UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, DC 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the

Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): December 15, 2011

Registrant’s telephone number, including area code: (858) 875-8600

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2. below):

8.01.    Other Events.

On December 15, 2011, Orexigen Therapeutics, Inc. (the “Company” or “Orexigen”) entered into a purchase agreement (the “Purchase Agreement”) with Leerink Swann LLC (“Leerink”), as the sole underwriter, relating to the issuance and sale of 5,646,173 shares of the Company’s common stock, par value $0.001 per share (“Common Stock”), and warrants to purchase 56,461,730 shares of Common Stock. The Common Stock and warrants to purchase Common Stock will be sold in units, with each unit consisting of (i) one share of Common Stock and (ii) a warrant to purchase ten shares of Common Stock. The warrants will be immediately exercisable at an exercise price of $0.001 per share and will expire ten years from the date of issuance. The shares of Common Stock and the warrants are immediately separable and will be issued separately. The price to the public in this offering is $1.45 per share of Common Stock and $14.49 per warrant to purchase ten shares of Common Stock, which together comprise the purchase price of $15.94 per unit. Leerink has agreed to purchase the units from the Company pursuant to the Purchase Agreement at a price of $15.4618 per unit. The net proceeds to the Company from this offering are expected to be approximately $86.9 million, after deducting underwriting discounts and commissions and estimated offering expenses payable by the Company and excluding the proceeds to the Company, if any, from the exercise of the warrants to be issued in the offering. The offering is expected to close on or about December 22, 2011, subject to customary closing conditions.

The offering is being made pursuant to the Company’s effective shelf registration statement on Form S-3 (Registration Statement No. 333-161736) previously filed with and declared effective by the Securities and Exchange Commission (the “SEC”) and a prospectus supplement and accompanying prospectus filed with the SEC.

The Purchase Agreement contains customary representations, warranties and agreements by the Company, customary conditions to closing, indemnification obligations of the Company and Leerink, including for liabilities under the Securities Act of 1933, as amended, other obligations of the parties and termination provisions. The representations, warranties and covenants contained in the Purchase Agreement were made only for purposes of such agreement and as of specific dates, were solely for the benefit of the parties to such agreement, and may be subject to limitations agreed upon by the contracting parties.

The foregoing descriptions of the Purchase Agreement and the warrants are not complete and are qualified in their entireties by reference to the full text of the Purchase Agreement and form of warrant, copies of which are filed as Exhibit 1.1 and Exhibit 4.1, respectively, to this report and are incorporated by reference herein. A copy of the opinion of Latham & Watkins LLP relating to the legality of the issuance and sale of the Common Stock and warrants to purchase Common Stock in the offering is attached as Exhibit 5.1 to this report.

The Company issued press releases on December 14, 2011 and December 15, 2011 announcing the commencement of the offering and the pricing of the offering, respectively, which press releases are attached as Exhibits 99.1 and 99.2, respectively, to this report.

Orexigen cautions you that statements included in this report that are not a description of historical facts are forward-looking statements. Words such as “believes,” “anticipates,” “plans,” “expects,” “indicates,” “will,” “intends,” “potential,” “suggests,” “assuming,” “designed” and similar expressions are intended to identify forward-looking statements. These statements are based on the Company’s current beliefs and expectations. These forward-looking statements include statements regarding Orexigen’s expectations regarding the completion of the public offering and the expected net proceeds therefrom. The inclusion of forward-looking statements should not be regarded as a representation by Orexigen that any of its plans will be achieved. Actual results may differ from those set forth in this release due to the risks and uncertainties associated with market conditions and the satisfaction of customary closing conditions related to the public offering as well as risks and uncertainties inherent in Orexigen’s business; and other risks described in the Company’s filings with the SEC. You are cautioned not to place undue reliance on these forward-looking statements, which speak only as of the date hereof, and Orexigen undertakes no obligation to revise or update this report to reflect events or circumstances after the date hereof. Further information regarding these and other risks is included under the heading “Risk Factors” in Orexigen’s Quarterly Report on Form 10-Q, which was filed with the SEC on November 8, 2011 and is available from the SEC’s website (www.sec.gov) and the Company’s website (www.orexigen.com) under the heading “Investor Relations”. All forward-looking statements are qualified in their entirety by this cautionary statement. This caution is made under the safe harbor provisions of Section 21E of the Private Securities Litigation Reform Act of 1995.

Item 9.01    Financial Statements and Exhibits.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

EXHIBIT INDEX