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): March 28, 2012

Horizon Pharma, Inc.

(Exact name of registrant as specified in its charter)

520 Lake Cook Road, Suite 520,

Deerfield, Illinois 60062

(Address of principal executive offices and zip code)

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 March 28, 2012, we filed suit in the United States District Court for the District of Delaware against Par Pharmaceutical, Inc. and Par Pharmaceutical Companies, Inc. (collectively, “Par”). The lawsuit alleges that Par has infringed U.S. Patent No. 8,067,033 (the “’033 patent”) by filing an Abbreviated New Drug Application (“ANDA”) seeking approval from the U.S. Food and Drug Administration (“FDA”) to market a generic version of DUEXIS^® tablets prior to the expiration of the patent. The ‘033 patent is 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 Par’s ANDA for 30 months or until an earlier district court decision that the ‘033 patent is 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.