UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 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): July 14, 2015

RECEPTOS, INC.

(Exact name of registrant as specified in its charter)

3033 Science Park Road, Suite 300

San Diego, California 92121

(Address of principal executive offices, including zip code)

(858) 652-5700

(Registrant’s telephone number, including area code)

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 July 14, 2015, Receptos, Inc. (the “Company”) and Celgene Corporation (“Parent”) issued a joint press release announcing the execution of the Agreement and Plan of Merger (the “Merger Agreement”) by and among Parent, Strix Corporation, a wholly owned subsidiary of Parent (“Acquisition Sub”), and the Company, pursuant to which Acquisition Sub will commence a tender offer (the “Offer”) to acquire all of the outstanding shares of common stock of the Company (the “Company Shares”) at a purchase price of $232.00 per Company Share net to the holder thereof in cash, subject to reduction for any applicable withholding taxes, without interest. A copy of the press release is filed herewith as Exhibit 99.1 and is incorporated herein by reference.

(d) Exhibits

Notice to Investors

The Offer described above has not yet commenced. This filing and the attached exhibit are not an offer to buy nor a solicitation of an offer to sell any the Company’s securities. The solicitation and the offer to buy the Company Shares will only be made pursuant to a tender offer statement on Schedule TO, including an offer to purchase, a letter of transmittal and other related materials that Parent and Acquisition Sub intend to file with the Securities and Exchange Commission (the “SEC”). In addition, the Company will file with the SEC a Solicitation/Recommendation Statement on Schedule 14D-9 with respect to the Offer. Once filed, investors will be able to obtain the tender offer statement on Schedule TO, the offer to purchase, the Company’s Solicitation/Recommendation Statement on Schedule 14D-9 and related materials with respect to the Offer and the merger of Acquisition Sub with and into the Company, with the Company surviving as a wholly owned subsidiary of Parent (the “Merger”), free of charge at the website of the SEC at www.sec.gov, and from the information agent named in the tender offer materials. Investors may also obtain, at no charge, any such documents filed with or furnished to the SEC by the Company under the “Investors” section of the Company’s website at http://ir.receptos.com/. INVESTORS AND SECURITY HOLDERS ARE ADVISED TO READ THESE DOCUMENTS WHEN THEY BECOME AVAILABLE, INCLUDING THE SOLICITATION/RECOMMENDATION STATEMENT OF THE COMPANY AND ANY AMENDMENTS THERETO, AS WELL AS ANY OTHER DOCUMENTS RELATING TO THE OFFER AND THE MERGER THAT ARE FILED WITH THE SEC, CAREFULLY AND IN THEIR ENTIRETY PRIOR TO MAKING ANY DECISIONS WITH RESPECT TO WHETHER TO TENDER THEIR SHARES PURSUANT TO THE OFFER BECAUSE THEY CONTAIN IMPORTANT INFORMATION, INCLUDING THE TERMS AND CONDITIONS OF THE OFFER.

Forward-Looking Statements

Statements included in this report that are not a description of historical facts are forward-looking statements. Words or phrases such as “believe,” “may,” “could,” “will,” “estimate,” “continue,” “anticipate,” “intend,” “seek,” “plan,” “expect,” “should,” “would” or similar expressions are intended to identify forward-looking statements, and are based on the Company’s current beliefs and expectations. These forward-looking statements include without limitation statements regarding the planned completion of the Offer and the Merger. The Company’s actual future results may differ materially from the Company’s current expectations due to the risks and uncertainties inherent in its business. These risks include, but are not limited to: uncertainties as to the timing of the Offer and the Merger; uncertainties as to the percentage of the Company’s stockholders tendering their shares in the Offer; the possibility that competing offers will be made; the possibility that various closing conditions for the Offer or the Merger may not be satisfied or waived, including that a governmental entity may prohibit, delay or refuse to grant approval for the consummation of the Merger; the effects of disruption caused by the transaction making it more difficult to maintain relationships with employees, collaborators, vendors and other business partners; the risk

that stockholder litigation in connection with the Offer or the Merger may result in significant costs of defense, indemnification and liability; and risks and uncertainties pertaining to the Company’s business, including those detailed under “Risk Factors” and elsewhere in the Company’s public periodic filings with the SEC, as well as the tender offer materials to be filed by Parent and Acquisition Sub and the Solicitation/Recommendation Statement to be filed by the Company in connection with the Offer.

The reader is cautioned not to place undue reliance on these forward-looking statements, which speak only as of the date hereof. All forward-looking statements are qualified in their entirety by this cautionary statement and the date hereof. All forward-looking statements are qualified in their entirety by this cautionary statement and the Company undertakes no obligation to revise or update this report to reflect events or circumstances after the date hereof, except as required by law.

SIGNATURES

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

EXHIBIT INDEX