EXHIBIT 5.1
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                           WEIL, GOTSHAL & MANGES LLP           Austin
                                767 Fifth Avenue                Boston
                               New York, NY 10153               Brussels
                                 (212) 310-8000                 Budapest
                              Fax: (212) 310-8007               Dallas
                                                                Frankfurt
                                                                Houston
                                                                London
                                                                Miami
                                                                Munich
                                                                Paris
                                  March 2, 2007                 Prague
                                                                Shanghai
                                                                Silicon Valley
                                                                Singapore
                                                                Warsaw
                                                                Washington, D.C.

Magellan Health Services, Inc.
55 Nod Road
Avon, Connecticut 06001


Ladies and Gentlemen:

            We have acted as counsel to Magellan Health Services, Inc., a
Delaware corporation (the "Company"), in connection with the preparation and
filing with the Securities and Exchange Commission of the Company's Registration
Statement on Form S-8 (the "S-8 Registration Statement") under the Securities
Act of 1933, as amended, relating to the registration of the following shares of
the Company's Ordinary Common Stock ("shares"): (i) 99,643 shares issuable upon
the exercise of options (the "Assumed Options") granted pursuant to amendments
to Incentive Stock Option Award Agreements (the "Assumed Option Agreements")
assumed by the Company pursuant to an Agreement and Plan of Merger dated as of
December 12, 2005 among the Company and various other parties (the "Merger
Agreement"), (ii) 100,000 shares issuable upon the exercise of options (the
"2003 Options") granted pursuant to Stock Option Agreements and Notices of Stock
Option Grants (collectively, the "2003 Option Agreements"), which were granted
on the same terms and conditions as provided for the grant of options pursuant
to the Company's 2003 Management Incentive Plan (the "2003 Plan") but were not
granted pursuant to the 2003 Plan, (iii) 137,500 shares issuable upon the
exercise of options (the "2006 Options") granted pursuant to Stock Option
Agreements and Notices of Stock Option Grants (collectively, the "2006 Option
Agreements"), which were granted upon the same terms and conditions as provided
for the grant of options pursuant to the Company's 2006 Management Incentive
Plan (the "2006 Plan", and together with the 2003 Plan, the "Plans") but were
not granted pursuant to the 2006 Plan, and (iv) 4,753 shares issuable upon the
settlement of a Restricted Stock Unit Award (the "RSU Shares") granted pursuant
to a Restricted Stock Unit Agreement and Notice of Restricted Stock Unit Award
(collectively, the "2006 RSU Award Agreement"), which were granted on the same







Magellan Health Services, Inc.
Page 2



terms and conditions as provided for the grant of Restricted Stock Units
pursuant to the 2006 Plan but were not granted pursuant to the 2006 Plan.

            In so acting, we have examined originals or copies (certified or
otherwise identified to our satisfaction) of the S-8 Registration Statement, the
Assumed Option Agreements, the 2003 Option Agreements, the 2006 Option
Agreements, the 2006 RSU Award Agreement, the Merger Agreement, the Plans, the
Company's Amended and Restated Certificate of Incorporation, the Company's
Bylaws and such corporate records, agreements, documents and other instruments,
and such certificates or comparable documents of public officials and of
officers and representatives of the Company, and have made such inquiries of
such officers and representatives, as we have deemed relevant and necessary as a
basis for the opinions hereinafter set forth.

            In such examination, we have assumed the genuineness of all
signatures, the legal capacity of all natural persons, the authenticity of all
documents submitted to us as originals, the conformity to original documents of
all documents submitted to us as certified, conformed or photostatic copies and
the authenticity of the originals of such latter documents. As to all questions
of fact material to these opinions that have not been independently established,
we have relied upon certificates or comparable documents of officers and
representatives of the Company.

            Based on the foregoing, and subject to the qualifications stated
herein, we are of the opinion that each of (i) the shares issuable upon the
exercise of (a) the Assumed Options, (b) the 2003 Options, and (c) the 2006
Options, and (ii) the RSU Shares have been duly authorized and, when issued and
delivered in accordance with the Assumed Option Agreements, the 2003 Option
Agreements, the 2006 Option Agreements or the 2006 RSU Award Agreement, as the
case may be, upon receipt by the Company of consideration constituting lawful
consideration therefor under Delaware law, will be validly issued, fully paid
and non-assessable.

            The opinions expressed herein are limited to the corporate laws of
the State of Delaware and the federal laws of the United States, and we express
no opinion as to the effect on the matters covered by this letter of the laws of
any other jurisdiction.

            We hereby consent to the use of this letter as an exhibit to the
Registration Statement and to any and all references to our firm in the
Registration Statement and the Prospectus which is a part of the Registration
Statement.

                                    Very truly yours,

                                    /s/ Weil, Gotshal & Manges LLP