Exhibit 5.1

Sean M. Clayton

T: +1 858 550 6034

sclayton@cooley.com

September 20, 2012

Horizon Pharma, Inc.

520 Lake Cook Road, Suite 520

Deerfield, IL 60015

Ladies and Gentlemen:

You have requested our opinion with respect to certain matters in connection with the issuance by Horizon Pharma, Inc., a Delaware corporation (the “Company”), of up to 24,638,750 shares of the Company’s common stock, par value $0.0001 (the “Shares”), and warrants (the “Warrants”) to purchase up to 12,319,375 shares of the Company’s common stock (the “Warrant Shares”), including up to 3,213,750 Shares and Warrants to purchase up to 1,606,875 Warrant Shares that may be sold pursuant to the exercise of an overallotment option, pursuant to a Registration Statement on Form S-3 (Registration No. 333-182975) (the “Registration Statement”) filed with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended (the “Act”), the prospectus filed with the Commission on August 9, 2012 and related to the Registration Statement (the “Base Prospectus”) and the prospectus supplement dated September 20, 2012, filed with the Commission pursuant to Rule 424(b) promulgated under the Act (the “Prospectus Supplement”). The Base Prospectus and the Prospectus Supplement are collectively referred to as the “Prospectus.” The Shares and Warrants are to be issued by the Company as described in the Registration Statement and the Prospectus.

In connection with this opinion, we have examined and relied upon the Registration Statement, the Prospectus, the form of Warrant, the Company’s Amended and Restated Certificate of Incorporation, its Amended and Restated Bylaws, the Underwriting Agreement, dated September 20, 2012, by and among the Company, Cowen and Company, LLC, JMP Securities LLC, and Stifel, Nicolaus & Company, Incorporated (the “Agreement”), and the originals or copies certified to our satisfaction of such records, documents, certificates, memoranda and other instruments as in our judgment are necessary or appropriate to enable us to render the opinion expressed below. We have assumed the genuineness and authenticity of all documents submitted to us as originals; the conformity to originals of all documents submitted to us as copies; the accuracy, completeness and authenticity of certificates of public officials; and the due authorization, execution and delivery of all documents where due authorization, execution and delivery are a prerequisite to the effectiveness thereof.

Our opinion is expressed only with respect to the federal laws of the United States of America, the General Corporation Law of the State of Delaware and, as to the Warrants constituting valid and legally binding obligations of the Company, with respect to the laws of the State of New York. Our opinion is based on these laws as in effect on the date hereof. We express no opinion to the extent that the laws of any particular jurisdiction other than those identified above are applicable to the subject matter hereof.

4401 EASTGATE MALL, SAN DIEGO, CA 92121 T: (858) 550-6000 F: (858) 550-6420 WWW.COOLEY.COM

Horizon Pharma, Inc.

September 20, 2012

Page Two

With respect to the Warrant Shares, we express no opinion to the extent that, notwithstanding its current reservation of shares of common stock, future issuances of securities, including the Warrant Shares, of the Company and/or adjustments to outstanding securities, including the Warrants, of the Company may cause the Warrants to be exercisable for more shares of common stock than the number that remain authorized but unissued. Further, we have assumed the Exercise Price (as defined in the Warrants) will not be adjusted to an amount below the par value per share of the Company’s common stock.

On the basis of the foregoing, and in reliance thereon, we are of the opinion that (i) the Shares, when issued in accordance with the Registration Statement, the Prospectus and the Agreement, will be validly issued, fully paid and nonassessable, (ii) provided that the Warrants have been duly executed and delivered by the Company and duly delivered to the purchasers thereof against payment therefor, the Warrants, when issued and sold as contemplated in the Registration Statement, the Prospectus and the Agreement, will be valid and legally binding obligations of the Company, enforceable against the Company in accordance with their terms, except as enforcement thereof may be limited by bankruptcy, reorganization, insolvency, moratorium, fraudulent conveyance, debtor and creditor, and similar laws which relate to or affect creditors’ rights generally, and by general principles of equity (including without limitation, concepts of materiality, reasonableness, good faith and fair dealing) regardless of whether considered in a proceeding in equity or at law, and (iii) the Warrant Shares, when issued and paid for in accordance with the terms of the Warrants, will be validly issued, fully paid and nonassessable.

We consent to the reference to our firm under the caption “Legal Matters” in the Prospectus and to the filing of this opinion as an exhibit to a Current Report on Form 8-K to be filed with the Commission for incorporation by reference into the Registration Statement.

4401 EASTGATE MALL, SAN DIEGO, CA 92121 T: (858) 550-6000 F: (858) 550-6420 WWW.COOLEY.COM