Exhibit 5.1

L. Kay Chandler

T: +1 858 550 6014

kchandler@cooley.com

April 10, 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 filing by Horizon Pharma, Inc., a Delaware corporation (the “Company”), of a Registration Statement on Form S-1 (the “Registration Statement”) with the Securities and Exchange Commission, including a related prospectus filed with the Registration Statement (the “Prospectus”), covering the registration for resale of up to 20,819,468 shares of the Common Stock, $0.0001 par value, of the Company on behalf of certain selling stockholders, including 14,033,829 shares of Common Stock (the “Shares”) and up to 6,785,639 shares of Common Stock (the “Warrant Shares”) issuable upon the exercise of certain warrants held by such selling stockholders (the “Warrants”).

In connection with this opinion, we have examined and relied upon the Registration Statement, the Prospectus, the Warrants, the Company’s Amended and Restated Certificate of Incorporation, its Amended and Restated Bylaws, 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 thereof and the due execution and delivery of all documents where due 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 and the General Corporation Law of the State of Delaware.

On the basis of the foregoing, and in reliance thereon, we are of the opinion that (i) the Shares are validly issued, fully paid and nonassessable, and (ii) the Warrant Shares, when issued and sold 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 the Registration Statement.

Very truly yours,

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