April 18, 2013

Acura Pharmaceuticals, Inc.

616 N. North Court

Palatine, IL 60067

Gentlemen:

Reference is made to the opinion letter dated March 6, 2013 and included as Exhibit 5.1 to the Registration Statement on Form S-3 (File No. 333-187075) (the “Registration Statement”) filed on March 6, 2013 by Acura Pharmaceuticals, Inc., a New York corporation (the “Company”), with the Securities and Exchange Commission (the “SEC”) under the Securities Act of 1933, as amended (the “Securities Act”), for the registration of, among other things, shares of the Company’s Common Stock, $0.01 par value per share (the “Common Stock”). The Registration Statement was declared effective by the SEC on March 15, 2013. We are delivering this supplemental opinion letter in connection with the prospectus supplement (“Prospectus Supplement”) filed on April 18, 2013 by the Company with the SEC pursuant to Rule 424 under the Securities Act. The Prospectus included in the Registration Statement (the “Prospectus”) and the Prospectus Supplement relate to the offering by the Company, from time to time, of shares of Common Stock (the “Shares”) having an aggregate offering price of up to $13,000,000 (the “Offering”). The Shares are to be sold from time to time pursuant to the At Market Issuance Sales Agreement, dated April 18, 2013 (the “Sales Agreement”) between the Company and MLV & Co. LLC (the “Agent”).

As counsel to the Company, in rendering the opinions hereinafter expressed, we have examined and relied upon originals or copies of such corporate and public records and agreements, instruments, certificates, and other documents as we have deemed necessary or appropriate for purposes of this opinion, including; (i) the Registration Statement; (ii) the Prospectus Supplement and the Prospectus; (iii) the Restated Certificate of Incorporation of the Company as certified by the Secretary of State of New York and certified by the Secretary of the Company, and the Amended and Restated By-laws of the Company as currently in effect and as certified by the Secretary of the Company; (iv) the Sales Agreement; (v); and (vi) a copy of certain resolutions of the board of directors of the Company (the “Board of Directors”), adopted on February 28, 2013, relating to the Registration Statement and resolutions of the Deal Committee of the Board of Directors, dated April 16, 2013 and April 17, 2013, relating to the Sales Agreement, in each case as certified by the Secretary of the Company. As to all matters of fact (including factual conclusions and characterizations and descriptions of purpose, intention or other state of mind), we have relied entirely upon certificates of certain of the officers of the Company and have assumed, without independent inquiry, the accuracy of those certificates.

5640092v2

CALIFORNIA \ CONNECTICUT \ MASSACHUSETTS \ MICHIGAN \ NEW JERSEY \ NEW YORK \ PENNSYLVANIA \ VIRGINIA \ WASHINGTON, D.C.

Paul D. Drobbin \ Attorney in charge, Newark office \ LeClairRyan is a Virginia professional corporation

Acura Pharmaceuticals, Inc.

April 18, 2013

Page 2

We have also assumed (i) the genuineness of all signatures, the conformity to the originals of all documents reviewed by us as copies, the authenticity and completeness of all original documents reviewed by us in original or copy form and the legal competence of each individual executing a document; (ii) that at the time of each issuance and sale of Shares in the Offering, the Company will continue to be validly existing and in good standing under the laws of the State of New York, with the requisite corporate power and authority to issue and sell all such Shares at such time; (iii) that any Shares issued from time to time in the Offering will not, in the aggregate, exceed (x) the maximum aggregate offering price set forth in the Prospectus Supplement and Prospectus or (y) the maximum authorized number of shares of Common Stock under the Restated Certificate of Incorporation of the Company, as amended from time to time, minus the number of shares of Common Stock that may have been issued and are outstanding, or are reserved for other purposes, at such time; (iv) that the execution, delivery and performance by the Company of the Sales Agreement will not constitute a breach or violation of any agreement or instrument that is binding upon the Company from time to time; and (v) that Placement Notices (as defined in the Sales Agreement), if any, will be delivered in accordance with the terms of the Sales Agreement and the authority granted by the Company’s Board of Directors and Deal Committee.

Acura Pharmaceuticals, Inc.

April 18, 2013

Page 3

This opinion is limited solely to the New York Business Corporation Law (the “NYBCL”), and the reported judicial decisions interpreting those laws as applied by courts located New York, in each case to the extent that the same may apply to or govern the transactions contemplated by the Sales Agreement, the Prospectus Supplement and the Prospectus, and we express no opinion as to the laws of any other jurisdiction. We express no opinion as to the effect of events occurring, circumstances arising, or changes in law becoming effective or occurring, after the date hereof on the matters addressed in this opinion and we assume no responsibility to inform you of additional or changed facts, or changes in law, of which we may become aware.

Based on such examination and subject to the foregoing, we are the opinion that the Shares have been duly authorized and, upon issuance and delivery of the Shares against payment therefor in accordance with the terms of the Sales Agreement (and provided that the purchase price per share paid by purchasers is equal to or in excess of the par value per share of Common Stock and in excess of any minimum purchase price, and within any other parameters, established by the authorized officers of the Company in accordance with the Deal Committee resolutions), such Shares will be validly issued, full paid and non-assessable.

We hereby consent to the inclusion of this opinion as Exhibit 5.1 to the Company’s Current Report on Form 8-K and to the references to our firm under the caption “Legal Matters” in the Prospectus Supplement. In giving our consent, we do not admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations thereunder.