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                                                                       EXHIBIT 5


                                  March 8, 2001


Board of Directors
Merck & Co., Inc.
One Merck Drive
Whitehouse Station, NJ  08889-0100

Re: Merck & Co., Inc. - Common Stock

Ladies and Gentlemen:

I am the Vice President, Secretary & Assistant General Counsel of Merck & Co.,
Inc., a New Jersey corporation (the "Corporation"), and in such capacity have
acted as counsel for the Corporation in connection with the proposed
registration under the Securities Act of 1933, as amended, of up to 450,000
shares of the Common Stock, $0.01 par value (the "Shares"), of the Corporation,
which may be issued by the Corporation pursuant to the 1996 Non-Employee
Directors Stock Option Plan (the "Plan"), or any successor plans thereto, as
such plans or successor plans may be amended from time to time. I have examined
such corporate records and other documents, including the Registration Statement
on Form S-8 relating to the Shares, and have reviewed such matters of law as I
have deemed necessary for this opinion.

I am admitted to the Bar of the State of New York. The opinions expressed herein
are limited in all respects to the federal laws of the United States of America,
the laws of the State of New York, and the Business Corporation Act of the State
of New Jersey.

On the basis of the foregoing examination and review, I advise you that, in my
opinion:

1.   The Corporation is a corporation duly organized and existing under the laws
     of the State of New Jersey.

2.   All necessary corporate action on the part of the Corporation has been
     taken to authorize the issuance of the Shares, and, when issued as
     described in the Registration Statement, the Shares will be legally and
     validly issued, fully paid and non-assessable.

The opinions expressed herein are rendered only to you and are solely for your
benefit and may not be relied upon by any person, firm or corporation for any
reason without my prior written consent.

I consent to the filing of this opinion as an exhibit to the Registration
Statement. In giving this consent I do not admit that I am in the category of
persons whose consent is required under


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Section 7 of the Securities Act of 1933 or the rules and regulations of the
Securities and Exchange Commission thereunder.



                                                  By:  /s/ Celia A. Colbert
                                                       ------------------------
                                                       Celia A. Colbert


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