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): October 24, 2012

OREXIGEN THERAPEUTICS, INC.

(Exact Name of Registrant as Specified in its Charter)

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

(Former Name or Former Address, if Changed Since Last Report.)

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):

Item 2.02 Results of Operations and Financial Condition.

As of September 30, 2012, Orexigen Therapeutics, Inc. (the “Company” or “Orexigen”) had $56.1 million in cash and cash equivalents and an additional $52.2 million in marketable securities, for a total of $108.3 million.

The information contained in Item 2.02 of this Current Report on Form 8-K is unaudited and preliminary, and does not present all information necessary for an understanding of the Company’s financial condition as of September 30, 2012. The review of the Company’s financial statements for the three months ended September 30, 2012 is ongoing and could result in changes to these amounts.

In accordance with General Instruction B.2 of Form 8-K, the information in this Current Report on Form 8-K shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or otherwise subject to the liability of that section, nor shall it be deemed incorporated by reference in any filing under the Securities Act of 1933, as amended, or the Exchange Act, whether made before or after the date hereof, except as expressly set forth by specific reference in such filing to this Current Report on Form 8-K.

* * *

Orexigen cautions you that statements included in this Current Report on Form 8-K that are not a description of historical facts are forward-looking statements. Words such as “believes,” “anticipates,” “plans,” “expects,” “indicates,” “should,” “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 estimates of Orexigen’s cash, cash equivalents and marketable securities as of September 30, 2012. 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 report due to risks related to changes in estimated financial amounts based on the completion of Orexigen’s review of its financial statements for the three months ended September 30, 2012 and other risks described in the Company’s filings with the Securities and Exchange Commission. 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 filed with the Securities and Exchange Commission August 9, 2012 and which is available from the SEC’s website (www.sec.gov) and on Orexigen’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.

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.