Exhibit 8.1

May 11, 2021

NeuroRx, Inc.

1201 N. Market Street, Suite 111

Wilmington, DE 19801

Ladies and Gentlemen:

We have acted as United States federal income tax counsel to NeuroRx, Inc., a Delaware corporation (the “Company”), in connection with the Agreement and Plan of Merger, dated as of December 13, 2020 (the “Merger Agreement”), by and among (i) the Company, (ii) Big Rock Partner Acquisition Corp., a Delaware corporation (“BRPA”), and (iii) Big Rock Merger Corp., a Delaware corporation and wholly owned subsidiary of BRPA (“Merger Sub”). Capitalized terms used herein but not defined shall have the meanings set forth in the Merger Agreement.

This opinion is being delivered in connection with the Registration Statement (as amended through the date hereof, the “Registration Statement”) initially filed by BRPA on January 27, 2021 relating to the transactions contemplated by the Merger Agreement.

In connection with this opinion, we have examined and relied on originals or copies, certified or otherwise identified to our satisfaction, of (i) the Merger Agreement, (ii) the Registration Statement, (iii) the representation letters dated May 11, 2021 and delivered to us by BRPA, Merger Sub and the Company for purposes of this opinion, and (iv) such other documents, certificates and records as we have deemed

necessary or appropriate as a basis for the opinion set forth herein. In addition, we have made such investigations of fact and law as we have deemed necessary as a basis for the opinion expressed below. We have assumed that the Merger will be consummated in accordance with the Merger Agreement.

For purposes of our opinion, we have assumed the legal capacity of all natural persons, the genuineness of all signatures, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as certified, conformed, photostatic or electronic copies, and the authenticity of the originals of such latter documents. We have assumed that the Merger Agreement and such other documents, certificates, and records are, and will continue to be, duly authorized, valid, and enforceable.

In rendering our opinion, we have relied upon statements and representations of officers and other representatives of BRPA, Merger Sub and the Company and we have assumed that such statements and representations are and will continue to be true, correct, and complete without regard to any qualification as to knowledge or belief.

Our opinion is based on the United States Internal Revenue Code of 1986, as amended (the “Code”), Treasury regulations promulgated thereunder, judicial decisions, published positions of the Internal Revenue Service, and such other authorities as we have considered relevant, all as in effect on the date of this opinion and all of which are subject to change or differing interpretations (possibly with retroactive effect). A change in the authorities upon which our opinion is based could affect the conclusions expressed herein. The opinion set forth herein has no binding effect on the United States Internal Revenue Service or the courts of the United States. No assurance can be given that, if the matter were contested, a court would agree with the opinion set forth herein.

Based upon and subject to the foregoing, and subject to the assumptions, limitations and qualifications set forth herein and in the Registration Statement, the discussion set forth under the caption “Material U.S. Federal Income Tax Consequences” in the Registration Statement, insofar as it expresses conclusions as to the application of United States federal income tax law, is our opinion as to the material United States federal income tax consequences of the Merger to U.S. Holders (as defined in the Registration Statement).

Except as set forth above, we express no opinion as to the tax consequences to any party, whether federal, state, local or foreign, of any transactions related to the Merger or contemplated by the Merger Agreement and this opinion may not be relied upon except with respect to the consequences specifically discussed herein. Furthermore, our opinion is based on current U.S. federal income tax law and administrative practice, and we do not undertake to advise you as to any changes after the date hereof of the Merger in U.S. federal income tax law that may affect our opinion. In addition, there can be no assurance that changes in the law will not take place which

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could affect the U.S. federal income tax consequences of the Merger or that contrary positions may not be taken by the Internal Revenue Service. To the extent any of the representations, warranties, statements and assumptions material to our opinion and upon which we have relied are not accurate and complete in all material respects at all the relevant times, our opinion would be adversely affected and should not be relied upon.

This opinion has been prepared in connection with the filing of the Registration Statement and may not be relied upon for any other purpose without our prior written consent.

This opinion is expressed as of the date hereof, and we are under no obligation to supplement or revise our opinion to reflect any legal developments, any factual matters arising subsequent to the date hereof, or the impact of any information, document, certificate, record, statement, representation, covenant, or assumption relied upon herein that becomes incorrect or untrue.

We hereby consent to use of this opinion as an exhibit to the Registration Statement, to the use of our name under the heading “Legal Matters” contained in the prospectus included in the Registration Statement and to the discussion of this opinion in the prospectus included in the Registration Statement. In giving this consent, we do not thereby admit that we come within the category of persons whose consent is required under Section 7 of the Securities Act of 1933, as amended, or the rules or regulations of the Securities and Exchange Commission promulgated thereunder.

Very truly yours,

/s/ Paul, Weiss, Rifkind, Wharton & Garrison LLP

PAUL, WEISS, RIFKIND, WHARTON & GARRISON LLP

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