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): August 26, 2013

Horizon Pharma, Inc.

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (224) 383-3000

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:

On August 26, 2013, we filed suit in the United States District Court for the District of New Jersey against Watson Laboratories, Inc. – Florida, Actavis Pharma, Inc., Andrx Corp., and Actavis, Inc. (collectively, “WLF”). The lawsuit alleges that WLF has infringed U.S. Patent Nos. 6,488,960, 6,677,326, 8,168,218, 8,309,124, and 8,394,407 by filing an Abbreviated New Drug Application (“ANDA”) seeking approval from the U.S. Food and Drug Administration (“FDA”) to market generic versions of RAYOS^® containing 1 mg, 2 mg, and 5 mg of prednisone prior to the expiration of the patents. The subject patents are listed in the FDA’s Approved Drug Products with Therapeutic Equivalence Evaluations, commonly known as the Orange Book. Our commencement of the patent infringement lawsuit stays, or bars, FDA approval of WLF’s ANDA for 30 months or until an earlier district court decision that the subject patents are not infringed or invalid.

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.