Exhibit 5.1

June 12, 2017

Regeneron Pharmaceuticals, Inc.
777 Old Saw Mill River Road
Tarrytown, New York 10591-6707

Re: Registration Statement on Form S-8

Ladies and Gentlemen:

We have acted as special counsel to Regeneron Pharmaceuticals, Inc., a New York corporation (the “Company”), in connection with the Registration Statement on Form S-8 of the Company (the “Registration Statement”) to be filed with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended (the “Securities Act”), on the date hereof, relating to the registration of 12,000,000 shares (the “Shares”) of the Company’s common stock, par value $0.001 per share (the “Common Stock”), issuable pursuant to the Regeneron Pharmaceuticals, Inc. Amended and Restated 2014 Long-Term Incentive Plan (the “Plan”).

This opinion is being furnished in accordance with the requirements of Item 601(b)(5) of Regulation S-K under the Securities Act.

In rendering the opinion stated herein, we have examined and relied upon the following:

(a) the Registration Statement in the form to be filed with the Commission on the date hereof;

(b) the Plan;

(c) an executed copy of a certificate of Joseph J. LaRosa, Senior Vice President, General Counsel and Secretary of the Company, dated as of the date hereof (the “Secretary’s Certificate”)

(d) a copy of the Company’s Restated Certificate of Incorporation, as amended, as certified by the Secretary of State of the State of New York as of June 2, 2017, and certified pursuant to the Secretary’s Certificate;

(e) a copy the Company’s Amended and Restated By-Laws, as in effect as of the date hereof, as certified pursuant to the Secretary’s Certificate;

(f) a specimen certificate evidencing the Common Stock;

(g) a copy of certain resolutions of the Board of Directors of the Company, adopted on April 6, 2017, certified pursuant to the Secretary’s Certificate; and

(h) the Final Report of the Inspector of Election for the Company’s 2017 Annual Meeting of Shareholders, reflecting approval of the Plan (Proposal No. 3) by a majority of the votes cast in person or by proxy at such meeting.

We have also examined originals or copies, certified or otherwise identified to our satisfaction, of such records of the Company and such agreements, certificates and receipts of public officials, certificates of officers or other representatives of the Company and others, and such other documents as we have deemed necessary or appropriate as a basis for the opinion stated below, including the facts and conclusions set forth in the Secretary’s Certificate.

In our examination, we have assumed the genuineness of all signatures, including endorsements, the legal capacity and competency of all natural persons, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as facsimile, electronic, certified or photostatic copies, and the authenticity of the originals of such copies. As to any facts relevant to the opinion expressed herein that we did not independently establish or verify, we have relied upon statements and representations of officers and other representatives of the Company and others and of public officials.

We do not express any opinion with respect to the law of any jurisdiction other than the Business Corporation Law of the State of New York (the “NYBCL”).

Based on the foregoing and subject to the qualifications and assumptions stated herein, we are of the opinion that the Shares have been duly authorized by all requisite corporate action on the part of the Company under the NYBCL and, when the Shares are issued and paid for in full in accordance with the terms and conditions of the Plan and any applicable award agreement, the Shares will be validly issued, fully paid and nonassessable.

In addition, in rendering the foregoing opinion, we have assumed that if issued in physical form, the certificates evidencing the Shares will be manually signed by one of the authorized officers of the transfer agent and registrar for the Common Stock and registered by such transfer agent and registrar and will conform to the specimen certificate examined by us evidencing the Common Stock or, if issued in book-entry form, an appropriate account statement evidencing the Shares credited to the recipient’s account maintained with the Company’s transfer agent has been issued by the Company’s transfer agent. We have also assumed that each award agreement setting forth the terms of each grant of options or other award under the Plan will be consistent with the Plan, duly authorized, and if applicable, validly executed and delivered by the

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parties thereto, and that the Shares will be issued in accordance with the terms of the Plan for consideration in an amount at least equal to the par value of such Shares.

We hereby consent to the filing of this opinion with the Commission as an exhibit to the Registration Statement. In giving this consent, we do not thereby admit that we are within the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the Commission. This opinion is expressed as of the date hereof unless otherwise expressly stated, and we disclaim any undertaking to advise you of any subsequent changes in the facts stated or assumed herein or of any subsequent changes in applicable laws.

Very truly yours,

/s/ Skadden, Arps, Slate, Meagher & Flom LLP

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