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): June 2, 2014

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):

On June 2, 2014, Orexigen Therapeutics, Inc. (the “Company” or “Orexigen”) announced that the United States Food and Drug Administration (the “FDA”) informed the Company that the FDA intends to reassign the Prescription Drug User Fee Act (“PDUFA”) action date for the NB32 New Drug Application to June 11, 2014, one day beyond the original assignment. The FDA noted that due to an inadvertant administrative error, the FDA had miscalculated the originally assigned PDUFA date.

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,” “should,” “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: the adjusted PDUFA date and the timing of potential approval of the NDA for NB32 in the United States. 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 materially from those expressed or implied in this report due to the risk and uncertainties inherent in Orexigen’s business, including, without limitation: the potential for additional delays in the regulatory review process for the NDA for NB32; the Special Protocol Assessment pertaining to NB32 is not binding on the FDA if public health concerns unrecognized at the time the SPA agreement was entered into become evident, other new scientific concerns regarding product safety or efficacy arise, or if Orexigen fails to comply with the agreed upon trial protocol; the potential for the FDA to not approve NB32 even after meeting the prespecified threshold and resubmitting the NDA; the possibility that the public announcement of the results of the interim analysis would later be deemed to jeopardize the integrity of the Light Study potentially resulting in the requirement to conduct additional, costly studies; additional analysis of the interim results or new data from the continuing Light Study, including safety-related data, may produce negative or inconclusive results, or may be inconsistent with the conclusion that the interim analysis was successful; the potential that the interim analysis may not be predictive of future results in the Light Study; the final results of the Light Study may not support continued approval of NB32; Orexigen’s ability to maintain sufficient capital to fund its operations through potential approval of NB32 in 2014; and other risks described in Orexigen’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 on May 9, 2014 and its other reports, which are 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.