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                                                                     EXHIBIT 5.1

                                 Latham & Watkins
   BOSTON                        ATTORNEYS AT LAW                  NEW YORK
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  CHICAGO                                                      NORTHERN VIRGINIA

 FRANKFURT                       -----------------               ORANGE COUNTY

  HAMBURG                                                          SAN DIEGO

 HONG KONG                                                       SAN FRANCISCO

   LONDON                                                       SILICON VALLEY

LOS ANGELES                                                       SINGAPORE

   MOSCOW                                                           TOKYO

 NEW JERSEY                                                     WASHINGTON, D.C.

                                  May 15, 2001

NeoTherapeutics, Inc.
157 Technology Drive
Irvine, California 92612

     Re: Registration of 900,000 shares of common stock, par value $.001 per
         share, of NeoTherapeutics, Inc., pursuant to a Registration Statement
         on Form S-3

Ladies and Gentlemen:

     In connection with the registration for resale of 900,000 shares of common
stock, par value $.001 per share, of NeoTherapeutics, Inc., a Delaware
corporation (the "Company"), under the Securities Act of 1933, as amended, on
Form S-3 (the "Registration Statement"), you have requested our opinion with
respect to the matters set forth below. The shares being registered for resale
include 900,000 shares of common stock issued upon exercise of adjustable
warrants issued to Montrose Investments Ltd. and Strong River Investments. Inc.
(the "Investors") pursuant to a Securities Purchase Agreement dated September
29, 2000 (the "Securities Purchase Agreement") and amended by a Letter Agreement
dated April 17, 2001 (the "Letter Agreement") by and between the Company and the
Investors (the "Shares").

     We have made such legal and factual examinations and inquiries, including
an examination of originals or copies certified or otherwise identified to our
satisfaction of such documents, corporate records and instruments, as we have
deemed necessary or appropriate for purposes of this opinion. In our
examination, we have assumed the genuineness of all signatures, the authenticity
of all documents submitted to us as originals, and the conformity to authentic
original documents of all documents submitted to us as copies. As to facts
material to the opinions, statements and assumptions expressed herein, we have,
with your consent, relied upon oral or written statements and representations of
officers and other representatives of the Company and others. In addition, we
have obtained and relied upon such certificates and assurances from public
officials as we have deemed necessary.

     We are opining herein as to the effect on the subject transaction only of
the General Corporation Law of the State of Delaware and we express no opinion
with respect to the applicability thereto, or the effect thereon, of the laws of
any other jurisdiction or, in the case of Delaware, any other laws, or as to any
matters of municipal law or the laws of any other local agencies within the
state.

     Subject to the foregoing, it is our opinion that the Shares have been duly
authorized and validly issued and are fully paid and nonassessable.

     This opinion is rendered only to the Company and is solely for the benefit
of the Company in connection with the transaction covered hereby. This opinion
may not be relied upon by you for any other purpose, or furnished to, quoted to
or relied upon, by any other person, firm or corporation for any purpose,
without our prior written consent.

     We consent to your filing this opinion as an exhibit to the Registration
Statement and to the reference to our firm contained under the heading "Validity
of Common Stock."

                                                Very truly yours,

                                                /s/ Latham & Watkins

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