EXHIBIT 5.1

                        OPINION OF BURNS & LEVINSON LLP

                              September 22, 2000

Interneuron Pharmaceuticals, Inc.
One Ledgemont Center
99 Hayden Avenue
Lexington, MA 02421

Gentlemen:

You have requested our opinion with respect to the issuance by the Company of an
aggregate of 250,000 shares (the "Shares") of Common Stock, par value $.001 per
share, of Interneuron Pharmaceuticals, Inc., a Delaware corporation (the
"Company"), issuable upon exercise of options pursuant to a Registration
Statement on Form S-8 (the "Registration Statement") under the Securities Act of
1933, as amended (the "Act"). We have examined originals, or copies certified or
otherwise identified to our satisfaction, of such documents and corporate and
public records as we deem necessary as a basis for the opinion hereinafter
expressed. With respect to such, we have assumed the genuineness of all
signatures appearing on all documents presented to us as originals, and the
conformity to the originals of all documents presented to us as conformed or
reproduced copies. Where factual matters relevant to such opinion were not
independently established, we have relied upon certificates of officers and
responsible employees and agents of the Company. Based upon the foregoing, it is
our opinion that the Shares issuable upon exercise of options will be, when
sold, paid for and issued as contemplated by the terms of the options duly and
validly issued and fully paid and nonassessable.

                                        Very truly yours,

                                        /s/ BURNS & LEVINSON LLP
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                                        BURNS & LEVINSON LLP


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