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 27, 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 27, 2014, the board of directors (the “Board”) of Orexigen Therapeutics, Inc. (the “Company”) determined that it was in the best interests of the Company and its stockholders to amend the Amended and Restated Bylaws of the Company (the “Bylaws”), and by resolution authorized, approved and adopted amendments to the Bylaws (the “Bylaw Amendments”). The Bylaw Amendments became effective immediately upon their adoption.

The Bylaw Amendments revise and enhance the advance notice provisions for stockholder proposals of business and director nominations. Specifically, the changes to the advance notice provisions include the following revisions:

In addition, the Bylaw Amendments change the voting standard for the election of directors in uncontested elections from a plurality standard to a majority standard. The new majority voting standard will apply to the election of directors at the Company’s 2015 annual meeting of stockholders. Under the Bylaw Amendments, a majority of votes cast means that the number of votes cast “for” a director’s election exceeds the number of votes cast “against” such nominee. For this purpose, abstentions and broker non-votes will not count as a vote cast. If an incumbent nominee does not receive a majority of the votes cast in an uncontested election, that director shall promptly offer to tender his or her resignation to the Board. The Nominating/Corporate Governance Committee of the Board will then consider whether to accept the director’s resignation and make a recommendation to the Board. The Board will then consider the resignation, and within 90 days after the date of certification of the election results, the Board shall publicly disclose its decision and the reasons for its decision. The director whose resignation is under consideration may not participate in any deliberation regarding his or her resignation unless none of the directors received a majority of the votes cast. If the Board accepts a director’s resignation, the Board may elect a replacement in accordance with the Bylaws.

In the case of a contested election, where the number of nominees for director exceeds the number of directors to be elected, a plurality of the votes standard will continue to apply.

The Bylaw Amendments also provide that unless the Company consents in writing to the selection of an alternative forum, the Court of Chancery of the State of Delaware (or, in the event that the Court of Chancery does not have jurisdiction, the federal district court for the District of Delaware or other state courts of the State of Delaware) shall be the sole and exclusive forum for (a) any derivative action or proceeding brought on behalf of the Company, (b) any action asserting a claim of breach of a fiduciary duty owed by any director, officer or other employee of the Company to the Company or the Company’s stockholders, (c) any action asserting a claim against the corporation or any director or officer or other employee of the corporation arising pursuant to any provision of the Delaware General Corporation Law or the Company’s certificate of incorporation or Bylaws (as either may be amended from time to time), or (d) any action asserting a claim against the corporation or any director or officer or other employee of the corporation governed by the internal affairs doctrine.

The above description of the Bylaw Amendments does not purport to be complete and is qualified in its entirety by reference to the Bylaw Amendments, which are attached hereto as Exhibit 3.1.

The Company held its annual meeting of stockholders on June 27, 2014. The following is a brief description of each matter voted upon at the meeting and the number of votes cast for, withheld or against, the number of abstentions and the numbers of broker non-votes with respect to each matter, as applicable.

In accordance with the above results, each nominee was elected to serve as a director.

In accordance with the above results, the flexible settlement feature in connection with the potential conversion of the Company’s Convertible Senior Notes was approved.

In accordance with the above results, the compensation paid to the Company’s named executive officers was not approved on an advisory basis.

In accordance with the above results, the appointment of Ernst & Young LLP was approved.

(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.

Date: July 3, 2014

EXHIBIT INDEX