Document:

<PAGE>

                                                                     Exhibit 4.1

                       [Form of common stock certificate
                        of Medco Health Solutions, Inc.]

Number                                                                    SHARES

                           Medco Health Solutions, Inc.
              INCORPORATED UNDER THE LAWS OF THE STATE OF DELAWARE

MHS

    THIS CERTIFICATE IS TRANSFERABLE IN DULUTH, MINNEAPOLIS OR NEW YORK CITY
                                  COMMON STOCK

                                                               SEE REVERSE FOR
                                                             CERTAIN DEFINITIONS

This Certifies that                           CUSIP 58405U102

is the owner of

     FULLY PAID AND NON-ASSESSABLE SHARES OF THE COMMON STOCK OF THE PAR VALUE
OF ONE CENT ($0.01) EACH OF
========================Medco Health Solutions, Inc.============================
Of Medco Health Solutions, Inc. transferable on the books of the Corporation by
the holder hereof in person or by duly authorized attorney upon surrender of
this certificate properly endorsed. This certificate and the shares represented
hereby are issued and shall be held subject to all the provisions of the
Certificate of Incorporation of Medco Health Solutions, Inc. and to any and all
amendments thereto, to all of which the holder by acceptance hereof assents.
This certificate is not valid until countersigned by the Transfer Agent and
registered by the Registrar.

     WITNESS the facsimile seal of the Corporation and the facsimile signature
of its proper officers.

Dated:

<TABLE>

<S>                                                 <C>
                                                    /s/ Richard T. Clark
                                                    CHIEF EXECUTIVE OFFICER

                  [MEDCO HEALTH SOLUTIONS, INC. CORPORATE SEAL]

COUNTERSIGNED AND REGISTERED:
WELLS FARGO BANK MINNESOTA, N.A.
                      TRANSFER AGENT
                       AND REGISTRAR,
BY
   /s/ Jennie M. Kaufman                            /s/ David S. Machlowitz
   AUTHORIZED OFFICER                               SENIOR VICE PRESIDENT, GENERAL COUNSEL
                                                    AND SECRETARY
</TABLE>

<PAGE>

  [Form of reverse of Common Stock Certificate of Medco Health Solutions, Inc.]

                           Medco Health Solutions, Inc.

        The corporation will furnish without charge to each stockholder who so
requests the powers, designations, preferences and relative, participating,
optional, or other special rights of each class of stock or series thereof and
the qualifications, limitations or restrictions of such preferences and/or
rights. Such requests may be made to the corporation or the transfer agent.

   The following abbreviations, when used in the inscription on the face of this
certificate, shall be construed as though they were written out in full
according to applicable laws or regulations:

<TABLE>
<S>                                          <C>
TEN COM - as tenants in common               UNIF GIFT MIN ACT - ___________ Custodian_________
                                                                  (Cust)              (Minor)
TEN ENT - as tenants by the entireties                            under Uniform Gifts to Minors

JT TEN  - as joint tenants with right                            Act___________________________
          of survivorship and not as                                         (State)
          tenants in common
</TABLE>

     Additional abbreviations may also be used though not in the above list

   For value received, ___________________ hereby sell, assign and transfer unto

 PLEASE INSERT SOCIAL SECURITY OR OTHER
    IDENTIFYING NUMBER OF ASSIGNEE

[___________________________________]___________________________________________

________________________________________________________________________________
  (PLEASE PRINT OR TYPEWRITE NAME AND ADDRESS, INCLUDING ZIP CODE, OF ASSIGNEE)

________________________________________________________________________________

________________________________________________________________________________

_________________________________________________________________________ Shares
of the capital stock represented by the within Certificate, and do hereby
irrevocably constitute and appoint

_______________________________________________________________________ Attorney
to transfer the said stock on the books of the within named Corporation with
full power of substitution in the premises.

Dated____________________________

                       _________________________________________________________
                       NOTICE: THE SIGNATURE TO THIS ASSIGNMENT MUST CORRESPOND
                               WITH THE NAME AS WRITTEN UPON THE FACE OF THE
                               CERTIFICATE IN EVERY PARTICULAR WITHOUT
                               ALTERATION OR ENLARGEMENT OR ANY CHANGE WHATEVER.

Signatures(s) Guaranteed:

_______________________________________________________
THE SIGNATURE(S) SHOULD BE GUARANTEED BY AN ELIGIBLE
GUARANTOR INSTITUTION (BANKS, STOCKBROKERS, SAVINGS AND
LOAN ASSOCIATIONS AND CREDIT UNIONS WITH MEMBERSHIP IN
AN APPROVED SIGNATURE GUARANTEE MEDALLION PROGRAM),
PURSUANT TO S.E.C. RULE 17AD-15.<PAGE>
                                                                   Exhibit 10.20

         J.P. MORGAN SECURITIES INC.       GOLDMAN SACHS CREDIT PARTNERS L.P.
               270 PARK AVENUE                      85 BROAD STREET
             NEW YORK, NY 10017                    NEW YORK, NY 10004

             JPMORGAN CHASE BANK                     CITIBANK, N.A.
               270 PARK AVENUE                    388 GREENWICH STREET
             NEW YORK, NY 10017                    NEW YORK, NY 10013

                                               SALOMON SMITH BARNEY INC.
                                                  388 GREENWICH STREET
                                                   NEW YORK, NY 10013

                                                         May 17, 2002

Merck-Medco Managed Care, L.L.C.
100 Parsons Pond Drive
Franklin Lakes, NJ 07417

Attention of:     Ms. JoAnn Reed
                  Chief Financial Officer

                        Merck-Medco Managed Care, L.L.C.
                        --------------------------------
                                Commitment Letter
                                -----------------

Ladies and Gentlemen:

                  You have advised J.P. Morgan Securities Inc. ("JPMorgan"),
                                                                 --------
Goldman Sachs Credit Partners L.P. ("Goldman") and Salomon Smith Barney Inc.
                                     -------
("Salomon") that, in connection with (a) the initial public offering of the
  -------
common stock (the "IPO") of Merck-Medco Managed Care, L.L.C., currently a
                   ---
Delaware limited liability company that intends to, on or prior to the effective
date of the registration statement relating to the Notes Offering (as defined
below), convert to a Delaware corporation and change its name to MedcoHealth
Solutions, Inc. (the "Borrower"), by its parent company, Merck & Co., Inc.
                      --------
("Merck"), (b) the public offering of $1,000,000,000 aggregate principal amount
  -----
of senior notes (the "Notes Offering") by the Borrower and (c) the payment by
                      --------------
the Borrower to Merck of a dividend in the approximate amount of $1,500,000,000
(the "Merck Dividend"), the Borrower wishes to put in place three new credit
      --------------
facilities in an aggregate principal amount of up to $1,250,000,000. In that
connection, you have requested that JPMorgan, Goldman and Salomon agree to
structure, arrange and syndicate loans proposed to be made pursuant to a five
year $500,000,000 senior unsecured revolving credit facility, a 364 day
$250,000,000 senior unsecured revolving credit facility, and a five year
$500,000,000 senior unsecured term loan facility (each, a "Facility" and
                                                           --------
together, the "Facilities"). This Commitment Letter is being delivered to the
               ----------
Borrower in connection with the IPO, the Notes Offering and the Merck Dividend.

<PAGE>

                                                                               2

                  JPMorgan Chase Bank ("JPMorgan Chase") is pleased to advise
                                        --------------
you of its several, and not joint, commitment to provide, at the Borrower's
election, up to $140 million of the principal amount of the Facilities; Goldman
is pleased to advise you of its several, and not joint, commitment to provide,
at the Borrower's election, up to $140 million of the principal amount of the
Facilities; and Citibank, N.A., ("Citibank") is pleased to advise you of its
                                  --------
several, and not joint, commitment to provide, at the Borrower's election, up to
$140 million of the principal amount of the Facilities, the commitment of each
being allocated to each Facility on a pro rata basis, and each will, in such
capacity, perform the duties and exercise the authority customarily performed
and exercised by entities performing such role, subject to the terms and subject
to the conditions set forth or referred to in this commitment letter (the
"Commitment Letter") and in the Summary of Principal Terms and Conditions
 -----------------
attached hereto as Exhibit A (the "Term Sheet"); provided that all such
                                   ----------    --------
commitments are subject to the successful syndication of the balance of the
Facilities by JPMorgan, Goldman and Salomon, and JPMorgan, Goldman and Salomon
undertake to use best efforts to syndicate the balance of the Facilities. It is
agreed that JPMorgan, Goldman and Salomon will act as the Lead Arrangers,
JPMorgan, Goldman and Salomon will act as the Bookrunners, JPMorgan Chase will
act as the Administrative Agent, and Goldman and Citibank will act as the
Syndication Agents for the Facilities, upon the terms and subject to the
conditions set forth or referred to in this Commitment Letter and in the Term
Sheet. It is agreed that (a) no additional lead arrangers, bookrunners,
administrative agent or syndication agents or other agents will be appointed and
no other titles will be awarded in connection with the Facilities other than
such as may be agreed upon by you, JPMorgan, Goldman and Salomon and (b) no
Lender (as defined below) will receive compensation outside the terms contained
herein and in the Fee Letter and the Administrative Fee Letter referred to below
in connection with the Facilities unless you, JPMorgan, Goldman and Salomon
shall so agree.

                  JPMorgan, Goldman and Salomon reserve the right, prior to or
after the execution of the Loan Agreement (as defined in the Term Sheet), to
syndicate portions of the several, and not joint, commitments of JPMorgan Chase,
Goldman and Citibank to lend hereunder, pursuant to a syndication to be managed
by JPMorgan, Goldman and Salomon, to one or more financial institutions
(together with JPMorgan Chase, Goldman and Citibank, the "Lenders") that will
                                                          -------
become parties to the Loan Agreement. You understand that JPMorgan, Goldman and
Salomon intend to commence syndication efforts promptly, and you agree actively
to assist, as reasonably requested by, JPMorgan, Goldman and Salomon in
completing a timely and orderly syndication satisfactory to JPMorgan, Goldman,
Salomon and you. Such assistance shall include (a) your using commercially
reasonable efforts to facilitate direct contact during the syndication between
your senior management, representatives and advisors and the proposed Lenders,
(b) assistance in the preparation of a Confidential Information Memorandum and
other marketing materials to be used in connection with the syndication of the
Facilities, all of which shall be subject to your approval (such approval not to
be unreasonably withheld) and (c) the hosting, with JPMorgan, Goldman and
Salomon, of one or more meetings of prospective Lenders.

                  It is agreed that JPMorgan, Goldman and Salomon will, in
consultation with the Borrower, manage all aspects of the syndication, including
selection of Lenders (which Lenders shall include the Lenders listed in the Term
Sheet and shall be reasonably acceptable to you), the determination of when
potential Lenders will be approached and the time of acceptance of the Lenders'
commitments, any naming rights and titles, which institutions will participate
and the final allocations of the commitments and fees among the Lenders. To
assist in the syndication effort, you agree promptly to prepare and provide to
JPMorgan, Goldman and Salomon all information with respect to the Borrower and
its subsidiaries, the IPO, the Notes Offering, the Merck Dividend and the other
transactions contemplated hereby, including such financial information and
projections (the "Projections") as JPMorgan, Goldman or Salomon may reasonably
                  -----------
request. You represent and covenant (and it shall be a condition to the
commitment of JPMorgan, Goldman and Salomon and their affiliates and the
agreements of JPMorgan, Goldman and Salomon and their affiliates hereunder) that
(a) all information other than the

<PAGE>

                                                                               3

Projections (the "Information") that has been or will be made available to
                  -----------
JPMorgan, Goldman or Salomon by or on behalf of you or any of your authorized
representatives is or will be, when furnished (or if subsequently revised, when
such revisions are furnished), complete and correct in all material respects and
does not or will not, when furnished, contain any untrue statement of a material
fact or omit to state a material fact necessary in order to make the statements
contained therein not materially misleading in light of the circumstances under
which such statements are made and (b) the Projections that have been or will be
made available to JPMorgan, Goldman or Salomon by or on behalf of you or any of
your authorized representatives have been or will be prepared in good faith
based upon assumptions believed by you to be reasonable when furnished. You
agree that if, at any time to and including the date on which the Loan Agreement
is executed, any of the representations in the preceding sentence would be
incorrect if the Information and Projections were being furnished, and such
representations were being made, at such time, then you will promptly supplement
the Information and the Projections so that such representations will be correct
under the circumstances. In arranging the Facilities, including the syndication
of the Facilities, JPMorgan, Goldman and Salomon will be relying primarily on
the Information and the Projections without responsibility for independent
verification thereof.

                  As consideration for JPMorgan's, Goldman's, Citibank's and
their affiliates' commitments and agreements hereunder and JPMorgan's, Goldman's
and Salomon's agreements to structure, arrange and syndicate the Facilities and
to provide advisory services in connection therewith, you agree to pay to
JPMorgan, JPMorgan Chase, Goldman, Citibank and Salomon the nonrefundable fees
set forth in, and if and to the extent required under the terms of, the Fee
Letter dated the date hereof among JPMorgan, JPMorgan Chase, Goldman, Citibank
and Salomon and you (the "Fee Letter") and in the Administrative Fee Letter
                          ----------
dated the date hereof among JPMorgan Chase, JPMorgan and you (the
"Administrative Fee Letter"), both of which are delivered herewith.
 -------------------------

                  The commitments of JPMorgan Chase, Goldman, Salomon and their
affiliates and the agreements of JPMorgan, Goldman and Salomon and their
affiliates hereunder are subject to (a) JPMorgan's, Goldman's and Salomon's
completion and reasonable satisfaction with the results of their respective
ongoing and confirmatory due diligence investigations of the business, assets,
liabilities, operations, condition (financial and otherwise), prospects and
material agreements of the Borrower and its subsidiaries, the IPO, the Notes
Offering and the Merck Dividend, (b) there not having been any material adverse
change in the structure and terms of and the documentation providing for the
IPO, the Notes Offering and the Merck Dividend since the date hereof, (c) there
not having occurred or come to the attention of JPMorgan, Goldman or Salomon any
event, condition or circumstance that has had or would reasonably be expected to
have a material adverse effect on, or that JPMorgan, Goldman and Salomon
reasonably believe in good faith to be materially inconsistent with its
understanding, based on information heretofore provided to it, of, the business,
operations, properties, assets, liabilities, condition (financial or otherwise)
or prospects of the Borrower and its subsidiaries, taken as a whole, (d) there
not having occurred a disruption of or an adverse change in financial or banking
market conditions that in the reasonable judgment of JPMorgan, Goldman and
Salomon would materially impair the syndication of the Facilities, (e) the
negotiation, execution and delivery of the Loan Agreement and Transaction
Documentation containing terms and conditions consistent with the Term Sheet and
otherwise reasonably satisfactory to JPMorgan, Goldman and Salomon and their
respective counsel, (f) JPMorgan's, Goldman's and Salomon's satisfaction that
prior to and during the syndication of the Facilities there shall be no
competing offering, placement or arrangement of any debt securities or bank
financing by or on behalf of the Borrower or any of its subsidiaries (other than
the Notes Offering and the Bridge Facility (as defined in the Term Sheet)
contemplated to be entered into by the parties hereto) that would be likely in
JPMorgan's, Goldman's or Salomon's judgment to have a material adverse effect on
the syndication of the Facilities and (g) the other conditions set forth in the
Term Sheet, the Fee Letter and the Administrative Fee Letter. This Commitment
Letter, the Term Sheet, the Fee Letter and the

<PAGE>

                                                                               4

Administrative Fee Letter do not purport to set forth all the terms and
conditions of the Facilities. Those matters that are not covered by or made
clear under the provisions of this Commitment Letter, the Term Sheet, the Fee
Letter and the Administrative Fee Letter are subject to the approval and
agreement of JPMorgan, Goldman, Salomon and the Borrower.

                  By executing this Commitment Letter, you agree (a) to
indemnify and hold harmless JPMorgan, Goldman, Salomon, their affiliates and the
respective officers, directors, employees, advisors and agents of such persons
(each, an "indemnified person" and collectively, "indemnified persons") from and
           ------------------                     -------------------
against any and all losses, claims, damages, liabilities and expenses, joint or
several, to which any indemnified person may become subject arising out of or in
connection with any actual or threatened claim, litigation, investigation or
proceeding relating to this Commitment Letter, the Facilities or any actual or
proposed use of the proceeds thereof, or the Fee Letter, the Administrative Fee
Letter, the Term Sheet, the IPO, the Notes Offering or the Merck Dividend,
regardless of whether any of such indemnified persons is a party thereto, and to
reimburse each of such indemnified persons upon demand for any reasonable legal
or other reasonable expenses incurred in connection with investigating or
defending any of the foregoing; provided that the foregoing indemnity will not,
                                --------
as to any indemnified person, apply to losses, claims, damages, liabilities or
related expenses to the extent they are determined by a final judgment of a
court of competent jurisdiction to have resulted from the willful misconduct,
bad faith or gross negligence of such indemnified person, and (b) to reimburse
JPMorgan, Goldman and Salomon and their affiliates from time to time for all
reasonable out-of-pocket expenses (including, without limitation, due diligence
expenses, syndication expenses, travel expenses and fees, charges and
disbursements of counsel), incurred by them in connection with the arrangement,
syndication and documentation (including, without limitation, this Commitment
Letter, the Term Sheet, the Fee Letter, the Administrative Fee Letter and the
Loan Agreement) of the Facilities or the administration, amendment, modification
or waiver thereof. In the event that any third party claim is made against an
indemnified person for which the foregoing indemnification may be sought, you
shall have the right to assume the defense of and the right to settle such
claim; provided that you may not settle any such claim without the consent of
       --------
JPMorgan, Goldman and Salomon (such consent not to be unreasonably withheld). If
you assume such defense, JPMorgan, Goldman and Salomon may, at your cost,
participate in (but not control) such defense. If for any reason the foregoing
indemnification is unavailable to any indemnified person or insufficient to hold
it harmless, then you shall contribute to the amount paid or payable by such
indemnified person as a result of such loss, claim, damage or liability in such
proportion as is appropriate to reflect the relative economic interests of you
and your stockholders on the one hand and such indemnified persons on the other
hand in the matters contemplated by this Commitment Letter, the Fee Letter, the
Administrative Fee Letter or the Term Sheet as well as the relative fault of you
and such indemnified person with respect to such loss, claim, damage or
liability and any other relevant equitable considerations. You also agree that
neither any indemnified person nor any of its affiliates nor the officers,
directors, managing directors, employees, advisors and agents of such persons
shall have any liability to you or any person asserting claims on behalf of or
in right of you or any other person in connection with or as a result of either
this arrangement or any matter referred to in this Commitment Letter, the Fee
Letter, the Administrative Fee Letter, the Term Sheet or the Facilities except
to the extent that any losses, claims, damages, liabilities or expenses incurred
by you have resulted from the willful misconduct, gross negligence or bad faith
of such indemnified person in performing the services that are the subject of
this Commitment Letter, the Fee Letter, the Administrative Fee Letter, the Term
Sheet or the Facilities; provided that no indemnified person shall be liable for
                         --------
any damages arising from the use by others of information or other materials
obtained through electronic telecommunications or other information transmission
systems (except for damages that result from an indemnified person's willful
misconduct, bad faith or gross negligence, but only to the extent thereof) or
for any special, indirect, consequential or punitive damages in connection with
its activities related to the Facilities.

<PAGE>

                                                                               5

                  As you know, each of JPMorgan, Goldman and Salomon may from
time to time effect transactions, for its own account or the account of
customers, and hold positions in loans or options on loans of the Borrower and
other companies that may be the subject of this arrangement. In addition, each
of JPMorgan, Goldman and Salomon or certain affiliates thereof, is a
full-service securities firm and as such may from time to time effect
transactions, for its own account or the account of customers, and hold
positions in securities or options on securities of the Borrower and other
companies that may be the subject of this arrangement.

                  You acknowledge that JPMorgan or Salomon and their affiliates
may be providing financing or other services to other companies that have or may
in the future have conflicting interests regarding the transactions described
herein and otherwise. JPMorgan and Salomon agree that they will keep
confidential and not disclose or use information obtained from you or your
representatives in the course of the transactions contemplated hereby in
connection with the performance by JPMorgan or Salomon of services for such
other companies or to any other person or entity without your approval except
that JPMorgan, Salomon and their affiliates each may disclose any of such
information (a) to potential Lenders who agree to be bound by the terms of this
paragraph as if they were JPMorgan or Salomon and (b) (i) to its directors,
officers, employees, agents and counsels who are working with or consulted in
connection with this matter and are advised of the disclosure and use
obligations hereunder, (ii) as may be compelled in a judicial or administrative
proceeding or otherwise required by law (in which case JPMorgan, Salomon or
their affiliate, as the case may be, agrees to inform you promptly thereof in
advance), (iii) to the extent requested by any governmental or regulatory
authority, and (iv) to the extent the foregoing becomes publicly available other
than as a result of a breach of this paragraph, was in the possession of
JPMorgan, Salomon or their affiliates prior to the date hereof (other than as a
result of their participation in the IPO, the Notes Offering or related
transactions) or was or is received by JPMorgan, Salomon or their affiliates
from a third party which, to the knowledge of JPMorgan, Salomon or their
affiliate, was not under an obligation of confidentiality with respect thereto,
in which case such information may be disclosed by JPMorgan, Salomon and their
affiliates. You also acknowledge that none of JPMorgan nor Salomon nor any of
their affiliates has any obligation to use in connection with the transactions
contemplated hereby, or to furnish to you, confidential information obtained by
them from other companies. The limitation on use of information and
confidentiality and disclosure obligations described in this paragraph shall
terminate on the date the Loan Agreement has been executed and delivered and be
superceded by the use of information and confidential and disclosure provisions
contained in the Loan Agreement. Notwithstanding the foregoing, nothing in this
letter agreement shall be deemed to (i) restrict or affect the rights or ability
of JPMorgan or Salomon to comply with all applicable disclosure laws,
regulations and principles in connection with the IPO, the Notes Offering or
related transactions, (ii) restrict the ability of JPMorgan or Salomon to share
information with other underwriters participating in the IPO, the Notes Offering
or related transactions, (iii) prevent JPMorgan or Salomon from retaining
documents or other information in connection with due diligence, including any
documents or other information disclosed to JPMorgan or Salomon hereunder, (iv)
restrict or affect the rights or ability of JPMorgan or Salomon to use any such
documents or other information in investigating or defending itself against
allegations or claims made or threatened by purchasers, regulatory authorities
or others in connection with the IPO, the Notes Offering or related
transactions, or (v) prevent JPMorgan or Salomon from disclosing any
confidential information required to be disclosed by an applicable statute,
rule, regulation or other legal requirement in connection with or arising out of
the IPO, the Notes Offering or related transactions.

                  In order to enable each of JPMorgan, Goldman and Salomon to
bring relevant expertise to bear on its engagement under this Commitment Letter
from among its global affiliates, the Borrower agrees that JPMorgan, Goldman and
Salomon each may perform the services contemplated hereby in conjunction with
its affiliates and may exchange with such affiliates information concerning the

<PAGE>

                                                                               6

Borrower, and that any such affiliate performing services hereunder shall be
subject to the provisions of the immediately preceding paragraph and entitled to
the benefits afforded to JPMorgan, Goldman and Salomon under this Commitment
Letter.

                  This Commitment Letter and the commitment and agreements of
JPMorgan, JPMorgan Chase, Goldman, Citibank and Salomon hereunder shall not be
assignable by you without the prior written consent of JPMorgan, Goldman and
Salomon, and any attempted assignment without such consent shall be void. This
Commitment Letter may not be amended nor may any provision hereof be waived or
modified except by an instrument in writing signed by JPMorgan, Goldman, Salomon
and you. This Commitment Letter may be executed in any number of counterparts,
each of which shall be an original and all of which, when taken together, shall
constitute one agreement. Delivery of an executed counterpart of a signature
page of this Commitment Letter by facsimile transmission shall be effective as
delivery of a manually executed counterpart of this Commitment Letter. This
Commitment Letter is intended to be solely for the benefit of the parties hereto
(and indemnified persons) and is not intended to confer any benefits upon, or
create any rights in favor of, any person other than the parties hereto. This
Commitment Letter, the Fee Letter and Administrative Fee Letter are the only
agreements that have been entered into among us with respect to the Facilities
and set forth the entire understanding of the parties with respect thereto. This
Commitment Letter shall be governed by, and construed in accordance with, the
laws of the State of New York. Any right to trial by jury with respect to any
action or proceeding arising in connection with or as a result of either this
arrangement or any matter referred to in this Commitment Letter, the Term Sheet,
the Fee Letter or the Administrative Fee Letter is hereby waived by the parties
hereto.

                  You agree that you will not disclose this Commitment Letter,
the Term Sheet, the Fee Letter, the Administrative Fee Letter or any of their
terms or substance, or the activities or advice of JPMorgan, Goldman, Salomon or
their affiliates pursuant hereto or thereto, directly or indirectly, to any
person without the prior written approval of JPMorgan, Goldman and Salomon,
except that (a) you may disclose any of the foregoing (i) on a need-to-know
basis to your directors, officers, employees and agents who are directly
involved in the consideration of this matter and who have been advised of the
disclosure limitations set forth above, (ii) as may be compelled in a judicial
or administrative proceeding or otherwise required by law (in which case you
agree to inform us promptly thereof in advance), (iii) to the extent requested
by any governmental or regulatory authority, and (iv) to the extent the
foregoing become publicly available other than as a result of a breach of this
paragraph, in which case the information so made publicly available may be
disclosed by you, and (b) following your acceptance of this Commitment Letter,
the Fee Letter and the Administrative Fee Letter, you may disclose this
Commitment Letter and the Term Sheet and the contents hereof and thereof (but
not the Fee Letter, the Administrative Fee Letter or the contents thereof) in
one or more filings with the Securities and Exchange Commission and other
regulatory bodies having jurisdiction over you.

                  In addition, please note that JPMorgan, Goldman and Salomon
and their affiliates do not provide accounting, tax or legal advice. The
Borrower is authorized, subject to applicable law, to disclose any and all
aspects of this potential transaction that are necessary to support any United
States federal income tax benefits expected to be claimed with respect to such
transaction, without JPMorgan, Goldman, Salomon or their respective affiliates
imposing any limitation.

                  If the foregoing correctly sets forth our agreement, please
indicate your acceptance of the terms hereof and of the Fee Letter and the
Administrative Fee Letter by signing in the appropriate space below and in the
Fee Letter and the Administrative Fee Letter and returning to JPMorgan, Goldman
and Salomon the enclosed duplicate originals of this Commitment Letter and the
Fee Letter and to JPMorgan the enclosed duplicate of the Administrative Fee
Letter not later than 5:00 p.m., New York City time, on

<PAGE>

                                                                               7

May 17, 2002, failing which the commitment and agreements of JPMorgan, JPMorgan
Chase, Goldman, Citibank and Salomon contained herein will expire at such time.
In the event the Loan Agreement has not been executed and delivered on or prior
to August 15, 2002, then this Commitment Letter and the commitment and
agreements of JPMorgan, JPMorgan Chase, Goldman, Citibank and Salomon contained
herein shall, subject to the next sentence, automatically terminate unless
JPMorgan, Goldman and Salomon shall agree to an extension. The compensation,
reimbursement, indemnification, confidentiality and limitations on use
provisions contained herein and in the Fee Letter and the Administrative Fee
Letter shall remain in full force and effect regardless of whether the Loan
Agreement shall be executed and delivered and notwithstanding the termination of
this Commitment Letter or the commitment and agreements of JPMorgan, JPMorgan
Chase, Goldman, Citibank and Salomon hereunder. Except as otherwise provided in
the Loan Agreement, the Facilities and commitments under the Loan Agreement
shall terminate if the Initial Funding Date (as defined in the Term Sheet) has
not occurred on or prior to August 15, 2002.

<PAGE>

                  JPMorgan, JPMorgan Chase, Goldman, Citibank and Salomon are
pleased to have been given the opportunity to assist you in connection with this
important financing.

                                            Very truly yours,

                                            J.P. MORGAN SECURITIES INC.,

                                            by

                                                 /s/  Irene C. Markus
                                               ---------------------------------
                                               Name:  Irene C. Markus
                                               Title:  Vice President

                                            JPMORGAN CHASE BANK,

                                            by

                                                 /s/  Thomas H. Kozlark
                                               ---------------------------------
                                               Name:  Thomas H. Kozlark
                                               Title:  Vice President

                                            GOLDMAN SACHS CREDIT PARTNERS L.P.,

                                            by

                                                 /s/  Ed Kearns
                                               ---------------------------------
                                               Name:  Ed Kearns
                                               Title:  Authorized Signatory

                                            CITIBANK, N.A.,

                                            by

                                                 /s/  William Clark
                                               ---------------------------------
                                               Name:  William Clark
                                               Title:  Managing Director and
                                                        Vice President

                                            SALOMON SMITH BARNEY INC.,

                                            by

                                                 /s/  Wajeeh Faheem
                                               ---------------------------------
                                               Name:  Wajeeh Faheem
                                               Title:  Attorney-in-fact

Accepted and agreed to as of
the date first above written:

MERCK-MEDCO MANAGED CARE, L.L.C.,

by

      /s/  JoAnn Reed
   ----------------------------------
   Name:  JoAnn Reed
   Title:  Senior Vice President

<PAGE>

                           MEDCOHEALTH SOLUTIONS, INC.
                $1,250,000,000 SENIOR UNSECURED CREDIT FACILITIES
                    SUMMARY OF PRINCIPAL TERMS AND CONDITIONS
                    -----------------------------------------

All terms and conditions not addressed in this Summary of Principal Terms and
Conditions (this "Summary") shall be reasonably satisfactory to the Lead
Arrangers and reflected in the definitive agreement for the Loans (the "Loan
Agreement") and other documentation for the Loans (the "Transaction
Documentation").

Borrower:                         MedcoHealth Solutions, Inc., a Delaware
--------
                                  corporation/1/ (the "Borrower").

Joint Lead Arrangers:             J.P. Morgan Securities Inc. ("JPMorgan"),
--------------------
                                  Goldman Sachs Credit Partners L.P. ("Goldman")
                                  and Salomon Smith Barney Inc. ("Salomon" and,
                                  together with JPMorgan and Goldman, the "Lead
                                  Arrangers").

Joint Bookrunners:                JPMorgan, Goldman and Salomon (collectively,
-----------------
                                  the "Bookrunners").

Administrative Agent:             JPMorgan Chase Bank (the "Administrative
--------------------
                                  Agent").

Co-Syndication Agents:            Goldman and Citibank, N.A. ("Citibank" and,
---------------------
                                  together with Goldman, the "Syndication
                                  Agents").

Lenders:                          A syndicate of banks, financial institutions
-------
                                  and other entities reasonably acceptable to
                                  the Borrower, arranged by the Lead Arrangers
                                  (collectively, the "Lenders"), subject to the
                                  terms of the commitment letter ("Commitment
                                  Letter") to which this Summary is attached.

Facilities:                       Total facilities of $1,250 million comprising
----------
                                  the following (loans made under the Facilities
                                  being herein referred to collectively as the
                                  "Loans"):

                                  1. Five year $500 million senior unsecured
                                     revolving credit facility (the "Five Year
                                     Revolving Credit Facility") to be made
                                     available on a revolving basis (loans made
                                     under the Five Year Revolving Credit
                                     Facility are herein referred to as "Five
                                     Year Revolving Loans"). Any amounts re-paid
                                     may be re-borrowed, subject to the terms
                                     and conditions to be set forth in the Loan
                                     Agreement;

                                  2. 364 day $250 million senior unsecured
                                     revolving credit facility (the "364 Day
                                     Revolving Credit Facility" and, together
                                     with the Five Year Revolving Credit
                                     Facility, the "Revolving Credit
                                     Facilities") to be made available on a
                                     revolving basis (loans made under the 364
                                     Day Revolving Credit Facility are herein
                                     referred to as "364 Day Revolving Loans"
                                     and, together with the Five Year Revolving
                                     Loans, the "Revolving Loans"). Any amounts
                                     repaid may be reborrowed, subject to the
                                     terms and conditions to be set forth in the
                                     Loan Agreement; and

_____________________
/1/      Currently a Delaware limited liability company named Merck-Medco
         Managed Care, L.L.C., which on or prior to the effective date of the
         IPO will convert to a Delaware corporation pursuant to Section 265 of
         the Delaware General Corporation Law and intends to change its name to
         MedcoHealth Solutions, Inc.

<PAGE>

                                  3. Five year $500 million senior unsecured
                                     term loan facility (the "Term Loan
                                     Facility", and loans made under the Term
                                     Loan Facility are herein referred to as the
                                     "Term Loan").

                                  (The Five Year Revolving Credit Facility, the
                                  364 Day Revolving Credit Facility and the Term
                                  Loan Facility are each herein referred to as a
                                  "Facility" and, collectively, the
                                  "Facilities".)

Swingline Facility:               A portion of the Revolving Credit Facilities
------------------
                                  may be used by the Borrower as part of a
                                  swingline facility available on same-day
                                  notice in an aggregate face amount up to but
                                  not to exceed $50 million.

Letters of Credit Sublimit:       A portion of the Revolving Credit Facilities
--------------------------
                                  may be used by the Borrower for the issuance
                                  of letters of credit by JPMorgan Chase in an
                                  aggregate face amount up to but not to exceed
                                  an amount to be agreed.

Use of Proceeds:                  The net proceeds of the Term Loan shall be
---------------
                                  used, together with the net proceeds from the
                                  Borrower's public offering of $1,000 million
                                  aggregate principal amount of senior notes
                                  (the "Notes Offering") or the net proceeds of
                                  the Borrower's $1,000 million senior unsecured
                                  bridge loan facility (the "Bridge Facility"),
                                  to pay for the approximately $1,500 million
                                  dividend (the "Merck Dividend") to Merck &
                                  Co., Inc. ("Merck") to be declared prior to,
                                  and paid shortly after, the closing of the
                                  initial public offering of the Borrower's
                                  common stock (the "IPO").

                                  The proceeds of the Revolving Loans shall be
                                  used for the Borrower's general corporate and
                                  working capital purposes.

Availability:                     The Borrower may request Revolving Loans,
------------
                                  subject to a notice and notice periods to be
                                  agreed between the Borrower and the Lenders as
                                  set forth in the Loan Agreement, at any time
                                  during the period commencing on the closing
                                  date of the IPO and continuing until the
                                  earlier to occur of (i) the termination of the
                                  Lenders' commitments under corresponding
                                  Revolving Credit Facility in accordance with
                                  the terms and conditions of the Loan Agreement
                                  and (ii) the Final Maturity of such Revolving
                                  Loans.

                                  The Term Loan Facility shall be drawn down in
                                  full upon the closing date of the IPO.

                                  The Facilities and commitments under the Loan
                                  Agreement shall terminate if the Initial
                                  Funding Date has not occurred on or prior to
                                  August 15, 2002.

Final Maturity of Loans:          The Final Maturity of the Five Year Revolving
-----------------------
                                  Loans is the earlier of (i) the five year
                                  anniversary of the last day of the quarter in
                                  which the closing date of the IPO occurs or
                                  (ii) the optional prepayment in full of the
                                  Five Year Revolving Loans and cancellation by
                                  the Borrower of the Five Year Revolving Credit
                                  Facility.

                                  The Final Maturity of the 364 Day Revolving
                                  Loans is the earlier of (i) the 364 day
                                  anniversary of the Closing Date or (ii) the
                                  optional

<PAGE>

                                  prepayment in full of the 364 Day Revolving
                                  Loans and cancellation by the Borrower of the
                                  364 Day Revolving Credit Facility.

                                  The Final Maturity of the Term Loan is the
                                  earlier of (i) the five year anniversary of
                                  the last day of the quarter in which the
                                  closing date of the IPO occurs or (ii) the
                                  optional prepayment in full of the Term Loan.

Amortization of
Term Loan:                        The Borrower shall repay principal on the Term
---------
                                  Loan according to the following schedule of
                                  annual percentages:

                                  ==============================================
                                  Quarterly, during the        Annual percentage
                                  five-year period
                                  beginning on the first
                                  day of the quarter after
                                  the quarter in which the
                                  closing date of the IPO
                                  occurs:
                                  ----------------------------------------------
                                  Year 1                              10%
                                  ----------------------------------------------
                                  Year 2                              15%
                                  ----------------------------------------------
                                  Year 3                              20%
                                  ----------------------------------------------
                                  Year 4                              25%
                                  ----------------------------------------------
                                  Year 5                              30%
                                  ==============================================

Interest Rates and Fees:          Interest rates and fees are determined based
-----------------------
                                  on the credit ratings of the Borrower as set
                                  forth in the pricing grid attached hereto as
                                  Annex A. Interest rates will be increased by
                                  2.0% if there is an Event of Default.

Interest Payment Dates:           Interest on ABR Loans is due on the last day
----------------------
                                  of each March, June, September and December.
                                  Interest on LIBOR Rate Loans is due on (a) as
                                  to any LIBOR Rate Loan having an interest
                                  period of three months or less, the last day
                                  of such interest period; (b) as to any LIBOR
                                  Rate Loan having an interest period longer
                                  than three months, each day which is three
                                  months, or a whole multiple thereof, after the
                                  first day of such interest period and the last
                                  day of such interest period; and (c) as to any
                                  LIBOR Rate Loan converted to an ABR Loan (as
                                  the case may be) or prepaid, the date of such
                                  conversion or prepayment.

<PAGE>

Optional Revolving Credit
Facility Reductions and
Optional Term
Loans Prepayments:                The Borrower may at any time permanently
-----------------
                                  reduce either of the unutilized Revolving
                                  Credit Facility commitments (and upon
                                  cancellation of all such commitments for the
                                  Revolving Credit Facility, the Lenders shall
                                  have no obligations to provide Revolving Loans
                                  corresponding to such Revolving Credit
                                  Facility, and upon cancellation of all such
                                  commitments for both of the Revolving Credit
                                  Facilities, the Lenders shall have no
                                  obligations to provide letters of credit) and
                                  the Term Loan may be prepaid by the Borrower,
                                  in each case in minimum aggregate principal
                                  amounts of $5 million and increments of $1
                                  million in excess thereof and subject to
                                  customary funding breakage fees and to a
                                  notice and notice period to be agreed between
                                  the Borrower and the Lenders as set forth in
                                  the Loan Agreement. Amounts prepaid at the
                                  option of the Borrower shall be applied as
                                  determined by the Borrower. Any indebtedness
                                  of the Borrower (other than the Loans) shall
                                  not be prepaid or repurchased unless the Loans
                                  have been repaid in full, unless the
                                  prepayment or repurchase is as a result of a
                                  permitted refinancing and subject to other
                                  exceptions to be agreed.

Mandatory Prepayments:            Loans under the Facilities will be required to
---------------------
                                  be prepaid, without premium or penalty, but
                                  subject to customary reimbursement of any
                                  breakage costs, with (a) 100% of the after-tax
                                  net cash proceeds received from Asset Sales
                                  (to be defined) or other dispositions of a
                                  material portion of the property of the
                                  Borrower and its subsidiaries (including
                                  insurance and condemnation proceeds in excess
                                  of a threshold amount to be agreed upon, but
                                  excluding sales and dispositions in the
                                  ordinary course of business) subject to the
                                  Borrower's option to reinvest or commit to
                                  reinvest in the business within 12 months of
                                  the receipt of such proceeds, and (b) if
                                  Borrower's leverage ratio is above a level to
                                  be agreed upon, 100% of the net cash proceeds
                                  received from issuances of debt obligations of
                                  the Borrower and its subsidiaries, subject, in
                                  the case of clauses (a) and (b), to payment of
                                  such amounts as mandatory prepayments under
                                  the Bridge Facility and other limited
                                  exceptions to be agreed upon. Mandatory
                                  prepayments shall be applied first pro rata to
                                  scheduled principal repayments of the Term
                                  Loan and then to repay Revolving Loans.

Security:                         None.
--------

Ranking:                          The Loans shall rank pari passu with each
-------
                                  other and with all other senior unsecured
                                  indebtedness of the Borrower.

Conditions to Closing:            The transactions outlined in this Summary are
---------------------
                                  conditioned upon satisfaction of, among other
                                  things, the conditions set forth in the
                                  Commitment Letter and the closing conditions
                                  (the "Closing Conditions") set forth below
                                  (the date upon which all such Closing
                                  Conditions shall be satisfied, the "Closing
                                  Date"):

                                  1. The negotiation, execution and delivery of
                                     a Loan Agreement, including schedules,
                                     exhibits and other Transaction

<PAGE>

                                            Documentation containing terms and
                                            conditions consistent with this
                                            Summary, and otherwise reasonably
                                            satisfactory in form and substance
                                            to the Lead Arrangers, including
                                            customary legal opinions of counsel
                                            to the Borrower and the Lenders, as
                                            the case may be, as to the
                                            transactions contemplated thereby,
                                            together with other customary
                                            closing documentation.

                                    2.      The terms and conditions of the IPO
                                            and the Notes Offering continue to
                                            be consistent with Annex B. The
                                            underwriting and exchange agreements
                                            that will give effect to the IPO
                                            shall be substantially in the form
                                            reviewed prior to the Closing Date
                                            by the Lead Arrangers, and none of
                                            the terms or conditions of such
                                            agreements shall be amended, waived
                                            or otherwise modified in a manner
                                            that would, in the reasonable
                                            judgment of the Lead Arrangers, be
                                            materially adverse to the Lenders
                                            without the consent of the Lead
                                            Arrangers.

                                    3.      Absence of any default or Event of
                                            Default in existence at the time of,
                                            or after giving effect to the making
                                            of, any Loan.

                                    4.      All representations and warranties
                                            (including, without limitation, a
                                            representation and warranty on no
                                            material adverse change) being true
                                            and correct in all material respects
                                            as of the Closing Date, except to
                                            the extent expressly made as of an
                                            earlier date.

                                    5.      The Closing Date shall be on the
                                            Initial Funding Date.

Conditions to
First Borrowing:                    Conditions  precedent to the first borrowing
---------------
                                    among any of the Facilities shall include
                                    satisfaction of, among other things, the
                                    conditions set forth in the Commitment
                                    Letter and the initial funding conditions
                                    (the "Initial Funding Conditions") set forth
                                    below (the date upon which all Initial
                                    Funding Conditions shall be satisfied and
                                    the first borrowing is funded, the "Initial
                                    Funding Date" (and such date shall be the
                                    Initial Funding Date even if there is no
                                    borrowing under any of the other Facilities
                                    on such date)):

                                    1.      All representations and warranties
                                            (including, without limitation, a
                                            representation and warranty on no
                                            material adverse change) being true
                                            and correct in all material respects
                                            as of the Initial Funding Date.

                                    2.      All costs, fees, expenses (including
                                            reasonable out-of-pocket legal fees
                                            and expenses and the reasonable
                                            out-of-pocket fees and expenses of
                                            appraisers, consultants and other
                                            advisors) and other compensation
                                            payable to the Lead Arrangers,
                                            including the Arrangement Fee, and
                                            the other Lenders shall have been
                                            paid in cash to the extent due.

                                    3.      The Borrower shall have delivered
                                            evidence of the final long-term
                                            ratings it received from each of S&P
                                            and Moody's for each

<PAGE>

                                            Facility, which shall be no less
                                            than BBB+ and Ba1, respectively, and
                                            with a stable or positive outlook.

                                    4.      The terms and conditions of the IPO
                                            shall be consistent with Annex B.
                                            The underwriting and exchange
                                            agreements giving effect to the IPO
                                            and the separation and transition
                                            agreements with Merck (described in
                                            the prospectus for the IPO) shall be
                                            substantially in the form reviewed
                                            prior to the borrowing by the Lead
                                            Arrangers and in full force and
                                            effect without any defaults or
                                            breaches thereunder that would cause
                                            any of the conditions to closing
                                            thereunder not to be satisfied, and
                                            none of the terms or conditions of
                                            such documentation shall be amended,
                                            waived or otherwise modified in a
                                            manner that would, in the reasonable
                                            judgment of the Lead Arrangers, be
                                            materially adverse to the Lenders
                                            without the consent of the Lead
                                            Arrangers.

                                    5.      The IPO shall have been completed,
                                            and the separation and transition
                                            agreements with Merck (described in
                                            the prospectus for the IPO) executed
                                            and delivered, each on terms and
                                            conditions consistent with Annex B.

                                    6.      The Borrower shall have converted
                                            from a Delaware limited liability
                                            company to a Delaware corporation
                                            pursuant to Section 265 of the
                                            Delaware General Corporation Law.

                                    7.      Satisfaction of the Continuing
                                            Conditions (as defined below).

Continuing Conditions:              Conditions precedent to any borrowing under
---------------------
                                    the Facilities shall include satisfaction of
                                    the continuing conditions ("Continuing
                                    Conditions") set forth below:

                                    1.      The actual uses for the Facilities
                                            shall be consistent in all material
                                            respects with the description of the
                                            uses set forth in the Commitment
                                            Letter.

                                    2.      Absence of any default or Event of
                                            Default in existence at the time of,
                                            or after giving effect to the making
                                            of, any Loan.

                                    3.      All representations and warranties
                                            (including, without limitation, a
                                            representation and warranty on no
                                            material adverse change) being true
                                            and correct in all material respects
                                            as of the date of the borrowing,
                                            except to the extent expressly made
                                            as of an earlier date.

Representations and
Warranties:                         Shall apply to the Borrower and each of its
----------
                                    subsidiaries. Usual and customary
                                    representations and warranties (including
                                    the usual and customary qualifications and
                                    limitations thereon) for transactions of
                                    this size, type and purpose, including,
                                    without limitation: financial statements
                                    (including pro forma financial statements
                                    and projections); absence of material
                                    undisclosed liabilities (including those
                                    related to Medicare, Medicaid or any other
                                    managed care recoupment or

<PAGE>

                                    recoupments of any third-party payor being
                                    sought, threatened, requested or claimed);
                                    no material adverse change; corporate
                                    existence and good standing; corporate power
                                    and authority; compliance with laws and
                                    regulations (including ERISA and
                                    environmental laws); enforceability of the
                                    Loan Agreement and Transaction Documentation
                                    (and, in the case of a drawing under the
                                    Facilities, the effectiveness and
                                    enforceability by the Borrower of the
                                    documentation relating to the IPO, including
                                    the separation and transition agreements
                                    with Merck (described in the prospectus for
                                    the IPO)); no conflict with law or
                                    contractual obligations, governmental
                                    authorizations, licenses and permits; no
                                    material litigation; no default under
                                    material contracts or licenses; ownership of
                                    property; liens; intellectual property; no
                                    burdensome restrictions; taxes;
                                    subsidiaries; environmental matters;
                                    solvency; maintenance of insurance; and
                                    accuracy and completeness of disclosure.

Affirmative Covenants:              Shall apply to the Borrower and each of its
---------------------
                                    subsidiaries. Usual and customary
                                    affirmative covenants (including the usual
                                    and customary qualifications and limitations
                                    thereon) for transactions of this size, type
                                    and purpose, including, without limitation:
                                    financial and other information such as
                                    certified quarterly and audited annual
                                    financial statements, reports to
                                    shareholders (provided that such financial
                                    statements and reports to shareholders shall
                                    be deemed delivered on the second business
                                    day following the day on which they are
                                    filed with the Securities and Exchange
                                    Commission, unless actually delivered on an
                                    earlier date), notices of defaults,
                                    litigation and other material events,
                                    budgets and other information customarily
                                    supplied in a transaction of this type;
                                    payment and performance of other
                                    obligations; continuation of business and
                                    maintenance of existence and material rights
                                    and privileges, including maintenance of
                                    licenses; compliance with all applicable
                                    laws and regulations (including
                                    environmental matters, taxation and ERISA)
                                    and material contractual obligations
                                    (including those with Merck); maintenance of
                                    property and insurance; right of Lenders to
                                    inspect property and books and records; and
                                    further assurances.

Financial Covenants:                Usual and customary for transactions of this
-------------------
                                    size, type and purpose, including, without
                                    limitation: leverage covenant and interest
                                    coverage covenant (in each case with
                                    definitions to be agreed upon).

Negative Covenants:                 Shall apply to the Borrower and each of its
------------------
                                    subsidiaries. Usual and customary negative
                                    covenants (including the usual and customary
                                    qualifications and limitations thereon and
                                    exceptions and baskets to be agreed) for
                                    transactions of this size, type and purpose
                                    including, without limitation: limitation on
                                    liens; limitation on sale and lease back
                                    transactions; limitation on fundamental
                                    changes in the corporate existence and
                                    principal business of the Borrower and its
                                    subsidiaries; limitation on indebtedness;
                                    limitation on capital expenditures;
                                    limitation on investments; limitation on
                                    dividends; limitation on other restricted
                                    payments; transactions with affiliates to be
                                    on an arm's-length basis (except for
                                    agreements with Merck to be entered into in
                                    connection with the IPO and subject to other
                                    exceptions to be agreed upon); limitation on
                                    entering into restrictive agreements;
                                    limitation on asset sales and divestitures;
                                    limitation on acquisitions; limitation on
                                    amendments to or

<PAGE>

                                    prepayments or redemptions of indebtedness;
                                    ERISA matters; and limitation on use of
                                    derivative obligations.

Events of Default:                  Usual and customary for transactions of this
-----------------
                                    size, type and purpose including, without
                                    limitation:

                                    1. Failure to pay the principal on a Loan
                                    under any Facility on its due date.

                                    2. Failure to pay interest or other amounts
                                    under the Loan Agreement on a Loan under any
                                    Facility or under the Transaction
                                    Documentation within 5 days of its due date.

                                    3. Breach of affirmative and/or negative
                                    covenants to be identified which shall
                                    continue unremedied for 30 days after notice
                                    to the Borrower.

                                    4. Breach of any other affirmative or
                                    negative covenant or financial covenant.

                                    5. Breach of representations and warranties.

                                    6. Cross default to debt exceeding $25
                                    million.

                                    7. Certain other events, such as certain
                                    events under Borrower's agreements with
                                    Merck, to be agreed.

                                    8. The Borrower files for bankruptcy or
                                    certain other events in bankruptcy,
                                    insolvency or reorganization occur.

                                    9. Change in control (to be defined) after
                                    the closing date for the IPO.

Required Lenders:                   Lenders having outstanding Loans and
----------------
                                    commitments under the Facilities
                                    representing more than 51% of the sum of the
                                    total of the outstanding Loans and
                                    commitments under the Facilities.

Voting:                             Amendments and waivers of the Loan Agreement
------
                                    will require the approval of Lenders holding
                                    more than 51% of the aggregate amount of the
                                    Loans under the Facilities; provided that
                                    (a) the consent of all affected Lenders will
                                    be required with respect to (i) reductions
                                    in the unpaid principal amount or extensions
                                    of the scheduled date for the final payment
                                    of principal of any Loan, (ii) reductions in
                                    interest rates or fees or extensions of the
                                    dates for payment thereof, (iii) increases
                                    in the amounts or extensions of the expiry
                                    date of the Lenders' commitments and (iv)
                                    modifications of the pro rata provisions of
                                    the credit agreements, and the consent of
                                    100% of the Lenders will be required with
                                    respect to modifications to any of the
                                    voting percentages and (b) the consent of
                                    Lenders holding a majority in interest of an
                                    adversely affected Facility shall be
                                    required with respect to any amendment that
                                    by its terms changes the allocation between
                                    the Facilities of any voluntary or mandatory
                                    prepayments.

<PAGE>

Assignments:                Each Lender may assign all or a portion of its Loans
-----------
                            and commitments under any Facility with the consent
                            of the Borrower (such consent not to be unreasonably
                            withheld, provided that the Borrower's consent shall
                            not be required during the continuation of an Event
                            of Default) and the Administrative Agent (such
                            consent not to be unreasonably withheld); provided
                            that each such assignment shall be in minimum
                            amounts equal to $5 million and subject to payment
                            by the Lender of an administrative fee of $3,500 to
                            the Administrative Agent. Assignments by a Lender to
                            a Federal Reserve Bank or affiliates of such Lender
                            shall not require the consent of the Administrative
                            Agent.

Yield Protection:           The Loan Agreement shall contain customary
----------------
                            provisions indemnifying the Lenders for "breakage
                            costs" incurred in connection with (i) prepayment of
                            a LIBOR Rate Loan on a day other than the last day
                            of an interest period with respect thereto
                            (including as a result of an Event of Default), (ii)
                            the conversion of a LIBOR Rate Loan other than on
                            the last day of an interest period, and (iii) the
                            failure to borrow or prepay any Loans on a date
                            specified in any notice under the Loan Agreement.
                            The Loan Agreement shall also contain customary
                            increased costs and illegality provisions.

Expenses and
Indemnification:            The Loan Agreement and the Transaction Documentation
---------------
                            shall contain expense and indemnification provisions
                            for the benefit and to the reasonable satisfaction
                            of the Lead Arrangers, Administrative Agent,
                            Syndication Agents and the Lenders customary for
                            transactions of this type including, without
                            limitation:

                            1.    All reasonable out-of-pocket expenses
                                  (including reasonable legal fees and
                                  reasonable expenses and expenses incurred in
                                  connection with due diligence and travel,
                                  courier, reproduction, printing and delivery
                                  expenses) of the Lead Arrangers and their
                                  affiliates incurred in connection with the
                                  arrangement and syndication of any Facility
                                  and with the preparation, execution and
                                  delivery, administration, amendment, waiver or
                                  modification (including proposed amendments,
                                  waivers or modifications) of the documentation
                                  contemplated hereby are to be paid by the
                                  Borrower. In addition, all out-of-pocket
                                  expenses (including legal fees and expenses)
                                  of the Lead Arrangers, the Administrative
                                  Agent and the Lenders for workout proceedings,
                                  enforcements costs and documentary taxes
                                  associated with any Facility are to be paid by
                                  the Borrower.

                            2.    The Borrower shall indemnify the Lead
                                  Arrangers and the Lenders and each of their
                                  respective affiliates, and hold them harmless
                                  from and against all reasonable out-of-pocket
                                  costs, expenses (including reasonable legal
                                  fees and expenses) and liabilities arising out
                                  of third-party claims relating to the proposed
                                  transactions, and any transactions related
                                  thereto and any actual or proposed use of the
                                  proceeds of any loans made under any Facility;
                                  provided that (a) no indemnified party shall

<PAGE>

                                  be indemnified for its willful misconduct, bad
                                  faith or gross negligence and (b) the Borrower
                                  shall have the right to assume the defense of
                                  a third party claim for which indemnity is
                                  sought.

                            3.    The Borrower shall indemnify the Lenders for
                                  withholding taxes imposed by any governmental
                                  authority. Such indemnification shall consist
                                  of customary tax gross-up provisions with
                                  customary exceptions.

Confidentiality:            The Loan Agreement and Transaction Documentation
---------------
                            shall contain customary confidentiality provisions.

Governing Law and Forum:    The laws of the State of New York.
-----------------------

Waiver of Trial by Jury:    Each party to the Loan Agreement shall waive the
-----------------------

                            right to trial by jury and shall consent to the
                            non-exclusive jurisdiction of the state and federal
                            courts located in The City of New York.

Counsel to Lead Arrangers:  Sullivan & Cromwell.
-------------------------

<PAGE>

                                                                         ANNEX A
                                                                         -------

                             INTEREST RATES AND FEES

"Applicable Commitment Fee Rate" means, for any period a percentage per annum
equal to the percentage set forth below determined by reference to the higher of
(i) the rating of the Borrower's long-term, senior unsecured Debt from S&P or
(ii) the rating of the Borrower's long-term, senior unsecured Debt from Moody's,
in each case as in effect from time to time during such period:

<TABLE>
<CAPTION>
-----------------------------------------------------------------------------------------------------------------------
                BORROWER'S
                                            Applicable                 Applicable
             LONG-TERM SENIOR             Commitment Fee             Commitment Fee
                                           Fee Rate (364                  Rate              Applicable
           UNSECURED DEBT RATING           Day Revolving               (Five Year            Interest     Applicable
                                          Credit Facility)          Revolving Credit           Rate        Base Rate
         S&P or Moody's ("Level")                                       Facility)             Margin         Spread
-----------------------------------------------------------------------------------------------------------------------
<S>                                       <C>                       <C>                     <C>           <C>
LEVEL I

BBB+ or better, or Baa1 or better              .125%                      .150%                 .75%            0
-----------------------------------------------------------------------------------------------------------------------
LEVEL II

BBB or Baa2                                    .150%                      .200%                1.00%            0
-----------------------------------------------------------------------------------------------------------------------
LEVEL III

BBB- or Baa3                                   .175%                      .225%                1.25%         .250%
-----------------------------------------------------------------------------------------------------------------------
LEVEL IV

BB+ or Ba1                                     .250%                      .300%                1.50%         .500%
-----------------------------------------------------------------------------------------------------------------------
LEVEL V

Worse than or equal to BB and Ba2              .300%                      .350%                2.25%         1.25%
-----------------------------------------------------------------------------------------------------------------------
</TABLE>

provided that if, at any time, no rating is available from S&P or Moody's, the
--------
Applicable Commitment Fee Rate shall be .300% for the 364 Day Revolving Credit
Facility and .350% for the Five Year Revolving Credit Facility; and provided
                                                                    --------
further that upon the occurrence of a ratings differential between S&P and
-------
Moody's that corresponds to a differential of two or more Levels, the Applicable
Commitment Fee Rate shall be based upon the Level that is one Level above the
Level corresponding to the lower rating.

"Applicable Interest Rate Margin" means, for each LIBOR Advance, for any
Interest Period, a percentage per annum equal to the percentage set forth above
determined by reference to the higher of (i) the rating of the Borrower's
long-term, senior unsecured Debt from S&P or (ii) the rating of the Borrower's
long-term, senior unsecured Debt from Moody's, in each case used for LIBOR
Advances, as in effect on the first day of such Interest Period; provided that
                                                                 --------
if, at any time, no rating is available from S&P or Moody's, the Applicable
Interest Rate Margin for LIBOR Advances shall be 2.25%; and provided further
                                                            ----------------
that upon the occurrence of a ratings differential between S&P and Moody's that
corresponds to a differential of two or more Levels, the Applicable Interest
Rate Margin shall be based upon the Level that is one Level above the Level
corresponding to the lower rating.

LIBOR loans to provide for interest periods of 1, 2, 3 and 6 months and, if
available from the Lenders, 9 and 12 months.

<PAGE>

                                                                         ANNEX B
                                                                         -------

                        SUMMARY OF IPO AND NOTES OFFERING

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