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): August 7, 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 August 7, 2014, Orexigen Therapeutics, Inc. (“Orexigen” or the “Company”) issued a press release announcing its financial results for the three months ended June 30, 2014. A copy of this press release is attached hereto as Exhibit 99.1.

In accordance with General Instruction B.2 of Form 8-K, the information in this Current Report on Form 8-K, including Exhibit 99.1, 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.

(d) 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

Exhibit 99.1

Orexigen Therapeutics Reports Corporate and Financial Results for the Second Quarter Ended June 30, 2014

San Diego, CA, August 7, 2014 – Orexigen^® Therapeutics, Inc. (Nasdaq: OREX), a biopharmaceutical company focused on the treatment of obesity, today announced corporate and financial results for the second quarter ended June 30, 2014.

“We are making progress with the FDA discussing the alternatives for the post-marketing obligation for evaluation of cardiovascular outcomes, and we are confident that we can reach agreement on these final details of the NB32 review,” said Michael Narachi, CEO of Orexigen.

Financial results for the three months ended June 30, 2014

For the three months ended June 30, 2014, Orexigen reported a net loss of $24.5 million, or $0.21 per share, as compared to a net loss of $18.2 million, or $0.19 per share, for the second quarter of 2013.

Total operating expenses for the second quarter of 2014 were $23.7 million compared to $19.1 million for the second quarter of 2013. This overall increase in operating expenses reflects an increase in raw materials, inventory, and manufacturing-related expenses and in stock based compensation expense. While Orexigen is responsible for manufacturing NB32, Takeda Pharmaceuticals will reimburse Orexigen for manufacturing related costs, including finished goods.

As of June 30, 2014, Orexigen had $31.1 million in cash and cash equivalents and an additional $102.2 million in marketable securities, for a total of $133.3 million. Orexigen is eligible to receive cash milestone payments of $100 million from Takeda Pharmaceuticals, between US approval and first commercial sale of NB32.

NB32 program update:

Orexigen has licensed North American NB32 rights to Takeda. Takeda is preparing a well resourced primary care U.S. launch for NB32 with approximately 900 sales representatives.

In October 2013 Orexigen submitted a Marketing Authorization Application for NB32 to the European Medicines Agency (EMA). Orexigen received the Day 180 List of Outstanding Issues from the EMA’s Committee for Medicinal Products for Human Use (CHMP) and will submit the Company’s responses in September.

Orexigen owns NB32 rights outside of North America and will seek a partner to commercialize NB32 in those territories.

Conference Call Today at 8 a.m. Eastern Time (5:00 a.m. Pacific Time)

The Orexigen management team will host a teleconference and webcast to discuss the second quarter 2014 financial results and recent business highlights. The live call may be accessed by calling (800) 708-4540 (domestic) or (847) 619-6397 (international), participant code 37792592. The webcast can be accessed live on the Investor Relations section of the Orexigen web site at www.orexigen.com and will be archived for 14 days following the call.

About Orexigen Therapeutics

Orexigen Therapeutics, Inc. is a biopharmaceutical company focused on the treatment of obesity. The Company’s lead product candidate is NB32. Based on successful results of the Light Study, an ongoing cardiovascular outcomes trial, Orexigen’s strategy for NB32 is to pursue approvals worldwide and pharmaceutical partnerships for global commercialization. The Company has submitted applications for marketing authorization in the United States and Europe, with potential approvals in 2014. If approved, North American partner Takeda Pharmaceuticals will commercialize NB32 in the United States. The Company’s other product candidate, Empatic™, has completed Phase 2 clinical trials. Further information about the Company can be found at www.orexigen.com.

Forward-Looking Statements

Orexigen cautions you that statements included in this press release 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 timing of potential approval of the NDA for NB32 in the U.S. and the MAA for NB32 by the European Medicines Agency (EMA); the ability to reach agreement with the FDA on a post-marketing requirement for cardiovascular outcomes; the timing of Orexigen’s response to the Day 180 List of Outstanding Issues; the timing and potential for a Takeda launch of NB32; the benefit risk profile for NB32; the use by Takeda of 900 sales representatives focusing on primary care physicians in the U.S.; the success of a U.S. primary care launch of NB32; and the potential to obtain a partner to commercialize NB32 in certain territories outside of North America. 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 release due to the risk and uncertainties inherent in the Orexigen business, including, without limitation: the potential for the FDA to not approve NB32 even after meeting the prespecified threshold and resubmitting the NDA; the possibility that public disclosure of the results of the interim analysis of the Light Study 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 results from the interim analysis may not be sufficient to successfully respond to the questions raised regarding safety of NB32; the possibility that the CHMP requests additional or new data to be provided; the possibility that the CHMP determines to not approve NB32; the potential for early termination of Orexigen’s North American collaboration agreement with Takeda; the possibility that Orexigen is unable to find a partner for NB32 in territories outside of North America, the final results of the Light Study may not support continued approval of NB32; the therapeutic and commercial value of NB32; the potential for additional costly clinical trials needed for Empatic; 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 news release 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 Annual Report on Form 10-K filed with the Securities and Exchange Commission March 13, 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.

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Orexigen Therapeutics, Inc.

Balance Sheets

(In thousands, except share and par value amounts)

Orexigen Therapeutics, Inc.

Statements of Operations

(In thousands, except per share amounts)

(Unaudited)