive Income.” Under ASU No. 2013-02, companies are required to provide information about the amounts reclassified out of accumulated other comprehensive income by component. In addition, companies are required to present, either on the face of the financial statements or in the accompanying notes, significant amounts reclassified out of accumulated other comprehensive income by the respective line items of net income, but only if the amount reclassified is required to be reclassified to net income in its entirety in the same reporting period. For amounts that are not required to be reclassified in their entirety to net income in the same reporting period, companies are required to cross-reference to other disclosures that provide additional detail on those amounts. ASU No. 2013-02 is effective prospectively for reporting periods beginning after December 15, 2012. ASU No. 2013-02 did not impact our disclosures because the balance included in accumulated other comprehensive income related only to foreign currency translation, for which there were no reclassifications in any periods reported.
	In March 2013, the FASB issued ASU No. 2013-05, “Parent’s Accounting for the Cumulative Translation Adjustment upon Derecognition of Certain Subsidiaries or Groups of Assets within a Foreign Entity or of an Investment in a Foreign Entity,” which requires the release of any related cumulative translation adjustment into net income when a parent ceases to have a controlling financial interest in a subsidiary or group of assets that is a business within a foreign entity. ASU No. 2013-05 is effective prospectively for fiscal years, and interim periods within those years, beginning after December 15, 2013. We do not expect the adoption of ASU No. 2013-05 to have a material impact on our consolidated financial statements.
	The preparation of financial statements in accordance 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.
	BELVIQ Launch
	In June 2013, BELVIQ, our internally discovered drug for chronic weight management in adults who are overweight with a comorbidity or obese, was made available to patients by prescription in the United States by Eisai Inc. under a marketing and supply agreement with our wholly owned subsidiary, Arena Pharmaceuticals GmbH, or Arena GmbH. (See Note 6 and, with regard to an expansion of such agreement and certain changes to the rights, obligations and other terms thereunder, Note 11.)

Fair_Value_Disclosures

Fair Value Disclosures	9 Months Ended
	Sep. 30, 2013
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 as of September 30, 2013, and December 31, 2012, in thousands:
			Fair Value Measurements at September 30, 2013
			Balance at			Quoted			Significant		Significant
			September 30,			Prices in			Other		Unobservable
			2013			Active			Observable		Inputs
						Markets			Inputs		(Level 3)
						(Level 1)			(Level 2)
	Assets:
	Money market funds and cash equivalents1		$	153,813		$	153,813		$	0	$	0
	Liabilities:
	Warrants		$	4,941		$	0		$	0	$	4,941
			Fair Value Measurements at December 31, 2012
			Balance at			Quoted			Significant		Significant
			December 31,			Prices in			Other		Unobservable
			2012			Active			Observable		Inputs
						Markets			Inputs		(Level 3)
						(Level 1)			(Level 2)
	Assets:
	Money market funds and cash equivalents1		$	143,747		$	143,747		$	0	$	0
	Liabilities:
	Warrants		$	15,042		$	0		$	0	$	15,042
	1	Included in cash and cash equivalents on our condensed consolidated balance sheets.
	The following table presents the activity for our derivative liabilities, which are classified as Level 3 in our valuation hierarchy, during the three and nine months ended September 30, 2013, in thousands:
			Three months ended			Nine months ended
			September 30,			September 30,
			2013			2013
	Beginning balance		$	9,041		$	15,042
	Gain from valuation of derivative liabilities			(4,100	)		(10,101	)
	Balance at September 30, 2013		$	4,941		$	4,941
	In July 2005, we amended the license agreement that we entered into in July 2001 with TaiGen Biotechnology Co., Ltd., or TaiGen. Under these agreements, we received equity in TaiGen in exchange for TaiGen’s right to select and obtain G protein-coupled receptors, or GPCRs, from us. Due to impairment charges, our investment in TaiGen has had a cost basis of zero since December 31, 2011.
	In September 2013, TaiGen’s common shares began to trade under the name “TaiGen Biopharmaceuticals Holding Limited” on the GreTai Securities Market, which is a thinly traded, over-the-counter market for emerging Taiwanese securities. In connection with such stock listing, our investment in TaiGen converted into shares of TaiGen’s common stock, representing approximately 4% of its outstanding common stock. As the prices of securities traded on this over-the-counter, foreign stock market do not reflect fair value pursuant to the accounting guidance over marketable securities, we have continued to value our investment in TaiGen at zero.

Inventory

Inventory	9 Months Ended
	Sep. 30, 2013
Inventory	'
	3. Inventory
	Upon receiving US Food and Drug Administration, or FDA, approval of BELVIQ in June 2012 (which approval was subject to the final scheduling designation of the US Drug Enforcement Administration, or DEA), we began to capitalize inventory costs for BELVIQ, which were recorded as research and development expenses prior to such approval. All of our inventory, which is stated at the lower of cost (using a first-in, first-out basis) or market, relates to BELVIQ. The costs of manufacturing BELVIQ associated with deferred revenues are recorded as deferred costs, which are included in finished goods inventory until such time as the related deferred revenue is recognized. Our inventory consisted of the following as of September 30, 2013, and December 31, 2012, in thousands:
		September 30,		December 31,
		2013		2012
	Raw materials	$	586	$	423
	Work in process		1,722		4,184
	Finished goods at Arena GmbH		1,222		0
	Finished goods at Eisai		6,509		1,451
	Total inventory	$	10,039	$	6,058

Accounts_Payable_and_Other_Acc

Accounts Payable and Other Accrued Liabilities	9 Months Ended
	Sep. 30, 2013
Accounts Payable and Other Accrued Liabilities	'
	4. Accounts Payable and Other Accrued Liabilities
	Accounts payable and other accrued liabilities consisted of the following as of September 30, 2013, and December 31, 2012, in thousands:
		September 30,		December 31,
		2013		2012
	Payable to collaborators (See Note 6)	$	18,698	$	0
	Accounts payable		3,042		3,884
	Accrued expenses		1,840		2,006
	Accrued clinical and preclinical study fees		839		566
	Loss provision		37		482
	Other accrued liabilities		67		185
	Total accounts payable and other accrued liabilities	$	24,523	$	7,123

Derivative_Liabilities

Derivative Liabilities	9 Months Ended
	Sep. 30, 2013
Derivative Liabilities	'
	5. 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. The Series B Warrants are related to our Series B Convertible Preferred Stock, which we redeemed in 2008 and is no longer outstanding. 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. As of September 30, 2013, 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. The outstanding August 2008 Series B Warrant is recorded as a derivative liability on our condensed consolidated balance sheets.
	Our derivative liabilities consisted of the following as of September 30, 2013, and December 31, 2012, in thousands:
		September 30,			December 31,
		2013			2012
	Series B Warrants - current portion	$	0		$	2,587
	Series B Warrants, less current portion		4,941			12,455
	Total derivative liabilities	$	4,941		$	15,042
	Our outstanding warrants are revalued on each balance sheet date, with changes in the fair value between reporting periods recorded as other income or expense. At September 30, 2013, and December 31, 2012, our outstanding warrants were valued using the Black-Scholes option pricing model and the following assumptions:
		September 30, 2013			December 31, 2012
		August 2008			June 2006			August 2008
		Series B			Series B			Series B
		Warrant			Warrant			Warrant
	Risk-free interest rate		0.3	%		0.1	%		0.3	%
	Dividend yield		0	%		0	%		0	%
	Expected volatility		81	%		66	%		93	%
	Expected life (years)		1.87			0.5			2.62
	The change in the fair value of our derivative liabilities between reporting periods is recorded in the interest and other income (expense) section of our condensed consolidated statements of operations and comprehensive income (loss). We recognized the following gain (loss) in the three and nine months ended September 30, 2013, and 2012, in thousands:
		Three months ended						Nine months ended
		September 30,						September 30,
		2013			2012			2013			2012
	Series B Warrants	$	4,100		$	5,259		$	10,101		$	(13,941	)
	Deerfield acceleration right		0			0			0			55
	Total gain (loss) from valuation of derivative liabilities	$	4,100		$	5,259		$	10,101		$	(13,886	)
	The “Deerfield acceleration right,” which we 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 May 2012, such right was revalued on each balance sheet date.

Marketing_and_Supply_Agreement

Marketing and Supply Agreement with Eisai Inc.	9 Months Ended
	Sep. 30, 2013
Marketing and Supply Agreement with Eisai Inc.	'
	6. Marketing and Supply Agreement with Eisai Inc.
	This Note describes our marketing and supply agreement with Eisai as of September 30, 2013. See Note 11 regarding an expansion of such agreement and certain changes to the rights, obligations and other terms thereunder.
	In July 2010, Arena GmbH and Eisai Inc. entered into the original Marketing and Supply Agreement, or Original Eisai Agreement, under which we granted Eisai Inc. exclusive commercialization rights for BELVIQ 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 Amended and Restated Marketing and Supply Agreement, or Eisai First Amended Agreement, which expanded Eisai Inc.’s exclusive commercialization rights to include most of North and South America, including Mexico, Canada and Brazil. Under the Eisai First Amended Agreement, we provide services related to development and regulatory activities, and we also manufacture and sell BELVIQ to Eisai Inc. We are also entitled to receive upfront payments, milestone payments based on the achievement of regulatory filings and approvals, one-time purchase price adjustment payments and other payments, and payments from sales of BELVIQ.
	The following table summarizes the revenues we have recognized under the Eisai First Amended Agreement in the three and nine months ended September 30, 2013, and 2012, in thousands:
		Three months ended						Nine months ended
		September 30,						September 30,
		2013			2012			2013		2012
	Net product sales	$	2,011		$	0		$	3,330	$	0
	Milestone payments		0			0			66,000		20,000
	Amortization of upfront payments		861			864			2,584		2,642
	Reimbursements of development and patent expenses		112			0			349		0
	Total	$	2,984		$	864		$	72,263	$	22,642
	Under the Eisai First Amended Agreement, we manufacture and sell BELVIQ to Eisai Inc. for marketing and distribution in the United States and, subject to applicable regulatory approval, in the additional territories under our agreement for a purchase price starting at 31.5% and 30.75%, respectively, of Eisai Inc.’s aggregate annual net sales (which are the gross invoiced sales less certain deductions described in the Eisai First Amended Agreement that are estimated by Eisai Inc. based on their experience, including deductions for certain taxes, credits, allowances, discounts, rebates, chargebacks, returns and other items), or the Product Purchase Price, in all of such territories on an aggregate basis. The Product Purchase Price will increase on a tiered basis in the United States and in the additional territories to as high as 36.5% and 35.75%, respectively, on the portion of Eisai Inc.’s annual net product sales exceeding $750.0 million, subject to reduction (for sales in a particular country), including in the event of generic competition in the applicable country. The Eisai First Amended Agreement includes payments by Eisai Inc. if annual minimum sales requirements in the additional territories are not met during the first ten years after initial commercial sale in Mexico, Canada or Brazil. In addition, we are eligible to receive up to an aggregate of $1.19 billion in one-time purchase price adjustment payments and other payments based on Eisai Inc.’s annual net product sales of BELVIQ in all of the territories under our 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 Inc. will pay us a total of $330.0 million for annual net product sales of up to $1.0 billion. We are also eligible to receive up to an additional $185.0 million in one-time purchase price adjustment payments based on Eisai Inc.’s annual net product sales of BELVIQ in the non-US territories under our agreement, with the first and last amounts payable upon first achievement of annual net sales of $100.0 million and $1.0 billion, respectively, in such territories.
	The amount that Eisai Inc. pays us for BELVIQ product supply is based on Eisai Inc.’s estimated price at the time the order is shipped, which is Eisai Inc.’s estimate of the Product Purchase Price, and is subject to change on April 1 and October 1 of each year. Eisai Inc.’s estimate of the Product Purchase Price was changed as of October 1, 2013. At the end of Eisai Inc.’s fiscal year (March 31), the estimated price paid to us for product that Eisai Inc. sold to their distributors is compared to the Product Purchase Price of such product, and the difference is either refunded back to Eisai Inc. (for overpayments) or paid to us (for underpayments). On a monthly basis, Eisai Inc. provides us the total amount of net sales for the month, details of the total deductions from gross sales to net sales and the sales in units. We recognize our revenues monthly based on our percentage of Eisai Inc.’s monthly net sales figures. When the revenues we recognize differ from the estimated price that Eisai Inc. 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 Inc. 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. At March 31 of each year, the amount recorded as receivable or payable will be settled by the applicable party.
	In June 2013, BELVIQ was made available to patients by prescription in the United States. We recognize revenues from net product sales 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. We recognize such revenues when Eisai Inc. ships BELVIQ to their distributors. Eisai Inc.’s estimated deductions used to determine net sales are subject to revision, and could result in material adjustments to our recognized revenues if actual deductions differ materially from Eisai Inc.’s original estimates.
	We recognized revenues from net product sales of $2.0 million and $3.3 million in the three and nine months ended September 30, 2013, respectively. The Product Purchase Price for the product Eisai Inc. has sold to date was lower than the estimated price that Eisai Inc. paid us, primarily because the estimated price that Eisai Inc. paid us did not include deductions for certain items related to product launch. This difference of $18.7 million at September 30, 2013, reflects both the amounts Eisai Inc. has sold to date and the product supply remaining in Eisai Inc.’s inventory at September 30, 2013, and is classified as payable to collaborators. (See Note 4.)
	Revenues from the upfront payments we received of $50.0 million when we entered into the Original Eisai Agreement and $5.0 million when we entered into the Eisai First Amended Agreement were deferred, as we determined that the exclusive rights did not have standalone value without our ongoing development and regulatory activities. These payments are being recognized ratably as revenue over the periods in which we expect the services to be rendered, which are approximately 16 years and 13 years, respectively.
	Revenues recognized in the nine months ended September 30, 2013, included (i) $65.0 million in milestones earned upon the final scheduling designation of the DEA for BELVIQ, (ii) a $0.5 million milestone earned upon Eisai Inc. filing for regulatory approval of BELVIQ in Canada and (iii) a $0.5 million milestone earned upon Eisai Inc. filing for regulatory approval of BELVIQ in Mexico. Under the Eisai First Amended Agreement, we are also entitled to receive from Eisai Inc. up to $53.5 million of additional non-refundable milestone payments based on achievement of regulatory filings and approvals. Under the milestone method of revenue recognition, we recognize revenue for the amount payable to us for achieving each substantive milestone payment, if any, in the period the milestone is achieved.
	The following table summarizes the deferred revenues attributable to Eisai Inc. as of September 30, 2013, and December 31, 2012, in thousands:
		September 30,			December 31,
		2013			2012
	Upfront payments	$	43,555		$	46,140
	Product sales		25,086			11,594
	Total deferred revenues attributable to Eisai Inc.		68,641			57,734
	Less current portion		(28,532	)		(15,040	)
	Deferred revenues attributable to Eisai Inc., less current portion	$	40,109		$	42,694
	Under the Eisai First Amended Agreement, with respect to the post-marketing studies Eisai Inc. and we committed to conduct as part of the FDA approval of BELVIQ, Eisai Inc. and we will be responsible for 90% and 10%, respectively, of the expenses for the cardiovascular outcomes trial and certain pediatric studies, and we will share equally with Eisai Inc. the expenses of certain other pediatric studies. Eisai Inc. is responsible for regulatory activities related to the BELVIQ New Drug Application, or NDA, and for the regulatory activities for obtaining regulatory approval in any country in the additional territories. If the regulatory authority for a country in the additional territories requires development work before or following approval of BELVIQ in such country, Eisai Inc. and we will be responsible for 90% and 10%, respectively, of the expenses for such work, with the exception of the expenses for stability testing, which we will share equally with Eisai Inc. In addition, with respect to any non-FDA required development work we may conduct relating to BELVIQ, we would expect to incur additional expenses, which may be significant.
	Under the Eisai First Amended Agreement, Eisai Inc. will indemnify Arena GmbH for losses resulting from certain third-party claims, including for (a) Eisai Inc.’s negligence, willful misconduct or violation of law, (b) Eisai Inc.’s breach of the Eisai First Amended Agreement or related agreements, (c) certain uses or misuses of BELVIQ, (d) certain governmental investigations of Eisai Inc. related to BELVIQ, and (e) infringement relating to Eisai Inc.’s use of certain trademarks related to BELVIQ. Arena GmbH will indemnify Eisai Inc. for losses resulting from certain US product liability claims and 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 Inc., (ii) Arena GmbH’s negligence or willful misconduct with respect to certain uses or misuses of BELVIQ outside of the agreement, (iii) certain uses or misuses of BELVIQ after the term of the agreement or in any territory no longer under the agreement, (iv) Arena GmbH’s negligence, willful misconduct or violation of law, (v) Arena GmbH’s breach of the Eisai First Amended Agreement or related agreements; (vi) certain infringement of intellectual rights of a third party; and (vii) infringement relating to Eisai Inc.’s use of certain trademarks related to BELVIQ. In addition, each of Arena GmbH and Eisai Inc. will share equally in losses resulting from third-party product liability claims in the territories added with the Eisai First Amended Agreement, except to the extent caused by one party’s negligence, willful misconduct, violation of law or breach or default of the Eisai First Amended Agreement or certain other agreements between the parties. We are unable to predict the maximum potential amount of any future payment for such product liability indemnification provisions. As of September 30, 2013, we have not incurred any significant costs under these indemnification provisions.
	Under the Eisai First Amended Agreement, Eisai Inc. may terminate such agreement with respect to the United States or any country in the additional territories following the later of the expiration of all issued BELVIQ patents in such country and 12 years after the first commercial sale of BELVIQ in such country. Either party has the right to terminate the Eisai First Amended Agreement early in certain circumstances, including (a) if the other party is in material breach, (b) for commercialization concerns, and (c) for certain intellectual property infringement. Eisai Inc. also has the right to terminate the Eisai First Amended 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 BELVIQ in such country exceed sales of BELVIQ in that country (based on volume), and (ii) if Eisai Inc. is acquired by a company that has a product that competes with BELVIQ. In addition, we can terminate the Eisai First Amended Agreement early in its entirety or with respect to each country in the additional territories in certain circumstances, including termination in each country if Eisai Inc. does not satisfy certain regulatory filing and commercialization diligence requirements in such country.

Stockholders_Equity

Stockholders' Equity	9 Months Ended
	Sep. 30, 2013
Stockholders' Equity	'
	7. Stockholders’ Equity
	Equity Compensation Plan
	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. When approved by our stockholders, there were 26,720,370 shares of common stock available for issuance under the 2013 LTIP. As of September 30, 2013, there were 26,622,722 shares of common stock available for issuance under 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.
	Issuances of Common Stock
	In the nine months ended September 30, 2013, we issued a total of 810,424 shares of our common stock pursuant to stock option exercises and a total of 267,994 shares of our common stock under our employee stock purchase plan, resulting in net proceeds to us of $2.0 million and $0.6 million, respectively. In addition, in June 2013, we issued 10,000 shares of our common stock pursuant to the exercise of a portion of the then-outstanding June 2006 Series B Warrant to purchase shares of our common stock at $8.76 per share, resulting in net proceeds to us of $0.1 million.
	Share-based Compensation
	We recognized share-based compensation expense as follows, in thousands:
		Three months ended					Nine months ended
		September 30,					September 30,
		2013			2012		2013		2012
	Cost of product sales	$	0		$	0	$	17	$	0
	Research and development		1,188			509		3,047		1,215
	General and administrative		1,131			843		3,478		2,498
	Total share-based compensation expense	$	2,319		$	1,352	$	6,542	$	3,713
	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. A total of $0.1 million of share-based compensation was capitalized into inventory as of September 30, 2013.
	Share-based Award Activity
	The following table summarizes our stock option activity during the nine months ended September 30, 2013, in thousands (except per share data):
		Options			Weighted-
					Average
					Exercise Price
	Outstanding at January 1, 2013		13,842		$	4.44
	Granted		1,449			8.5
	Exercised		(810	)		2.49
	Forfeited/cancelled/expired		(216	)		5.93
	Outstanding at September 30, 2013		14,265		$	4.94
	The following table summarizes activity with respect to our restricted stock unit awards, or RSUs, which have a service condition, during the nine months ended September 30, 2013, in thousands (except per share data):
		RSUs			Weighted-Average
					Grant-Date Fair
					Value
	Unvested at January 1, 2013		165		$	8.87
	Granted		131			8.81
	Vested		(33	)		8.81
	Forfeited/cancelled		0
	Unvested at September 30, 2013		263		$	8.85
	Outstanding at September 30, 2013		296		$	8.84
	In March 2013, we granted our executive officers Total Stockholder Return, or TSR, performance restricted stock unit, or 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, 2013, of the NASDAQ Biotech Index. In the aggregate, the target number of shares of common stock that may be earned under the PRSUs is 780,000; however, the actual number of shares that may be earned ranges from 0% to 200% of such amount. As these awards contain a market condition, we used a Monte Carlo simulation model to estimate the grant-date fair value of the PRSUs and determined the related share-based compensation expense. The table below sets forth the assumptions used to value the awards at the date of grant and the estimated grant-date fair value:
	Risk-free interest rate		0.4	%
	Dividend yield		0	%
	Expected volatility		89	%
	Remaining performance period (years)		2.99
	Estimated fair value per share of PRSUs granted	$	7.5

Concentration_of_Credit_Risk_a

Concentration of Credit Risk and Major Customers	9 Months Ended
	Sep. 30, 2013
Concentration of Credit Risk and Major Customers	'
	8. Concentration of Credit Risk and Major Customers
	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 and 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.
	Eisai Inc. is the exclusive distributor and our only customer for BELVIQ in the United States, which is the only jurisdiction for which BELVIQ has been approved for marketing. We also produce drug products for Siegfried AG, or Siegfried, under a manufacturing services agreement, and all of our manufacturing services revenues are attributable to Siegfried.
	Percentages of our total revenues are as follows:
		Three months ended						Nine months ended
		September 30,						September 30,
	Source of Revenue	2013			2012			2013			2012
	Eisai marketing and supply agreement		83.4	%		58.2	%		96.5	%		88.3	%
	Manufacturing services agreement with Siegfried		12.4	%		40.6	%		2.9	%		11.4	%
	Other collaborative agreements		4.2	%		1.2	%		0.6	%		0.3	%
	Total percentage of revenues		100	%		100	%		100	%		100	%

Net_Income_Loss_Per_Share

Net Income (Loss) Per Share	9 Months Ended
	Sep. 30, 2013
Net Income (Loss) Per Share	'
	9. Net Income (Loss) Per Share
	We calculate basic and diluted net income (loss) per share allocable to common stockholders using the weighted-average number of shares of common stock outstanding during the period, less any shares subject to repurchase or forfeiture. There were no shares of our common stock outstanding subject to repurchase or forfeiture for the three and nine months ended September 30, 2013, or 2012.
	When we are in a net loss position, we exclude outstanding stock options, RSUs and PRSUs, as well as warrants and unvested restricted stock in our deferred compensation plan, from our calculation of diluted net loss per share, and our diluted net loss per share is the same as our basic net loss per share for such periods. When we report net income, the weighted-average number of shares used to calculate our diluted net income per share includes the potential dilutive effect of in-the-money securities, using the treasury stock method.
	The table below reconciles the number of shares used to calculate basic and diluted net income (loss) per share for the periods presented, in thousands.
		Three months ended				Nine months ended
		September 30,				September 30,
		2013		2012		2013		2012
	Weighted-average shares outstanding		218,316		213,881		217,923		189,545
	Potentially dilutive common shares outstanding:
	Stock options						5,489
	Warrants						876
	RSUs and unvested restricted stock						66
	Weighted-average shares used to calculate diluted net income (loss) per share		218,316		213,881		224,354		189,545
	The table below presents the potentially dilutive securities that would have been included in the above calculation of diluted net income (loss) per share allocable to common stockholders if they were not antidilutive for the periods presented, in thousands.
		Three months ended				Nine months ended
		September 30,				September 30,
		2013		2012		2013		2012
	Stock options		4,796		6,228		3,757		3,539
	Warrants		694		1,054		0		400
	RSUs		0		0		0		0
	Unvested restricted stock		79		79		0		79
	Total		5,569		7,361		3,757		4,018
	Because the market condition for the PRSUs was not satisfied at September 30, 2013, such securities are excluded from the tables above.

Legal_Proceedings

Legal Proceedings	9 Months Ended
	Sep. 30, 2013
Legal Proceedings	'
	10. 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 have been 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 are seeking unspecified monetary damages and other relief. On November 19, 2010, eight prospective lead plaintiffs filed motions to consolidate, appoint a lead plaintiff, and appoint lead counsel. The Court took the motions to consolidate under submission on January 14, 2011. 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 December 30, 2011, we filed a motion to dismiss the consolidated amended complaint. On March 28, 2013, the Court granted our motion to dismiss the consolidated amended complaint without prejudice. On May 13, 2013, the plaintiff filed a new consolidated amended complaint. On June 14, 2013, we filed a motion to dismiss the new consolidated amended complaint. On July 15, 2013, the plaintiff filed an opposition to our motion to dismiss. On July 29, 2013, we filed our reply. On October 25, 2013, the Court heard oral argument on the motion to dismiss. On November 5, 2013, the Court granted our motion to dismiss the new consolidated amended complaint without prejudice as to all parties except for Robert E. Hoffman, who was dismissed from the action with prejudice. The Court further instructed lead plaintiff that it could file a motion for leave to amend the now-dismissed new consolidated amended complaint on or before November 27, 2013. In addition to the class actions, a complaint involving similar legal and factual issues has been brought by at least one individual stockholder and is pending in federal court. On December 30, 2011, we filed a motion to dismiss the stockholder’s complaint. On March 29, 2013, the Court granted our motion to dismiss, in part without prejudice. On May 13, 2013, the plaintiff filed a new amended complaint. On June 14, 2013, we filed a motion to dismiss the new amended complaint. On July 10, 2013, the plaintiff filed an opposition to our motion to dismiss. On July 29, 2013, we filed our reply. 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.

Subsequent_Event

Subsequent Event	9 Months Ended
	Sep. 30, 2013
Subsequent Event	'
	11. Subsequent Event
	On November 7, 2013, Arena GmbH, Eisai Inc., and Eisai Inc.’s parent company, Eisai Co., Ltd. (collectively with Eisai Inc., Eisai) entered into a Second Amended and Restated Marketing and Supply Agreement, or Eisai Second Amended Agreement, which expands Eisai’s exclusive commercialization rights for BELVIQ to all of the countries in the world, except for South Korea, Taiwan, Australia, New Zealand and Israel. Eisai’s commercialization rights are subject to applicable regulatory approval.
	In July 2010, Arena GmbH and Eisai Inc. entered into the Original Eisai Agreement, under which we granted Eisai Inc. exclusive commercialization rights for BELVIQ solely in the United States and its territories and possessions. In May 2012, we and Eisai Inc. amended and restated such agreement by entering into the Eisai First Amended Agreement, which expanded Eisai Inc.’s exclusive commercialization rights to include most of North and South America. The Eisai Second Amended Agreement amends and restates the Eisai First Amended Agreement.
	Upfront and Milestones
	In connection with entering into the Eisai Second Amended Agreement, we will receive from Eisai an upfront payment of $60.0 million. This upfront payment is in addition to the $55.0 million we received in aggregate upfront payments in connection with entering into the Original Eisai Agreement and Eisai First Amended Agreement. We are also eligible to receive up to an aggregate of $176.5 million in regulatory and development milestone payments. These milestone payments include an aggregate of $53.5 million in such potential milestone payments that remained available for us to achieve under the Eisai First Amended Agreement and an aggregate of $123.0 million in additional potential milestone payments under the Eisai Second Amended Agreement.
	Product Purchase Price Payments
	As under the Eisai First Amended Agreement, we will manufacture BELVIQ at our facility in Switzerland, Eisai will purchase all of its requirements of BELVIQ from us, and we will sell BELVIQ to Eisai for commercialization in the United States and in the other territories in North and South America for a Product Purchase Price starting at 31.5% and 30.75%, respectively. With respect to the new territories added under the Eisai Second Amended Agreement, the Product Purchase Price starts at 27.5% in Europe, China and Japan, and at 30.75% in all of the other new territories under the Eisai Second Amended Agreement. All such purchase prices are a percentage of Eisai’s net product sales. 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. Eisai’s annual net product sales are subject to reduction (for sales in a particular country), including in the event of generic competition in the applicable country.
	Our eligibility to receive up to an aggregate of $1.19 billion in one-time purchase price adjustment payments and other payments will now be based on Eisai’s annual net product sales in all of the territories under the Eisai Second Amended Agreement on an aggregate basis, with the first and last amounts payable, as they were under the Eisai First Amended Agreement, with annual net 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 sales of up to $1.0 billion.
	We continue to be eligible to receive up to an additional $185.0 million in one-time purchase price adjustment payments based on Eisai’s annual net product sales in non-US territories in North and South America, with the first and last amounts payable upon first achievement of annual net sales of $100.0 million and $1.0 billion in such territories, respectively. Under the Eisai Second Amended Agreement, we are now also eligible to receive up to an additional $185.0 million in one-time purchase price adjustment payments based on Eisai’s annual net product sales in the territories outside of North and South America, with the first and last amounts payable upon first achievement of annual net sales of $100.0 million and $1.0 billion in such territories, respectively. In addition, the Eisai Second Amended Agreement continues to include 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.
	Development Payments
	With respect to any post-approval development work for BELVIQ for weight management required by the FDA, we and Eisai will continue to be responsible for 10% and 90%, respectively, of the expenses for such work other than the expenses of certain pediatric or adolescent studies, which we and Eisai will share equally. With respect to the rest of North and South America, we and Eisai will continue to be responsible for 10% and 90%, respectively, of the expenses for any pre- or post-approval development work with respect to BELVIQ for weight management required by a regulatory authority other than the expenses for stability testing, which we and Eisai will share equally.
	With respect to the territories outside of North and South America, we and Eisai will each fund 50% of any required pre-approval development expenses related to BELVIQ for weight management, up to an aggregate of $50.0 million by each of Eisai and us for all territories outside of North and South America combined. Thereafter, Eisai will be responsible for 100% of the expenses for such work.
	With respect to lorcaserin products other than BELVIQ for weight management, we and Eisai will equally fund the pre-approval development throughout the territories under the Eisai Second Amended Agreement. We and Eisai have initially prioritized the development areas of smoking cessation, a once-daily formulation, a fixed-dose combination with phentermine, as well as exploring BELVIQ’s impact on diabetes and cardiovascular outcomes. To the extent we include in the FDA-required cardiovascular outcomes trial diabetes or any non-FDA required endpoints as part of such trial, Eisai and we will share those expenses equally up to an aggregate of $40.0 million each, and, thereafter, Eisai will be responsible for 100% of such expenses.
	With respect to any required post-approval development expenses related to BELVIQ for weight management for territories outside of North and South America, we and Eisai will each fund 50% of such expenses, up to an aggregate of $25.0 million by each of Eisai and us for all territories outside of North and South America combined. Thereafter, we and Eisai will be responsible for 10% and 90%, respectively, of the expenses for such work. With respect to any required post-approval development expenses related to lorcaserin products other than BELVIQ for weight management, we and Eisai will each fund 50% of such expenses, up to an aggregate of $50.0 million by each of Eisai and us for all such products combined. Thereafter, we and Eisai will be responsible for 10% and 90%, respectively, of the expenses for such work.
	Certain Other Terms
	We and Eisai have agreed to not commercialize outside of the Eisai Second Amended Agreement any weight management product in the territories under the Eisai Second Amended Agreement. The Eisai Second Amended Agreement continues to include a stand-still provision limiting Eisai’s ability to acquire Arena’s or Arena GmbH’s securities and assets.
	Eisai may terminate the Eisai Second Amended Agreement with respect to any country in the territory following the later of the expiration of all issued BELVIQ patents in such country and 12 years after the first commercial sale of BELVIQ in such country. We and Eisai each have the right to terminate the Eisai Second Amended 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 Second Amended 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 BELVIQ in such country exceed sales of BELVIQ in that country (based on volume), and (ii) if Eisai is acquired by a company that has a product that competes with BELVIQ. In addition, we can terminate the Eisai Second Amended 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 Arena GmbH 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 Second Amended Agreement or related agreements, but excluding product liability claims, (c) certain uses or misuses of BELVIQ, (d) certain governmental investigations of Eisai related to BELVIQ, and (e) infringement relating to Eisai’s use of certain trademarks, tag lines and logos related to BELVIQ. 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 BELVIQ outside of the agreement, (iii) certain uses or misuses of BELVIQ 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 Second Amended 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 BELVIQ. In addition, Arena GmbH and Eisai will 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 Second Amended Agreement or certain other agreements between Arena GmbH and Eisai were the sole and direct cause of the product liability claim.

Fair_Value_Disclosures_Tables

Fair Value Disclosures (Tables)	9 Months Ended
	Sep. 30, 2013
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 as of September 30, 2013, and December 31, 2012, in thousands:
			Fair Value Measurements at September 30, 2013
			Balance at			Quoted			Significant		Significant
			September 30,			Prices in			Other		Unobservable
			2013			Active			Observable		Inputs
						Markets			Inputs		(Level 3)
						(Level 1)			(Level 2)
	Assets:
	Money market funds and cash equivalents1		$	153,813		$	153,813		$	0	$	0
	Liabilities:
	Warrants		$	4,941		$	0		$	0	$	4,941
			Fair Value Measurements at December 31, 2012
			Balance at			Quoted			Significant		Significant
			December 31,			Prices in			Other		Unobservable
			2012			Active			Observable		Inputs
						Markets			Inputs		(Level 3)
						(Level 1)			(Level 2)
	Assets:
	Money market funds and cash equivalents1		$	143,747		$	143,747		$	0	$	0
	Liabilities:
	Warrants		$	15,042		$	0		$	0	$	15,042
	1	Included in cash and cash equivalents on our condensed consolidated balance sheets.
Activity for Derivative Liabilities	'
	The following table presents the activity for our derivative liabilities, which are classified as Level 3 in our valuation hierarchy, during the three and nine months ended September 30, 2013, in thousands:
			Three months ended			Nine months ended
			September 30,			September 30,
			2013			2013
	Beginning balance		$	9,041		$	15,042
	Gain from valuation of derivative liabilities			(4,100	)		(10,101	)
	Balance at September 30, 2013		$	4,941		$	4,941

Inventory_Tables

Inventory (Tables)	9 Months Ended
	Sep. 30, 2013
Components of Inventory	'
	Our inventory consisted of the following as of September 30, 2013, and December 31, 2012, in thousands:
		September 30,		December 31,
		2013		2012
	Raw materials	$	586	$	423
	Work in process		1,722		4,184
	Finished goods at Arena GmbH		1,222		0
	Finished goods at Eisai		6,509		1,451
	Total inventory	$	10,039	$	6,058

Accounts_Payable_and_Other_Acc1

Accounts Payable and Other Accrued Liabilities (Tables)	9 Months Ended
	Sep. 30, 2013
Accounts Payable and Other Accrued Liabilities	'
	Accounts payable and other accrued liabilities consisted of the following as of September 30, 2013, and December 31, 2012, in thousands:
		September 30,		December 31,
		2013		2012
	Payable to collaborators (See Note 6)	$	18,698	$	0
	Accounts payable		3,042		3,884
	Accrued expenses		1,840		2,006
	Accrued clinical and preclinical study fees		839		566
	Loss provision		37		482
	Other accrued liabilities		67		185
	Total accounts payable and other accrued liabilities	$	24,523	$	7,123

Derivative_Liabilities_Tables

Derivative Liabilities (Tables)	9 Months Ended
	Sep. 30, 2013
Components of Derivative Liabilities	'
	Our derivative liabilities consisted of the following as of September 30, 2013, and December 31, 2012, in thousands:
		September 30,			December 31,
		2013			2012
	Series B Warrants - current portion	$	0		$	2,587
	Series B Warrants, less current portion		4,941			12,455
	Total derivative liabilities	$	4,941		$	15,042
Assumptions used to Value Warrants using Black-Scholes Option Pricing Model	'
	Our outstanding warrants are revalued on each balance sheet date, with changes in the fair value between reporting periods recorded as other income or expense. At September 30, 2013, and December 31, 2012, our outstanding warrants were valued using the Black-Scholes option pricing model and the following assumptions:
		September 30, 2013			December 31, 2012
		August 2008			June 2006			August 2008
		Series B			Series B			Series B
		Warrant			Warrant			Warrant
	Risk-free interest rate		0.3	%		0.1	%		0.3	%
	Dividend yield		0	%		0	%		0	%
	Expected volatility		81	%		66	%		93	%
	Expected life (years)		1.87			0.5			2.62
Gain (Loss) from Change in Fair Value of Derivative Liabilities	'
	The change in the fair value of our derivative liabilities between reporting periods is recorded in the interest and other income (expense) section of our condensed consolidated statements of operations and comprehensive income (loss). We recognized the following gain (loss) in the three and nine months ended September 30, 2013, and 2012, in thousands:
		Three months ended						Nine months ended
		September 30,						September 30,
		2013			2012			2013			2012
	Series B Warrants	$	4,100		$	5,259		$	10,101		$	(13,941	)
	Deerfield acceleration right		0			0			0			55
	Total gain (loss) from valuation of derivative liabilities	$	4,100		$	5,259		$	10,101		$	(13,886	)

Marketing_and_Supply_Agreement1

Marketing and Supply Agreement with Eisai Inc. (Tables)	9 Months Ended
	Sep. 30, 2013
Revenues Recognized under Eisai First Amended Agreement	'
	The following table summarizes the revenues we have recognized under the Eisai First Amended Agreement in the three and nine months ended September 30, 2013, and 2012, in thousands:
		Three months ended						Nine months ended
		September 30,						September 30,
		2013			2012			2013		2012
	Net product sales	$	2,011		$	0		$	3,330	$	0
	Milestone payments		0			0			66,000		20,000
	Amortization of upfront payments		861			864			2,584		2,642
	Reimbursements of development and patent expenses		112			0			349		0
	Total	$	2,984		$	864		$	72,263	$	22,642
Deferred Revenues Attributable to Eisai	'
	The following table summarizes the deferred revenues attributable to Eisai Inc. as of September 30, 2013, and December 31, 2012, in thousands:
		September 30,			December 31,
		2013			2012
	Upfront payments	$	43,555		$	46,140
	Product sales		25,086			11,594
	Total deferred revenues attributable to Eisai Inc.		68,641			57,734
	Less current portion		(28,532	)		(15,040	)
	Deferred revenues attributable to Eisai Inc., less current portion	$	40,109		$	42,694

Stockholders_Equity_Tables

Stockholders' Equity (Tables)	9 Months Ended
	Sep. 30, 2013
Share Based Compensation Expense	'
	We recognized share-based compensation expense as follows, in thousands:
		Three months ended					Nine months ended
		September 30,					September 30,
		2013			2012		2013		2012
	Cost of product sales	$	0		$	0	$	17	$	0
	Research and development		1,188			509		3,047		1,215
	General and administrative		1,131			843		3,478		2,498
	Total share-based compensation expense	$	2,319		$	1,352	$	6,542	$	3,713
Summary of Stock Option Activity	'
	The following table summarizes our stock option activity during the nine months ended September 30, 2013, in thousands (except per share data):
		Options			Weighted-
					Average
					Exercise Price
	Outstanding at January 1, 2013		13,842		$	4.44
	Granted		1,449			8.5
	Exercised		(810	)		2.49
	Forfeited/cancelled/expired		(216	)		5.93
	Outstanding at September 30, 2013		14,265		$	4.94
Summary of Restricted Stock Activity	'
	The following table summarizes activity with respect to our restricted stock unit awards, or RSUs, which have a service condition, during the nine months ended September 30, 2013, in thousands (except per share data):
		RSUs			Weighted-Average
					Grant-Date Fair
					Value
	Unvested at January 1, 2013		165		$	8.87
	Granted		131			8.81
	Vested		(33	)		8.81
	Forfeited/cancelled		0
	Unvested at September 30, 2013		263		$	8.85
	Outstanding at September 30, 2013		296		$	8.84
Assumption used to Value Awards and Estimated Grant Date Fair Value	'
	The table below sets forth the assumptions used to value the awards at the date of grant and the estimated grant-date fair value:
	Risk-free interest rate		0.4	%
	Dividend yield		0	%
	Expected volatility		89	%
	Remaining performance period (years)		2.99
	Estimated fair value per share of PRSUs granted	$	7.5

Concentration_of_Credit_Risk_a1

Concentration of Credit Risk and Major Customers (Tables)	9 Months Ended
	Sep. 30, 2013
Percentages of Total Revenues	'
	Percentages of our total revenues are as follows:
		Three months ended						Nine months ended
		September 30,						September 30,
	Source of Revenue	2013			2012			2013			2012
	Eisai marketing and supply agreement		83.4	%		58.2	%		96.5	%		88.3	%
	Manufacturing services agreement with Siegfried		12.4	%		40.6	%		2.9	%		11.4	%
	Other collaborative agreements		4.2	%		1.2	%		0.6	%		0.3	%
	Total percentage of revenues		100	%		100	%		100	%		100	%

Net_Income_Loss_Per_Share_Tabl

Net Income (Loss) Per Share (Tables)	9 Months Ended
	Sep. 30, 2013
Reconciliation of Number of Shares Used to Calculate Basic and Diluted Net Income (Loss) Per Share	'
	The table below reconciles the number of shares used to calculate basic and diluted net income (loss) per share for the periods presented, in thousands.
		Three months ended				Nine months ended
		September 30,				September 30,
		2013		2012		2013		2012
	Weighted-average shares outstanding		218,316		213,881		217,923		189,545
	Potentially dilutive common shares outstanding:
	Stock options						5,489
	Warrants						876
	RSUs and unvested restricted stock						66
	Weighted-average shares used to calculate diluted net income (loss) per share		218,316		213,881		224,354		189,545
Potentially Dilutive Securities Excluded from Calculation of Diluted Net Loss Per Share	'
	The table below presents the potentially dilutive securities that would have been included in the above calculation of diluted net income (loss) per share allocable to common stockholders if they were not antidilutive for the periods presented, in thousands.
		Three months ended				Nine months ended
		September 30,				September 30,
		2013		2012		2013		2012
	Stock options		4,796		6,228		3,757		3,539
	Warrants		694		1,054		0		400
	RSUs		0		0		0		0
	Unvested restricted stock		79		79		0		79
	Total		5,569		7,361		3,757		4,018

Financial_Assets_and_Liabiliti

Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $)	Sep. 30, 2013		Dec. 31, 2012
In Thousands, unless otherwise specified
Liabilities:	'		'
Warrants	$4,941		$15,042
Fair Value, Measurements, Recurring	'		'
Assets:	'		'
Money market funds and cash equivalents	153,813	[1]	143,747	[1]
Liabilities:	'		'
Warrants	4,941		15,042
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1)	'		'
Assets:	'		'
Money market funds and cash equivalents	153,813	[1]	143,747	[1]
Liabilities:	'		'
Warrants	0		0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)	'		'
Assets:	'		'
Money market funds and cash equivalents	0	[1]	0	[1]
Liabilities:	'		'
Warrants	0		0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)	'		'
Assets:	'		'
Money market funds and cash equivalents	0	[1]	0	[1]
Liabilities:	'		'
Warrants	$4,941		$15,042
[1]	Included in cash and cash equivalents on our condensed consolidated balance sheets.

Activity_for_Derivative_Liabil

Activity for Derivative Liabilities (Detail) (Significant Unobservable Inputs (Level 3), USD $)	3 Months Ended	9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2013
Significant Unobservable Inputs (Level 3)	'	'
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]	'	'
Beginning balance	$9,041	$15,042
Gain from valuation of derivative liabilities	-4,100	-10,101
Ending balance	$4,941	$4,941

Fair_Value_Disclosures_Additio

Fair Value Disclosures - Additional Information (Detail) (TaiGen Biotechnology Co., Ltd., USD $)	Sep. 30, 2013	Dec. 31, 2011
In Millions, unless otherwise specified
TaiGen Biotechnology Co., Ltd.	'	'
Fair Value Measurements Disclosure [Line Items]	'	'
Equity investment	$0	$0
Ownership percentage in TaiGen	4.00%	'

Components_of_Inventory_Detail

Components of Inventory (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Inventory [Line Items]	'	'
Raw materials	$586	$423
Work in process	1,722	4,184
Total inventory	10,039	6,058	[1]
Arena GmbH	'	'
Inventory [Line Items]	'	'
Finished goods	1,222	0
Eisai BELVIQ Agreement	'	'
Inventory [Line Items]	'	'
Finished goods	$6,509	$1,451
[1]	The balance sheet data at December 31, 2012, has been derived from audited financial statements at that date. It does not include, however, all of the information and notes required by US generally accepted accounting principles for complete financial statements.

Accounts_Payable_and_Other_Acc2

Accounts Payable and Other Accrued Liabilities (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Accounts Payable and Accrued Liabilities, Current [Abstract]	'	'
Payable to collaborators (See Note 6)	$18,698	$0
Accounts payable	3,042	3,884
Accrued expenses	1,840	2,006
Accrued clinical and preclinical study fees	839	566
Loss provision	37	482
Other accrued liabilities	67	185
Total accounts payable and other accrued liabilities	$24,523	$7,123	[1]
[1]	The balance sheet data at December 31, 2012, has been derived from audited financial statements at that date. It does not include, however, all of the information and notes required by US generally accepted accounting principles for complete financial statements.

Derivative_Liabilities_Additio

Derivative Liabilities - Additional Information (Detail) (USD $)	9 Months Ended		1 Months Ended
In Thousands, except Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Jun. 30, 2013	Sep. 30, 2013	Jun. 30, 2006	Sep. 30, 2013	Aug. 31, 2008	Sep. 30, 2013
			June 2006 Series B Warrants	June 2006 Series B Warrants	June 2006 Series B Warrants	August 2008 Series B Warrants	August 2008 Series B Warrants	Series B Warrants
Derivative [Line Items]	'	'	'	'	'	'	'	'
Issuance of warrants to purchase stock, number of shares	'	'	'	'	829,856	'	1,106,344	'
Original exercise price of warrants	'	'	'	'	15.49	'	7.71	'
Warrants outstanding	'	'	'	0	'	1,965,418	'	'
Revised exercise price of warrants	'	'	'	'	'	4.34	'	'
Adjustment price of warrants in anti-dilution provision	'	'	'	'	'	'	'	$6.72
Common stock issued pursuant to the exercise of series B Warrants	'	'	10,000	'	'	'	'	'
Proceeds from exercise of series B Warrants	$2,685	$150,657	$100	'	'	'	'	'
Number of shares of common stock into which expired warrant would have converted	'	'	1,457,405	'	'	'	'	'

Components_of_Derivative_Liabi

Components of Derivative Liabilities (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Derivative [Line Items]	'	'
Series B Warrants - current portion	$0	$2,587	[1]
Series B Warrants, less current portion	4,941	12,455	[1]
Total derivative liabilities	$4,941	$15,042
[1]	The balance sheet data at December 31, 2012, has been derived from audited financial statements at that date. It does not include, however, all of the information and notes required by US generally accepted accounting principles for complete financial statements.

Assumptions_used_to_Value_Warr

Assumptions used to Value Warrants using Black-Scholes Option Pricing Model (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
June 2006 Series B Warrants	'	'
Derivative [Line Items]	'	'
Risk-free interest rate	'	0.10%
Dividend yield	'	0.00%
Expected volatility	'	66.00%
Expected life (years)	'	'6 months
August 2008 Series B Warrants	'	'
Derivative [Line Items]	'	'
Risk-free interest rate	0.30%	0.30%
Dividend yield	0.00%	0.00%
Expected volatility	81.00%	93.00%
Expected life (years)	'1 year 10 months 13 days	'2 years 7 months 13 days

Gain_Loss_from_Change_in_Fair_

Gain (Loss) from Change in Fair Value of Derivative Liabilities (Detail) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Derivative [Line Items]	'	'	'	'
Total gain (loss) from valuation of derivative liabilities	$4,100	$5,259	$10,101	($13,886)
Series B Warrants	'	'	'	'
Derivative [Line Items]	'	'	'	'
Total gain (loss) from valuation of derivative liabilities	4,100	5,259	10,101	-13,941
Deerfield Acceleration Right	'	'	'	'
Derivative [Line Items]	'	'	'	'
Total gain (loss) from valuation of derivative liabilities	$0	$0	$0	$55

Marketing_and_Supply_Agreement2

Marketing and Supply Agreement with Eisai Inc. - Additional Information (Detail) (USD $)

1 Months Ended

3 Months Ended

9 Months Ended

9 Months Ended

3 Months Ended

1 Months Ended

3 Months Ended

9 Months Ended

12 Months Ended

1 Months Ended

9 Months Ended

Jul. 31, 2010

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Dec. 31, 2012

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

31-May-12

Mar. 31, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Dec. 31, 2012

Jul. 31, 2010

Sep. 30, 2013

UNITED STATES

CANADA

MEXICO

Eisai First Amended Agreement

Eisai First Amended Agreement

Eisai First Amended Agreement

Eisai First Amended Agreement

Eisai First Amended Agreement

Eisai First Amended Agreement

Original Agreement

Arena Pharmaceuticals

UNITED STATES

Non-US Territories

Non-US Territories

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Collaborative agreement initiation date

'2010-07

'

'

'

'

'

'

'

'

'2012-05

'

'

'

'

'

'2010-07

'

Purchase price range minimum

'

'

'

'

'

'

'

'

'

'

'

'

31.50%

30.75%

'

'

'

Purchase price range maximum

'

'

'

'

'

'

'

'

'

'

'

'

36.50%

35.75%

'

'

'

Annual net product sales threshold for maximum purchase price

'

'

'

'

'

'

'

'

'

'

'

$750,000,000

'

'

'

'

'

Aggregate one-time purchase price adjustments

'

'

'

'

'

'

'

'

'

'

'

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

'

'

'

'

'

Additional one-time purchase price adjustment payments outside US

'

'

'

'

'

'

'

'

'

'

'

'

'

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

'

'

Net product sales

'

2,011,000

0

3,330,000

0

'

'

'

'

'

'

'

'

'

'

'

'

Payable to collaborators

'

18,698,000

'

18,698,000

'

0

'

'

'

'

'

'

'

'

'

'

'

Collaborative agreement upfront payments

'

'

'

'

'

'

'

'

'

5,000,000

'

'

'

'

'

50,000,000

'

Collaborative agreement revenue recognition period

'

'

'

'

'

'

'

'

'

'13 years

'

'

'

'

'

'16 years

'

Revenue recognized from milestone payments

'

'

'

'

'

'

65,000,000

500,000

500,000

'

'

'

'

'

'

'

'

Additional potential regulatory and development milestone payments

'

'

'

'

'

'

'

'

'

'

$53,500,000

'

'

'

'

'

'

Portion of expenses

'

'

'

'

'

'

'

'

'

'

'

90.00%

'

'

'

'

10.00%

Expiration period after first commercial sale of BELVIQ

'

'

'

'

'

'

'

'

'

'

'

'12 years

'

'

'

'

'

Revenues_Recognized_under_Eisa

Revenues Recognized under Eisai BELVIQ Agreement (Detail) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'	'	'	'
Net product sales	$2,011	$0	$3,330	$0
Total revenues	3,578	1,485	74,878	25,651
Eisai First Amended Agreement	'	'	'	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'	'	'	'
Net product sales	2,011	0	3,330	0
Milestone payments	0	0	66,000	20,000
Amortization of upfront payments	861	864	2,584	2,642
Reimbursements of development and patent expenses	112	0	349	0
Total revenues	$2,984	$864	$72,263	$22,642

Deferred_Revenues_Recognized_u

Deferred Revenues Recognized under Eisai Agreement (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'	'
Less current portion	($29,162)	($15,453)	[1]
Deferred revenues, less current portion	46,389	47,282	[1]
Eisai BELVIQ Agreement	'	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'	'
Total deferred revenues	68,641	57,734
Less current portion	-28,532	-15,040
Deferred revenues, less current portion	40,109	42,694
Eisai BELVIQ Agreement | Up-front Payment Arrangement	'	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'	'
Total deferred revenues	43,555	46,140
Eisai BELVIQ Agreement | Sales Revenue, Goods, Net	'	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'	'
Total deferred revenues	$25,086	$11,594
[1]	The balance sheet data at December 31, 2012, has been derived from audited financial statements at that date. It does not include, however, all of the information and notes required by US generally accepted accounting principles for complete financial statements.

Stockholders_Equity_Additional

Stockholders' Equity - Additional Information (Detail) (USD $)

9 Months Ended

1 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

9 Months Ended

In Millions, except Share data, unless otherwise specified

Sep. 30, 2013

Jun. 30, 2013

Sep. 30, 2013

Mar. 31, 2013

Mar. 31, 2013

Mar. 31, 2013

Sep. 30, 2013

Jun. 10, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

June 2006 Series B Warrants

Restricted Stock Units (RSU)

Performance restricted stock units

Performance restricted stock units

Performance restricted stock units

Long Term Incentive Plan Twenty Thirteen

Long Term Incentive Plan Twenty Thirteen

Long Term Incentive Plan Twenty Thirteen

Long Term Incentive Plan Twenty Thirteen

Long Term Incentive Plan Twenty Thirteen

Long Term Incentive Plan Twenty Thirteen

Long Term Incentive Plan Twenty Thirteen

Long Term Incentive Plan Twenty Thirteen

Long Term Incentive Plan Twenty Thirteen

Minimum

Maximum

Stock Options And Stock Appreciation Rights

Other Than Stock Options And Stock Appreciation Rights

Stock Options

Restricted Stock Units (RSU)

Restricted Stock Units (RSU)

Performance restricted stock unit

Stock Options and Stock Appreciation Rights (SARs)

Minimum

Maximum

Minimum

Minimum

Stockholders Equity [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Shares authorized

'

'

'

'

'

'

'

30,000,000

'

'

'

'

'

'

'

Shares available for issuance

'

'

'

'

'

'

26,622,722

26,720,370

'

'

'

'

'

'

'

Reduction in number of shares available for grant for every share issued

'

'

'

'

'

'

'

'

1

1.25

'

'

'

'

'

Increase in number of shares available for grant for every share issued

'

'

'

'

'

'

'

'

1

1.25

'

'

'

'

'

Vesting percentage

'

'

'

'

'

'

'

'

'

'

25.00%

'

'

'

'

Vesting period

'

'

'

'3 years

'

'

'

'

'

'

'4 years

'1 year

'4 years

'12 months

'

Stock options exercisable period

'

'

'

'

'

'

'7 years

'

'

'

'

'

'

'

'

Purchase price of common shares as a percentage of fair market value

'

'

'

'

'

'

'

'

'

'

'

'

'

'

100.00%

Issuance of common stock upon exercise of options

810,424

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Issuance of common stock under employee stock purchase plan

267,994

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Net proceeds from issuance of common stock upon exercise of options

$2

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Net proceeds from issuance of common stock under employee stock purchase plan

0.6

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Common stock issued pursuant to the exercise of series B Warrants

'

10,000

'

'

'

'

'

'

'

'

'

'

'

'

'

Exercise price of warrants

'

8.76

'

'

'

'

'

'

'

'

'

'

'

'

'

Net proceeds from common stock issued pursuant to the exercise of series B Warrants

'

0.1

'

'

'

'

'

'

'

'

'

'

'

'

'

Share-based compensation expense capitalized

$0.10

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Number of performance share awards granted

'

'

131,000

780,000

'

'

'

'

'

'

'

'

'

'

'

Expected actual number of shares to be awarded as a percentage of target amounts

'

'

'

'

0.00%

200.00%

'

'

'

'

'

'

'

'

'

Share_Based_Compensation_Expen

Share Based Compensation Expense (Detail) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'	'
Share-based compensation expense	$2,319	$1,352	$6,542	$3,713
Cost of product sales	'	'	'	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'	'
Share-based compensation expense	0	0	17	0
Research and development	'	'	'	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'	'
Share-based compensation expense	1,188	509	3,047	1,215
General and administrative	'	'	'	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'	'
Share-based compensation expense	$1,131	$843	$3,478	$2,498

Summary_of_Stock_Option_Activi

Summary of Stock Option Activity (Detail) (USD $)	9 Months Ended
	Sep. 30, 2013
Number of stock options	'
Outstanding at January 1, 2013	13,842,000
Granted	1,449,000
Exercised	-810,424
Forfeited/cancelled/expired	-216,000
Outstanding at September 30, 2013	14,265,000
Weighted Average Exercise Price	'
Outstanding at January 1, 2013	$4.44
Granted	$8.50
Exercised	$2.49
Forfeited/cancelled/expired	$5.93
Outstanding at September 30, 2013	$4.94

Summary_of_Restricted_Stock_Ac

Summary of Restricted Stock Activity (Detail) (Restricted Stock Units (RSU), USD $)	9 Months Ended
	Sep. 30, 2013
Restricted Stock Units (RSU)	'
Restricted Stock Units	'
Unvested at January 1, 2013	165,000
Granted	131,000
Vested	-33,000
Forfeited/cancelled	0
Unvested at September 30, 2013	263,000
Outstanding at September 30, 2013	296,000
Weighted-Average Grant-Date Fair Value	'
Unvested at January 1, 2013	$8.87
Granted	$8.81
Vested	$8.81
Forfeited/cancelled	'
Unvested at September 30, 2013	$8.85
Outstanding at September 30, 2013	$8.84

Assumption_used_to_Value_Award

Assumption used to Value Awards and Estimated Grant date fair Value (Detail) (Performance restricted stock units, USD $)	3 Months Ended
	Mar. 31, 2013
Performance restricted stock units	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'
Risk-free interest rate	0.40%
Dividend yield	0.00%
Expected volatility	89.00%
Remaining performance period (years)	'2 years 11 months 27 days
Estimated fair value per share of PRSUs granted	$7.50

Percentages_of_Total_Revenues_

Percentages of Total Revenues (Detail) (Revenue)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Concentration Risk [Line Items]	'	'	'	'
Percentage of revenues	100.00%	100.00%	100.00%	100.00%
Eisai marketing and supply agreement	'	'	'	'
Concentration Risk [Line Items]	'	'	'	'
Percentage of revenues	83.40%	58.20%	96.50%	88.30%
Manufacturing services agreement with Siegfried	'	'	'	'
Concentration Risk [Line Items]	'	'	'	'
Percentage of revenues	12.40%	40.60%	2.90%	11.40%
Other collaborative agreements	'	'	'	'
Concentration Risk [Line Items]	'	'	'	'
Percentage of revenues	4.20%	1.20%	0.60%	0.30%

Reconciliation_of_Number_of_Sh

Reconciliation of Number of Shares Used to Calculate Basic and Diluted Net Income (Loss) Per Share (Detail)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Net income (loss) per share allocable to common stockholders:	'	'	'	'
Weighted-average shares outstanding	218,316	213,881	217,923	189,545
Potentially dilutive common shares outstanding:	'	'	'	'
Weighted average diluted shares outstanding	218,316	213,881	224,354	189,545
Stock Options	'	'	'	'
Potentially dilutive common shares outstanding:	'	'	'	'
Potential shares outstanding	'	'	5,489	'
Series B Warrants	'	'	'	'
Potentially dilutive common shares outstanding:	'	'	'	'
Potential shares outstanding	'	'	876	'
Restricted Stock Units (RSU)	'	'	'	'
Potentially dilutive common shares outstanding:	'	'	'	'
Potential shares outstanding	'	'	66	'

Potentially_Dilutive_Securitie

Potentially Dilutive Securities Excluded From Calculation of Diluted Net Loss Per Share (Detail)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'	'	'	'
Total antidilutive securities excluded from calculation of diluted net loss per share	5,569	7,361	3,757	4,018
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	4,796	6,228	3,757	3,539
Series B Warrants	'	'	'	'
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'	'	'	'
Total antidilutive securities excluded from calculation of diluted net loss per share	694	1,054	0	400
Restricted Stock Units (RSU)	'	'	'	'
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'	'	'	'
Total antidilutive securities excluded from calculation of diluted net loss per share	0	0	0	0
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	79	79	0	79

Legal_Proceedings_Additional_I

Legal Proceedings - Additional Information (Detail)	Jul. 31, 2013	Nov. 19, 2010
	Shareholder	Plaintiff
Commitments and Contingencies Disclosure [Line Items]	'	'
Number of lead plantiffs filed	'	8
Minimum number of stockholders involving in legal and factual issues	1	'

Subsequent_Event_Additional_In

Subsequent Event - Additional Information (Detail) (USD $)

1 Months Ended

9 Months Ended

1 Months Ended

3 Months Ended

9 Months Ended

23 Months Ended

1 Months Ended

Jul. 31, 2010

Sep. 30, 2013

Jul. 31, 2010

31-May-12

Mar. 31, 2013

Sep. 30, 2013

Sep. 30, 2013

31-May-12

Nov. 07, 2013

Nov. 07, 2013

Nov. 07, 2013

Nov. 07, 2013

Nov. 07, 2013

Nov. 07, 2013

Nov. 07, 2013

Nov. 07, 2013

Nov. 07, 2013

Nov. 07, 2013

Nov. 07, 2013

Nov. 07, 2013

Nov. 07, 2013

Nov. 07, 2013

Nov. 07, 2013

Nov. 07, 2013

Nov. 07, 2013

Nov. 07, 2013

Nov. 07, 2013

Nov. 07, 2013

Nov. 07, 2013

Nov. 07, 2013

Nov. 07, 2013

Nov. 07, 2013

Nov. 07, 2013

Nov. 07, 2013

Arena Pharmaceuticals

Original Agreement

Eisai First Amended Agreement

Eisai First Amended Agreement

Eisai First Amended Agreement

Eisai First Amended Agreement

Eisai Original Agreement And First Amended Agreement

Subsequent Event

Subsequent Event

Subsequent Event

Subsequent Event

Subsequent Event

Subsequent Event

Subsequent Event

Subsequent Event

Subsequent Event

Subsequent Event

Subsequent Event

Subsequent Event

Subsequent Event

Subsequent Event

Subsequent Event

Subsequent Event

Subsequent Event

Subsequent Event

Subsequent Event

Subsequent Event

Subsequent Event

Subsequent Event

Subsequent Event

Subsequent Event

Subsequent Event

Subsequent Event

UNITED STATES

Pre Approval Development For Weight Management Outside North and South America Up to Fifty Million Dollars

Pre Approval Development For Weight Management Outside North and South America

Post Approval Development For Weight Management Outside North and South America

Post Approval Development Other Than Weight Management For Outside North and South America

Non BELVIQ product for weight management

Eisai Second Amended Agreement

Eisai Second Amended Agreement

Eisai Second Amended Agreement

Eisai Second Amended Agreement

Eisai Second Amended Agreement

Eisai Second Amended Agreement

Eisai Second Amended Agreement

Eisai Second Amended Agreement

Eisai Second Amended Agreement

Eisai Second Amended Agreement

Eisai Second Amended Agreement

Eisai Second Amended Agreement

Eisai Second Amended Agreement

Arena Pharmaceuticals

Arena Pharmaceuticals

Arena Pharmaceuticals

Arena Pharmaceuticals

Arena Pharmaceuticals

Eisai First Amended Agreement

Eisai First Amended Agreement

UNITED STATES

Non-US territories

Europe, China and Japan

Other New Territories Under Second Amended Agreement

Territories Outside of North and South America

Pre Approval Development For Weight Management Outside North and South America Up to Fifty Million Dollars

Pre Approval Development For Weight Management Outside North and South America More Than Fifty Million Dollars

Post Approval Development For Weight Management Outside North and South America Up To Twenty Five Million Dollars

Post Approval Development For Weight Management Outside North and South America More Than Twenty Five Million Dollars

Post Approval Development Other Than Weight Management Outside North and South America Up To Fifty Million Dollars

Post Approval Development Other Than Weight Management Outside North and South America More Than Fifty Million Dollars

Non BELVIQ Product for Weight Management More than Forty Million Dollars

Post Approval Development For Weight Management Outside North and South America Up To Twenty Five Million Dollars

Post Approval Development For Weight Management Outside North and South America More Than Twenty Five Million Dollars

Post Approval Development Other Than Weight Management Outside North and South America Up To Fifty Million Dollars

Post Approval Development Other Than Weight Management Outside North and South America More Than Fifty Million Dollars

UNITED STATES

Non-US territories

Subsequent Event [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Collaborative agreement initiation date

'2010-07

'

'2010-07

'2012-05

'

'

'

'

'

'

'

'

'

'

'2013-11

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Collaborative agreement upfront payments

'

'

$50,000,000

$5,000,000

'

'

'

$55,000,000

'

'

'

'

'

'

$60,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Additional potential regulatory and development milestone payments

'

'

'

'

53,500,000

'

'

'

176,500,000

'

'

'

'

'

123,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Purchase price range minimum

'

'

'

'

'

'

31.50%

'

'

'

'

'

'

'

'

31.50%

'

27.50%

30.75%

'

'

'

'

'

'

'

'

'

'

'

'

'

31.50%

30.75%

Purchase price range maximum

'

'

'

'

'

'

36.50%

'

'

'

'

'

'

'

'

36.50%

'

35.00%

35.75%

'

'

'

'

'

'

'

'

'

'

'

'

'

36.50%

35.75%

Annual net product sales threshold for maximum purchase price

'

'

'

'

'

750,000,000

'

'

'

'

'

'

'

'

'

750,000,000

'

500,000,000

750,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Aggregate one-time purchase price adjustments

'

'

'

'

'

1,190,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

First annual net sales threshold to earn purchase price adjustments in all territories

'

'

'

'

'

250,000,000

'

'

'

'

'

'

'

'

250,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Last annual net sales threshold to earn purchase price adjustments in all territories

'

'

'

'

'

2,500,000,000

'

'

'

'

'

'

'

'

2,500,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Portion of purchase price adjustment payments

'

'

'

'

'

330,000,000

'

'

'

'

'

'

'

'

330,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Annual net sales threshold to earn portion of purchase price adjustments in all territories

'

'

'

'

'

1,000,000,000

'

'

'

'

'

'

'

'

1,000,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Additional one-time purchase price adjustment payments outside US

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

185,000,000

'

'

185,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

First annual net sales threshold to earn purchase price adjustments outside of the US

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

100,000,000

'

'

100,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Last annual net sales threshold to earn additional purchase price adjustments outside of the US

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

1,000,000,000

'

'

1,000,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Portion of expenses

'

10.00%

'

'

'

90.00%

'

'

'

50.00%

'

'

'

'

90.00%

'

'

'

'

'

50.00%

100.00%

50.00%

90.00%

50.00%

90.00%

100.00%

10.00%

50.00%

10.00%

50.00%

10.00%

'

'

Aggregate expense cap on equal split of development expenses

'

'

'

'

'

'

'

'

'

'

$50,000,000

$25,000,000

$50,000,000

$40,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'