UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 8-K/A

(Amendment No. 1)

CURRENT REPORT

Pursuant to Section 13 or 15(d)

of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): April 9, 2015 (March 29, 2015)

HYPERION THERAPEUTICS, INC.

(Exact Name of Registrant as Specified in its Charter)

Registrant’s telephone number, including area code: (650) 745-7802

Not Applicable

(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:

The purpose of this Amendment No. 1 is to file a corrected version of the Agreement and Plan of Merger (the “Merger Agreement”) (Exhibit 2.1), dated as of March 29, 2015, by and among Horizon Pharma, Inc., Ghrian Acquisition Inc. (“Purchaser”) and Hyperion Therapeutics, Inc. (the “Company”), the previously filed version of which contained typographical errors.

A copy of the corrected Merger Agreement is attached to this Amendment No. 1 as Exhibit 2.1.

(d) Exhibits.

Notice to Investors

This report is not a recommendation, an offer to purchase or a solicitation of an offer to sell shares of the Company stock. Purchaser has not commenced the tender offer for shares of Company stock described in this report. Upon commencement of the tender offer, Purchaser will file with the SEC a tender offer statement on Schedule TO and related exhibits, including an offer to purchase, letter of transmittal, and other related documents. Following commencement of the tender offer, the Company will file with the SEC a solicitation/recommendation statement on Schedule 14D-9. INVESTORS AND SECURITY HOLDERS ARE URGED TO READ BOTH THE TENDER OFFER STATEMENT AND THE SOLICITATION/RECOMMENDATION STATEMENT REGARDING THE OFFER, AS THEY MAY BE AMENDED FROM TIME TO TIME, WHEN THEY BECOME AVAILABLE BECAUSE THEY WILL CONTAIN IMPORTANT INFORMATION. Investors and security holders may obtain a free copy of these statements (when available) and other documents filed with the SEC at the website maintained by the SEC at www.sec.gov or by directing such requests to the Information Agent for the tender offer, which will be named in the tender offer statement.

Forward-Looking Statements

This report may contain forward-looking statements. These are forward-looking statements for purposes of the safe harbor provisions under the Private Securities Litigation Reform Act of 1995. The actual results of the transaction could vary materially as a result of a number of factors, including: uncertainties as to how many of the Company’s stockholders will tender their stock in the Offer; the possibility that competing offers will be made; the possibility that various closing conditions for the transaction may not be satisfied or waived, including the possibility that required regulatory approvals are not obtained, are delayed or are subject to unanticipated conditions that could adversely affect the combined company or the expected benefits of the transaction; the possibility that the transaction may not be timely completed, if at all; and the possibility that, prior to the completion of the transaction, if at all, the Company’s business may experience significant disruptions due to transaction-related uncertainty. Other factors that may cause actual results to differ materially include those set forth in the reports that the Company files from time to time with the SEC, including the Company’s annual report on Form 10-K for the fiscal year ended December 31, 2014 and subsequent quarterly and current reports on Forms 10-Q and 8-K. These forward-looking statements reflect the Company’s expectations as of the date of this report. The Company undertakes no obligation to update the information provided herein.

SIGNATURE

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.

Date: April 9, 2015