Exhibit 51 Opinion

Exhibit 5.1

2000 Pennsylvania Ave., NW
Washington, D.C.
20006-1888

Telephone: 202.887.1500

Facsimile: 202.887.0763

www.mofo.com

morrison & foerster llp

beijing, berlin, brussels,
denver, hong kong, london,
los angeles, new york,
northern virginia, palo alto,
san diego, san francisco, shanghai,
singapore, tokyo, washington, d.c.

June 18, 2018

Board of Directors

Pain Therapeutics, Inc.

7801 N. Capital of Texas Highway, Suite 260

Austin, TX 78731

Re: Registration Statement on Form S-8

Ladies and Gentlemen:

We are acting as counsel to Pain Therapeutics, Inc., a Delaware corporation (the “Company”), in connection with its registration statement on Form S-8 (the “Registration Statement”), filed with the Securities and Exchange Commission under the Securities Act of 1933, as amended (the “Securities Act”), relating to the proposed offering of up to 1,000,000 shares (the “Shares”) of the Company’s common stock, $0.001 par value per share. The Shares may be issued from time to time pursuant to awards granted under the Company’s 2018 Omnibus Incentive Plan (the “Plan”).

As counsel for the Company, we have examined originals or copies, certified or otherwise identified to our satisfaction, of such documents, corporate records, certificates of public officials, and other instruments as we have deemed necessary for the purposes of rendering this opinion and we are familiar with the proceedings taken and proposed to be taken by the Company in connection with the authorization, issuance and sale of the Shares. In our examination, we have assumed the genuineness of all signatures, the legal capacity and competency of all natural persons, the authenticity of all documents submitted to us as originals, and the conformity with the originals of all documents submitted to us as copies.

Based upon, subject to and limited by the foregoing, we are of the opinion that following (i) the effectiveness of the Registration Statement, (ii) the issuance of the Shares pursuant to awards granted under the terms of the Plan, and (iii) the receipt by the Company of the consideration for the Shares specified in the applicable resolutions of the Board of Directors of the Company or a duly authorized committee thereof and the Plan, the Shares will be validly issued, fully paid and nonassessable.

NY2-810549

June 18, 2018

Page Two

We express no opinion as to matters governed by laws of any jurisdiction other than the General Corporation Law of the State of Delaware, as in effect on the date hereof.

We consent to the use of this opinion as an exhibit to the Registration Statement, and we consent to the reference of our name wherever appearing in the Registration Statement and any amendments thereto. In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act. We disclaim any undertaking to advise you of any subsequent changes of the facts stated or assumed herein or any subsequent changes in applicable law.

Very truly yours,

/s/ Morrison & Foerster LLP

Morrison & Foerster LLP

NY2-810549