Orexigen Therapeutics (OREX) 8-K Orexigen® Therapeutics Announces Proposed Public Offering of Common Stock

Exhibit 5.1

	12636 High Bluff Drive, Suite 400
	San Diego, California 92130-2071
	Tel: +1.858.523.5400 Fax: +1.858.523.5450
	www.lw.com
	FIRM / AFFILIATE OFFICES
	Abu Dhabi	Moscow
	Barcelona	Munich
	Beijing	New Jersey
	Boston	New York
	Brussels	Orange County
October 25, 2012	Chicago	Paris
	Doha	Riyadh
	Dubai	Rome
	Frankfurt	San Diego
	Hamburg	San Francisco
	Hong Kong	Shanghai
	Houston	Silicon Valley
	London	Singapore
	Los Angeles	Tokyo
Orexigen Therapeutics, Inc.	Madrid	Washington, D.C.
3344 N. Torrey Pines Court	Milan

Suite 200

La Jolla, CA 92037

Re: Registration Statement on Form S-3 (No. 333-183918);

12,650,000 shares of Common Stock, par value $0.001 per share

Ladies and Gentlemen:

We have acted as counsel to Orexigen Therapeutics, Inc., a Delaware corporation (the “Company”), in connection with the proposed issuance of up to 12,650,000 shares of common stock of the Company, par value $0.001 per share, including up to 1,650,000 shares of common stock of the Company issuable upon exercise of the underwriters’ option to purchase additional shares (the “Shares”). The Shares are included in a registration statement on Form S-3 under the Securities Act of 1933, as amended (the “Act”), filed with the Securities and Exchange Commission (the “Commission”) on September 14, 2012 (File No. 333-183918), including the information deemed to be a part of the registration statement pursuant to Rule 430B of the Act (as so filed, the “Registration Statement”), a base prospectus dated October 4, 2012 (the “Base Prospectus”), a preliminary prospectus supplement dated October 24, 2012 filed with the Commission pursuant to Rule 424(b) under the Act (together with the Base Prospectus, the “Preliminary Prospectus”), the document that the Company has identified as an “issuer free writing prospectus” (as defined in Rules 433 and 405 under the Act) (the “Specified IFWP”) and a prospectus supplement dated October 25, 2012 filed with the Commission pursuant to Rule 424(b) under the Act (together with the Base Prospectus, the “Prospectus”). This opinion is being furnished in connection with the requirements of Item 601(b)(5) of Regulation S-K under the Act, and no opinion is expressed herein as to any matter pertaining to the contents of the Registration Statement, the Preliminary Prospectus, the Specified IFWP or Prospectus, other than as expressly stated herein with respect to the issue of the Shares.

As such counsel, we have examined such matters of fact and questions of law as we have considered appropriate for purposes of this letter. With your consent, we have relied upon certificates and other assurances of officers of the Company and others as to factual matters without having independently verified such factual matters. We are opining herein as to the General Corporation Law of the State of Delaware, and we express no opinion with respect to any other laws.

October 25, 2012

Page 2

Subject to the foregoing and the other matters set forth herein, it is our opinion that, as of the date hereof, the issue and sale of the Shares have been duly authorized by all necessary corporate action of the Company and, when the Shares shall have been duly registered on the books of the transfer agent and registrar therefor in the name or on behalf of the purchasers, and have been issued by the Company against payment therefor (not less than par value) in the circumstances contemplated by the form of underwriting agreement most recently filed as an exhibit to the Registration Statement, the Shares will be validly issued, fully paid and nonassessable. In rendering the foregoing opinion, we have assumed that the Company will comply with all applicable notice requirements regarding uncertificated shares provided in the General Corporation Law of the State of Delaware.

This opinion is for your benefit in connection with the Registration Statement and may be relied upon by you and by persons entitled to rely upon it pursuant to the applicable provisions of the Act. We consent to your filing this opinion as an exhibit to the Company’s Form 8-K dated October 25, 2012 and to the reference to our firm in the Prospectus under the heading “Legal Matters.” In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Act or the rules and regulations of the Commission thereunder.

Very truly yours,

/s/ Latham & Watkins LLP