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                                                                     EXHIBIT 5.1


                        [LETTERHEAD OF LATHAM & WATKINS]

                                  May 15, 2001
                                                                     029455-0018

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.

        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