<PAGE>   1
                                                                     EXHIBIT 4.8
                               GUARANTEE AGREEMENT

                                     BETWEEN

                                CAREMARK RX, INC.

                                 (AS GUARANTOR)

                                       AND

                            WILMINGTON TRUST COMPANY

                                  (AS TRUSTEE)

                                   DATED AS OF

                               SEPTEMBER 29, 1999

<PAGE>   2


                            CROSS-REFERENCE TABLE(1)


<TABLE>
<CAPTION>

----------------------------------------------- -------------------------
                                                Section of
Section of Trust Indenture Act Of               Guarantee
1939, as amended                                Agreement
----------------------------------------------- -------------------------
<S>                                             <C>
310(a)                                          4.1(a)
310(b)                                          4.1(c), 2.8
310(c)                                          Inapplicable
311(a)                                          2.2(b)
311(b)                                          2.2(b)
311(c)                                          Inapplicable
312(a)                                          2.2(a)
312(b)                                          2.2(b)
313                                             2.3
314(a)                                          2.4
314(b)                                          Inapplicable
314(c)                                          2.5
314(d)                                          Inapplicable
314(e)                                          1.1, 2.5, 3.2
314(f)                                          2.1, 3.2
315(a)                                          3.1(d)
315(b)                                          2.7
315(c)                                          3.1
315(d)                                          3.1(d)
316(a)                                          1.1, 2.6, 5.4
316(b)                                          5.3
316(c)                                          8.2
317(a)                                          Inapplicable
317(b)                                          Inapplicable
318(a)                                          2.1(b)
318(b)                                          2.1
318(c)                                          2.1(a)
</TABLE>



         (1) This Cross-Reference Table does not constitute part of the
Guarantee Agreement and shall not affect the interpretation of any of its terms
or provisions.

<PAGE>   3


                                TABLE OF CONTENTS

<TABLE>
<CAPTION>

                                                                                                                 PAGE


   <S>           <C>                                                                                             <C>
                                              ARTICLE I.  DEFINITIONS

   SECTION 1.1.  Definitions........................................................................................1

                                         ARTICLE II.  TRUST INDENTURE ACT

   SECTION 2.1.  Trust Indenture Act; Application...................................................................4
   SECTION 2.2.  List of Holders....................................................................................4
   SECTION 2.3.  Reports by Guarantee Trustee.......................................................................4
   SECTION 2.4.  Periodic Reports to Guarantee Trustee..............................................................4
   SECTION 2.5.  Evidence of Compliance with Conditions Precedent...................................................5
   SECTION 2.6.  Events of Default; Waiver..........................................................................5
   SECTION 2.7.  Event of Default; Notice...........................................................................5
   SECTION 2.8.  Conflicting Interests..............................................................................5

                           ARTICLE III.  POWERS, DUTIES AND RIGHTS OF GUARANTEE TRUSTEE

   SECTION 3.1.  Powers and Duties of Guarantee Trustee.............................................................6
   SECTION 3.2.  Certain Rights of Guarantee Trustee................................................................7
   SECTION 3.3.  Indemnity..........................................................................................8

                                          ARTICLE IV.  GUARANTEE TRUSTEE

   SECTION 4.1.  Guarantee Trustee: Eligibility.....................................................................9
   SECTION 4.2.  Appointment, Removal and Resignation of Guarantee Trustee..........................................9

                                               ARTICLE V.  GUARANTEE

   SECTION 5.1.  Guarantee.........................................................................................10
   SECTION 5.2.  Waiver of Notice and Demand.......................................................................10
   SECTION 5.3.  Obligations Not Affected..........................................................................10
   SECTION 5.4.  Rights of Holders.................................................................................11
   SECTION 5.5.  Guarantee of Payment..............................................................................11
   SECTION 5.6.  Subrogation.......................................................................................11
   SECTION 5.7.  Independent Obligations...........................................................................12

                                            ARTICLE VI.  SUBORDINATION

   SECTION 6.1.  Subordination.....................................................................................12

                                             ARTICLE VII.  TERMINATION

   SECTION 7.1.  Termination.......................................................................................12

                                           ARTICLE VIII.  MISCELLANEOUS

   SECTION 8.1.  Successors and Assign.............................................................................12
   SECTION 8.2.  Amendments........................................................................................13
   SECTION 8.3.  Notice............................................................................................13
   SECTION 8.4.  Benefit...........................................................................................14
   SECTION 8.5.  Interpretation....................................................................................14
</TABLE>


                                       i

<PAGE>   4

<TABLE>
   <S>           <C>                                                                                               <C>
   SECTION 8.6.  Governing Law.....................................................................................14
   SECTION 8.7.  Counterparts......................................................................................14
</TABLE>


                                       ii

<PAGE>   5

                               GUARANTEE AGREEMENT

         THIS GUARANTEE AGREEMENT, dated as of September 29, 1999, is executed
and delivered by CAREMARK RX, INC., a Delaware corporation (the "Guarantor"),
having its principal office at 3000 Galleria Tower, Suite 1000, Birmingham,
Alabama 35244, and WILMINGTON TRUST COMPANY, a Delaware corporation, as trustee
(the "Guarantee Trustee"), for the benefit of the Holders (as defined herein)
from time to time of the Preferred Securities and Common Securities (each as
defined herein and together, the "Trust Securities") of CAREMARK RX CAPITAL
TRUST I, a Delaware statutory business trust (the "Trust").

         WHEREAS, pursuant to an Amended and Restated Trust Agreement, dated as
of September 29, 1999 (the "Trust Agreement"), among the Guarantor, as
Depositor, the Property Trustee and the Delaware Trustee named therein, the
Administrative Trustees named therein and the Holders from time to time of
undivided beneficial interests in the assets of the Trust, the Trust is issuing
$200,000,000 aggregate Liquidation Amount (as defined in, and subject to
increase as provided in, the Trust Agreement) of its 7.0% Shared Preference
Redeemable Securities, Liquidation Amount $50 per security (the "Preferred
Securities"), representing preferred undivided beneficial interests in the
assets of the Trust and having the terms set forth in the Trust Agreement;

         WHEREAS, the Preferred Securities will be issued by the Trust and the
proceeds thereof, together with the proceeds from the issuance of the Common
Securities of the Trust will be used to purchase the Debentures (as defined in
the Trust Agreement) of the Guarantor which will be deposited with Wilmington
Trust Company, as Property Trustee under the Trust Agreement, as trust assets;
and

         WHEREAS, as incentive for the Holders to purchase the Preferred
Securities, the Guarantor desires irrevocably and unconditionally to agree, to
the extent set forth herein, to pay to the Holders of the Preferred Securities
the Guarantee Payments (as defined herein) and to make certain other payments on
the terms and conditions set forth herein.

         NOW, THEREFORE, in consideration of the purchase by each Holder of the
Preferred Securities, which purchase the Guarantor hereby agrees shall benefit
the Guarantor, the Guarantor executes and delivers this Guarantee Agreement for
the benefit of the Holders from time to time of the Preferred Securities.


                                   ARTICLE I.

                                   DEFINITIONS

         SECTION 1.1.      Definitions. As used in this Guarantee Agreement, the
terms  set forth below shall, unless the context otherwise requires, have the
following meanings. Capitalized or otherwise defined terms used but not
otherwise defined herein shall have the meanings assigned to such terms in the
Trust Agreement as in effect on the date hereof.

         "Affiliate" of any specified Person means any other Person directly or
indirectly controlling or controlled by or under direct or indirect common
control with such specified Person; provided, however, that an Affiliate of the
Guarantor shall not be deemed to be an Affiliate of the Trust. For the purposes
of this definition, "control" when used with respect to any specified Person
means the

<PAGE>   6

power to direct the management and policies of such Person, directly or
indirectly, whether through the ownership of voting securities, by contract or
otherwise; and the terms "controlling" and "controlled" have meanings
correlative to the foregoing.

         "Board of Directors" means either the board of directors of the
Guarantor or any committee of that board duly authorized to act hereunder or any
directors or officers of the Guarantor to whom such board of directors or such
committee shall have duly delegated its authority.

         "Common Security" means an undivided common beneficial interest in the
assets of the Trust, having a Liquidation Amount of $50 and having the rights
provided therefor in the Trust Agreement, including the right to convert to
shares of the Common Stock and the right to receive Distributions and a
Liquidation Distribution as provided therein.

         "Common Stock" means the common stock of the Guarantor, par value $.001
per share.

         "Event of Default" means a default by the Guarantor on any of its
payment or other obligations under this Guarantee Agreement; provided, however,
that, except with respect to a default in payment of any Guarantee Payments, the
Guarantor shall have received notice of default and shall not have cured such
default within 90 days after receipt of such notice.

         "Guarantee Payments" means the following payments or distributions,
without duplication, with respect to the Preferred Securities, to the extent not
paid or made by or on behalf of the Trust: (i) any accrued and unpaid
Distributions required to be paid on the Preferred Securities, to the extent
that the Trust shall have funds on hand available therefor at such time, (ii)
the redemption price, including all accrued and unpaid Distributions (and
Additional Amounts and/or Liquidated Damages, if applicable) to the date of
redemption (the "Redemption Price"), with respect to any Securities called for
redemption by the Trust, to the extent the Trust shall have funds on hand
available therefor at such time, (iii) the repurchase price, including all
accrued and unpaid Distributions (and Additional Amounts and/or Liquidated
Damages, if applicable) to the date of repurchase (the "Repurchase Price"), with
respect to any Preferred Securities required to be repurchased by the Trust in
the event of a Change in Control (as defined in the Trust Agreement), to the
extent the Trust shall have funds on hand available therefor at such time and
(iv) upon a voluntary or involuntary termination, winding up or liquidation of
the Trust, unless Debentures are distributed to the Holders or all of the
Preferred Securities are redeemed, the lesser of (a) the aggregate of the
Liquidation Amount plus accrued and unpaid Distributions (and Additional Amounts
and/or Liquidated Damages, if applicable) to the date of payment and (b) the
amount of assets of the Trust remaining available for distribution to Holders in
liquidation of the Trust after satisfaction of liabilities to creditors of the
Trust as required by applicable law (in either case, the "Liquidation
Distribution").

         "Guarantee Trustee" means Wilmington Trust Company, until a Successor
Guarantee Trustee has been appointed and has accepted such appointment pursuant
to the terms of this Guarantee Agreement, and thereafter means each such
Successor Guarantee Trustee.

         "Holder" means any holder, as registered on the books and records of
the Trust, of any Preferred Securities; provided, however, that in determining
whether the holders of the requisite percentage of Preferred Securities have
given any request, notice, consent or waiver hereunder,


                                       2
<PAGE>   7

"Holder" shall not include the Guarantor, the Guarantee Trustee or any Affiliate
of the Guarantor or the Guarantee Trustee.

         "Indenture" means the Indenture dated as of September 29, 1999, between
the Guarantor and Wilmington Trust Company, as trustee, as amended or
supplemented from time to time..

         "Majority in Aggregate Liquidation Amount of the Preferred Securities"
means, except as provided by the Trust Indenture Act, a vote by the Holders,
voting separately as a class, of more than 50% of the aggregate Liquidation
Amount of all then outstanding Preferred Securities issued by the Trust.

         "Officers' Certificate" means, with respect to any Person, a
certificate signed by the Chairman or a Vice Chairman of the Board of Directors
of such Person or the President, a Managing Director or a Vice President of such
Person, and by the Treasurer, an Assistant Treasurer, the Secretary or an
Assistant Secretary of such Person, and delivered to the Guarantee Trustee. Any
Officers' Certificate delivered with respect to compliance with a condition or
covenant provided for in this Guarantee Agreement shall include:

         (a) a statement that each officer signing the Officers' Certificate has
         read the covenant or condition and the definitions relating thereto;

         (b) a brief statement of the nature and scope of the examination or
         investigation undertaken by each officer in rendering the Officers'
         Certificate;

         (c) a statement that each officer has made such examination or
         investigation as, in such officer's opinion, is necessary to enable
         such officer to express an informed opinion as to whether or not such
         covenant or condition has been complied with; and

         (d) a statement as to whether, in the opinion of each officer, such
         condition or covenant has been complied with.

         "Person" means any individual, corporation, partnership, joint venture,
trust, limited liability company or corporation, unincorporated organization or
government or any agency or political subdivision thereof.

         "Responsible Officer" when used with respect to the Guarantee Trustee
means any officer of the Guarantee Trustee assigned by the Guarantee Trustee
from time to time to administer its corporate trust matters.

         "Successor Guarantee Trustee" means a successor Guarantee Trustee
possessing the qualifications to act as Guarantee Trustee under Section 4.1.

         "Trust Indenture Act" means the Trust Indenture Act of 1939 as in force
at the date as of which this instrument was executed; provided, however, that in
the event the Trust Indenture Act of 1939 is amended after such date, "Trust
Indenture Act" means, to the extent required by any such amendment, the Trust
Indenture Act of 1939 as so amended.


                                       3
<PAGE>   8

                                  ARTICLE II.

                               TRUST INDENTURE ACT

         SECTION 2.1.      Trust Indenture Act; Application.

         (a) This Guarantee Agreement shall be subject to the provisions of the
Trust Indenture Act whether or not such provisions are required or deemed to be
a part of this Guarantee Agreement and shall, to the extent applicable, be
governed by such provisions.

         (b) If any provision hereof limits, qualifies or conflicts with another
provision hereof which is or would be required or deemed to be included in this
Guarantee Agreement by any of the provisions of the Trust Indenture Act, such
required or deemed provision shall control. If any provision of this Guarantee
Agreement modifies or excludes any provision of the Trust Indenture Act which
may be so modified or excluded, the latter provision shall be deemed to apply to
this Guarantee Agreement as so modified or excluded, as the case may be.

         SECTION 2.2.      List of Holders.

         (a) The Guarantor will furnish or cause to be furnished to the
Guarantee Trustee:

                  (i)      semi-annually, not more than 15 days after January 15
         and July 15 in each year, a list in such form as the Guarantee Trustee
         may reasonably require, of the names and addresses of the Holders as of
         such January 1 and July 1, and

                  (ii)     at such other times as the Guarantee Trustee may
         request in writing, within 30 days after the receipt by the Guarantor
         of any such request, a list of similar form and content as of a date
         not more than 15 days prior to the time such list is furnished,
         excluding from any such list names and addresses received by the
         Guarantee Trustee in its capacity as Securities Registrar.

         (b) The Guarantee Trustee shall comply with its obligations under
Section 311(a), Section 311(b) and Section 312(b) of the Trust Indenture Act.

         SECTION 2.3.      Reports by Guarantee Trustee.

         The Guarantee Trustee shall transmit to Holders such reports concerning
the Guarantee Trustee and its actions under this Guarantee Agreement as may be
required pursuant to the Trust Indenture Act at the times and in the manner
provided pursuant thereto. If required by Section 313(a) of the Trust Indenture
Act, the Guarantee Trustee shall, within 60 days after each May 15 following the
date of this Guarantee Agreement deliver to Holders a brief report, dated as of
such May 15, which complies with the provisions of such Section 313(a).

         SECTION 2.4.      Periodic Reports to Guarantee Trustee.

         The Guarantor shall provide to the Guarantee Trustee such documents,
reports and information, if any, as required by Section 314 of the Trust
Indenture Act and the compliance certificate required by Section 314 of the
Trust Indenture Act, in the form, in the manner and at the


                                       4
<PAGE>   9

times required by Section 314 of the Trust Indenture Act. Delivery of such
reports, information and documents to the Guarantee Trustee shall be for
informational purposes only and the Guarantee Trustee's receipt of such shall
not constitute constructive notice of any information contained therein,
including the Guarantor's compliance with any of its covenants hereunder (as to
which the Guarantee Trustee shall be entitled to rely exclusively on Officers'
Certificates).

         SECTION 2.5.      Evidence of Compliance with Conditions Precedent.

         The Guarantor shall provide to the Guarantee Trustee such evidence of
compliance with such conditions precedent, if any, provided for in this
Guarantee Agreement that relate to any of the matters set forth in Section
314(c) of the Trust Indenture Act. Any certificate or opinion required to be
given by an officer pursuant to Section 314(c)(1) may be given in the form of an
Officers' Certificate.

         SECTION 2.6.      Events of Default; Waiver.

         The Holders of a Majority in Aggregate Liquidation Amount of the
Preferred Securities may, by vote, on behalf of the Holders, waive any past
Event of Default and its consequences. Upon such waiver, any such Event of
Default shall cease to exist, and any Event of Default arising therefrom shall
be deemed to have been cured, for every purpose of this Guarantee Agreement, but
no such waiver shall extend to any subsequent or other default or Event of
Default or impair any right consequent therefrom.

         SECTION 2.7.      Event of Default; Notice.

         (a) The Guarantee Trustee shall, within 90 days after the occurrence of
an Event of Default, transmit by mail, first class postage prepaid, to the
Holders, notices of all Events of Default actually known to the Guarantee
Trustee, unless such defaults have been cured before the giving of such notice,
provided, that, except in the case of a default in the payment of a Guarantee
Payment, the Guarantee Trustee shall be protected in withholding such notice if
and so long as the Board of Directors, the executive committee or a trust
committee of directors and/or Responsible Officers of the Guarantee Trustee in
good faith determines that the withholding of such notice is in the interests of
the Holders.

         (b) The Guarantee Trustee shall not be deemed to have knowledge of any
Event of Default unless the Guarantee Trustee shall have received written
notice, or a Responsible Officer charged with the administration of this
Guarantee Agreement shall have obtained written notice, of such Event of
Default.

         SECTION 2.8.      Conflicting Interests.

         The Trust Agreement shall be deemed to be specifically described in
this Guarantee Agreement for the purposes of clause (i) of the first proviso
contained in Section 310(b) of the Trust Indenture Act.


                                       5
<PAGE>   10

                                  ARTICLE III.

                 POWERS, DUTIES AND RIGHTS OF GUARANTEE TRUSTEE

         SECTION 3.1.      Powers and Duties of Guarantee Trustee.

         (a) This Guarantee Agreement shall be held by the Guarantee Trustee for
the benefit of the Holders, and the Guarantee Trustee shall not transfer this
Guarantee Agreement to any Person except a Holder exercising his or her rights
pursuant to Section 5.4(iv) or to a Successor Guarantee Trustee on acceptance by
such Successor Guarantee Trustee of its appointment to act as Successor
Guarantee Trustee. The right, title and interest of the Guarantee Trustee shall
automatically vest in any Successor Guarantee Trustee, upon acceptance by such
Successor Guarantee Trustee of its appointment hereunder, and such vesting and
cessation of title shall be effective whether or not conveyancing documents have
been executed and delivered pursuant to the appointment of such Successor
Guarantee Trustee.

         (b) If an Event of Default has occurred and is continuing, the
Guarantee Trustee shall enforce this Guarantee Agreement for the benefit of the
Holders.

         (c) The Guarantee Trustee, before the occurrence of any Event of
Default and after the curing of all Events of Default that may have occurred,
shall undertake to perform only such duties as are specifically set forth in
this Guarantee Agreement, and no implied covenants shall be read into this
Guarantee Agreement against the Guarantee Trustee. In case an Event of Default
has occurred (that has not been cured or waived pursuant to Section 2.6), the
Guarantee Trustee shall exercise such of the rights and powers vested in it by
this Guarantee Agreement, and use the same degree of care and skill in its
exercise thereof, as a prudent person would exercise or use under the
circumstances in the conduct of his or her own affairs.

         (d) No provision of this Guarantee Agreement shall be construed to
relieve the Guarantee Trustee from liability for its own negligent action, its
own negligent failure to act or its own willful misconduct, except that:

                  (i)      prior to the occurrence of any Event of Default and
         after the curing or waiving of all such Events of Default that may have
         occurred:

                           (A) the duties and obligations of the Guarantee
         Trustee shall be determined solely by the express provisions of this
         Guarantee Agreement, and the Guarantee Trustee shall not be liable
         except for the performance of such duties and obligations as are
         specifically set forth in this Guarantee Agreement; and

                           (B) in the absence of bad faith on the part of the
         Guarantee Trustee, the Guarantee Trustee may conclusively rely, as to
         the truth of the statements and the correctness of the opinions
         expressed therein, upon any certificates or opinions furnished to the
         Guarantee Trustee and conforming to the requirements of this Guarantee
         Agreement; but in the case of any such certificates or opinions that by
         any provision hereof or of the Trust Indenture Act are specifically
         required to be furnished to the Guarantee Trustee, the Guarantee
         Trustee shall be under a duty to examine the same to determine whether
         or not they conform to the requirements of this Guarantee Agreement;


                                       6
<PAGE>   11

                  (ii)     the Guarantee Trustee shall not be liable for any
         error of judgment made in good faith by a Responsible Officer of the
         Guarantee Trustee, unless it shall be proved that the Guarantee Trustee
         was negligent in ascertaining the pertinent facts upon which such
         judgment was made;

                  (iii)    the Guarantee Trustee shall not be liable with
         respect to any action taken or omitted to be taken by it in good faith
         in accordance with the direction of the Holders of not less than a
         Majority in Aggregate Liquidation Amount of the Preferred Securities
         relating to the time, method and place of conducting any proceeding for
         any remedy available to the Guarantee Trustee, or exercising any trust
         or power conferred upon the Guarantee Trustee under this Guarantee
         Agreement; and

                  (iv)     no provision of this Guarantee Agreement shall
         require the Guarantee Trustee to expend or risk its own funds or
         otherwise incur personal financial liability in the performance of any
         of its duties or in the exercise of any of its rights or powers, if the
         Guarantee Trustee shall have reasonable grounds for believing that the
         repayment of such funds or liability is not reasonably assured to it
         under the terms of this Guarantee Agreement or adequate indemnity
         against such risk or liability is not reasonably assured to it.

         SECTION 3.2.      Certain Rights of Guarantee Trustee.

         (a) Subject to the provisions of Section 3.1:

                  (i)      The Guarantee Trustee may rely and shall be fully
         protected in acting or refraining from acting upon any resolution,
         certificate, statement, instrument, opinion, report, notice, request,
         direction, consent, order, bond, debenture, note, other evidence of
         indebtedness or other paper or document reasonably believed by it to be
         genuine and to have been signed, sent or presented by the proper party
         or parties.

                  (ii)     Any direction or act of the Guarantor contemplated by
         this Guarantee Agreement shall be sufficiently evidenced by an
         Officers' Certificate unless otherwise prescribed herein.

                  (iii)    Whenever, in the administration of this Guarantee
         Agreement, the Guarantee Trustee shall deem it desirable that a matter
         be proved or established before taking, suffering or omitting to take
         any action hereunder, the Guarantee Trustee (unless other evidence is
         herein specifically prescribed) may, in the absence of bad faith on its
         part, request and rely upon an Officers' Certificate which, upon
         receipt of such request from the Guarantee Trustee, shall be promptly
         delivered by the Guarantor.

                  (iv)     The Guarantee Trustee may consult with legal counsel
         of its selection, and the advice or opinion of such legal counsel with
         respect to legal matters shall be full and complete authorization and
         protection in respect of any action taken, suffered or omitted to be
         taken by it hereunder in good faith and in accordance with such advice
         or opinion. Such legal counsel may be legal counsel to the Guarantor or
         any of its Affiliates and may be one of its employees. The Guarantee
         Trustee shall have the right at any time to seek instructions


                                       7
<PAGE>   12

         concerning the administration of this Guarantee Agreement from any
         court of competent jurisdiction.

                  (v)      The Guarantee Trustee shall be under no obligation to
         exercise any of the rights or powers vested in it by this Guarantee
         Agreement at the request or direction of any Holder, unless such Holder
         shall have provided to the Guarantee Trustee such adequate security and
         indemnity, as would satisfy a reasonable person in the position of the
         Guarantee Trustee, against the costs, expenses (including attorneys'
         fees and expenses) and liabilities that might be incurred by it in
         complying with such request or direction, including such reasonable
         advances as may be requested by the Guarantee Trustee; provided that,
         nothing contained in this Section 3.2(a)(v) shall be taken to relieve
         the Guarantee Trustee, upon the occurrence of an Event of Default, of
         its obligation to exercise the rights and powers vested in it by this
         Guarantee Agreement.

                  (vi)     The Guarantee Trustee shall not be bound to make any
         investigation into the facts or matters stated in any resolution,
         certificate, statement, instrument, opinion, report, notice, request,
         direction, consent, order, bond, debenture, note, other evidence of
         indebtedness or other paper or document, but the Guarantee Trustee, in
         its discretion, may make such further inquiry or investigation into
         such facts or matters as it may see fit.

                  (vii)    The Guarantee Trustee may execute any of the trusts
         or powers hereunder or perform any duties hereunder either directly or
         by or through its agents or attorneys, and the Guarantee Trustee shall
         not be responsible for any misconduct or negligence on the part of any
         such agent or attorney appointed with due care by it hereunder.

                  (viii)   Whenever in the administration of this Guarantee
         Agreement the Guarantee Trustee shall deem it desirable to receive
         written instructions with respect to enforcing any remedy or right or
         taking any other action hereunder, the Guarantee Trustee (A) may
         request instructions from the Holders, (B) may refrain from enforcing
         such remedy or right or taking such other action until such written
         instructions are received and (C) shall be protected in acting in
         accordance with such written instructions.

                  (ix)     The Guarantee Trustee shall not be liable for any
         action taken, suffered, or omitted to be taken by it in good faith and
         reasonably believed by it to be authorized or within the discretion or
         rights or powers conferred upon it by this Guarantee Agreement.

         (b) No provision of this Guarantee Agreement shall be deemed to impose
any duty or obligation on the Guarantee Trustee to perform any act or acts or
exercise any right, power, duty or obligation conferred or imposed on it in any
jurisdiction in which it shall be illegal, or in which the Guarantee Trustee
shall be unqualified or incompetent in accordance with applicable law, to
perform any such act or acts or to exercise any such right, power, duty or
obligation. No permissive power or authority available to the Guarantee Trustee
shall be construed to be a duty to act in accordance with such power and
authority.

         SECTION 3.3. Indemnity. The Guarantor agrees to indemnify the Guarantee
Trustee for, and to hold it harmless against, any loss, liability or expense
incurred without negligence or bad faith on the part of the Guarantee Trustee,
arising out of or in connection with the acceptance or


                                       8
<PAGE>   13

administration of this Guarantee Agreement, including the costs and expenses of
defending itself against any claim or liability in connection with the exercise
or performance of any of its powers or duties hereunder. The Guarantee Trustee
will not claim or exact any lien or payment on any Guarantee Payments as a
result of any amount due to it under this Guarantee Agreement.

                                  ARTICLE IV.

                                GUARANTEE TRUSTEE

         SECTION 4.1.      Guarantee Trustee: Eligibility.

         (a) There shall at all times be a Guarantee Trustee which shall:

                  (i)      not be an Affiliate of the Guarantor; and

                  (ii)     be a Person that is eligible pursuant to the Trust
         Indenture Act to act as such and has a combined capital and surplus of
         at least $50,000,000, and shall be a corporation meeting the
         requirements of Section 310(a) of the Trust Indenture Act. If such
         corporation publishes reports of condition at least annually, pursuant
         to law or to the requirements of the supervising or examining
         authority, then, for the purposes of this Section 4.1 and to the extent
         permitted by the Trust Indenture Act, the combined capital and surplus
         of such corporation shall be deemed to be its combined capital and
         surplus as set forth in its most recent report of condition so
         published.

         (b) If at any time the Guarantee Trustee shall cease to be eligible to
so act under Section 4. 1(a), the Guarantee Trustee shall immediately resign in
the manner and with the effect set out in Section 4.2(c).

         (c) If the Guarantee Trustee has or shall acquire any "conflicting
interest" within the meaning of Section 310(b) of the Trust Indenture Act, the
Guarantee Trustee and Guarantor shall in all respects comply with the provisions
of Section 310(b) of the Trust Indenture Act.

         SECTION 4.2. Appointment, Removal and Resignation of Guarantee Trustee.

         (a) Subject to Section 4.2(b), the Guarantee Trustee may be appointed
or removed without cause at any time by the Guarantor.

         (b) The Guarantee Trustee shall not be removed until a Successor
Guarantee Trustee has been appointed and has accepted such appointment by
written instrument executed by such Successor Guarantee Trustee and delivered to
the Guarantor.

         (c) The Guarantee Trustee appointed hereunder shall hold office until a
Successor Guarantee Trustee shall have been appointed or until its removal or
resignation. The Guarantee Trustee may resign from office (without need for
prior or subsequent accounting) by an instrument in writing executed by the
Guarantee Trustee and delivered to the Guarantor, which resignation shall not
take effect until a Successor Guarantee Trustee has been appointed and has
accepted such appointment by instrument in writing executed by such Successor
Guarantee Trustee and delivered to the Guarantor and the resigning Guarantee
Trustee.


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         (d) If no Successor Guarantee Trustee shall have been appointed and
accepted appointment as provided in this Section 4.2 within 60 days after
delivery to the Guarantor of an instrument of removal or resignation, the
resigning Guarantee Trustee may petition, at the expense of the Guarantor, any
court of competent jurisdiction for appointment of a Successor Guarantee
Trustee. Such court may thereupon, after prescribing such notice, if any, as it
may deem proper, appoint a Successor Guarantee Trustee.

                                    ARTICLE V.

                                    GUARANTEE

         SECTION 5.1. Guarantee. The Guarantor irrevocably and unconditionally
agrees to pay in full to the Holders the Guarantee Payments (without duplication
of amounts theretofore paid by or on behalf of the Trust), as and when due,
regardless of any defense, right of set-off or counterclaim which the Trust may
have or assert. The Guarantor's obligation to make a Guarantee Payment may be
satisfied by direct payment of the required amounts by the Guarantor to the
Holders or by causing the Trust to pay such amounts to the Holders.

         SECTION 5.2. Waiver of Notice and Demand. The Guarantor hereby waives
notice of acceptance of the Guarantee Agreement and of any liability to which it
applies or may apply, presentment, demand for payment, any right to require a
proceeding first against the Guarantee Trustee, the Trust or any other Person
before proceeding against the Guarantor, protest, notice of nonpayment, notice
of dishonor, notice of redemption and all other notices and demands.

         SECTION 5.3. Obligations Not Affected.

         The obligations, covenants, agreements and duties of the Guarantor
under this Guarantee Agreement shall in no way be affected or impaired by reason
of the happening from time to time of any of the following:

         (a) the release or waiver, by operation of law or otherwise, of the
performance or observance by the Trust of any express or implied agreement,
covenant, term or condition relating to the Preferred Securities to be performed
or observed by the Trust;

         (b) the extension of time for the payment by the Trust of all or any
portion of the Distributions (other than an extension of time for payment of
Distributions that results from the extension of any interest payment period on
the Debentures as provided in the Indenture), Redemption Price, Repurchase
Price, Liquidation Distribution or any other sums payable (including Additional
Amounts and/or Liquidated Damages, if applicable) under the terms of the
Preferred Securities or the extension of time for the performance of any other
obligation under, arising out of, or in connection with, the Preferred
Securities;

         (c) any failure, omission, delay or lack of diligence on the part of
the Holders to enforce, assert or exercise any right, privilege, power or remedy
conferred on the Holders pursuant to the terms of the Preferred Securities, or
any action on the part of the Trust granting indulgence or extension of any
kind;


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<PAGE>   15

         (d) the voluntary or involuntary liquidation, dissolution, sale of any
collateral, receivership, insolvency, bankruptcy, assignment for the benefit of
creditors, reorganization, arrangement, composition or readjustment of debt of,
or other similar proceedings affecting, the Trust or any of the assets of the
Trust;

         (e) any invalidity of, or defect or deficiency in, the Preferred
Securities;

         (f) the settlement or compromise of any obligation guaranteed hereby or
hereby incurred; or

         (g) any other circumstance whatsoever that might otherwise constitute a
legal or equitable discharge or defense of a guarantor, it being the intent of
this Section 5.3 that the obligations of the Guarantor hereunder shall be
absolute and unconditional under any and all circumstances.

         There shall be no obligation of the Holders to give notice to, or
obtain the consent of, the Guarantor with respect to the happening of any of the
foregoing.

         SECTION 5.4. Rights of Holders.

         The Guarantor expressly acknowledges that: (i) this Guarantee Agreement
will be deposited with the Guarantee Trustee to be held for the benefit of the
Holders; (ii) the Guarantee Trustee has the right to enforce this Guarantee
Agreement on behalf of the Holders; (iii) the Holders of a Majority in Aggregate
Liquidation Amount of the Preferred Securities have the right to direct the
time, method and place of conducting any proceeding for any remedy available to
the Guarantee Trustee in respect of this Guarantee Agreement or exercising any
trust or power conferred upon the Guarantee Trustee under this Guarantee
Agreement; and (iv) any Holder may institute a legal proceeding directly against
the Guarantor to enforce its rights under this Guarantee Agreement, without
first instituting a legal proceeding against the Guarantee Trustee, the Trust or
any other Person.

         SECTION 5.5. Guarantee of Payment.

         This Guarantee Agreement creates a guarantee of payment and not of
collection. This Guarantee Agreement will not be discharged except by payment of
the Guarantee Payments in full (without duplication of amounts theretofore paid
by the Trust) or upon distribution of Debentures to Holders as provided in the
Trust Agreement.

         SECTION 5.6. Subrogation.

         The Guarantor shall be subrogated to all (if any) rights of the Holders
against the Trust in respect of any amounts paid to the Holders by the Guarantor
under this Guarantee Agreement and shall have the right to waive payment by the
Trust pursuant to Section 5.1; provided, however, that the Guarantor shall not
(except to the extent required by mandatory provisions of law) be entitled to
enforce or exercise any rights which it may acquire by way of subrogation or any
indemnity, reimbursement or other agreement, in all cases as a result of payment
under this Guarantee Agreement, if, at the time of any such payment, any amounts
are due and unpaid under this Guarantee Agreement. If any amount shall be paid
to the Guarantor in violation of the preceding sentence, the Guarantor agrees to
hold such amount in trust for the Holders and to pay over such


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<PAGE>   16

amount to the Holders. Any amounts paid over to the Holders pursuant to this
Guarantee Agreement and not subsequently recovered from the Holders pursuant to
any insolvency law shall be deemed to have been applied by the Holders to the
Guarantee Payments

         SECTION 5.7. Independent Obligations. The Guarantor acknowledges that
its obligations hereunder are independent of the obligations of the Trust with
respect to the Preferred Securities and that the Guarantor shall be liable as
principal and as debtor hereunder to make Guarantee Payments pursuant to the
terms of this Guarantee Agreement notwithstanding the occurrence of any event
referred to in subsections (a) through (g), inclusive, of Section 5.3 hereof,
but subject to Section 6.1 hereof.

                                   ARTICLE VI.

                                  SUBORDINATION

         SECTION 6.1. Subordination.

         The obligations of the Guarantor under this Guarantee Agreement will
constitute unsecured obligations of the Guarantor and will rank subordinate and
junior in right of payment to all Senior Indebtedness (as defined in the
Indenture) of the Guarantor, except those made pari passu or subordinate to such
obligations expressly by their terms in the same manner as set forth in Article
XIII of the Indenture.

                                   ARTICLE VII.

                                   TERMINATION

         SECTION 7.1. Termination.

         This Guarantee Agreement shall terminate and be of no further force and
effect upon (i) full payment of the Redemption Price (including Additional
Amounts and any Liquidated Damages) of all Preferred Securities, (ii) the
distribution of Debentures to the Holders in exchange for all of the Preferred
Securities or (iii) full payment of the amounts payable in accordance with the
Trust Agreement upon liquidation of the Trust. Notwithstanding the foregoing,
this Guarantee Agreement will continue to be effective or will be reinstated, as
the case may be, if at any time any Holder must restore payment of any sums paid
with respect to Preferred Securities or this Guarantee Agreement.

                                  ARTICLE VIII.

                                  MISCELLANEOUS

         SECTION 8.1. Successors and Assign.

         All guarantees and agreements contained in this Guarantee Agreement
shall bind the successors, assigns, receivers, trustees and representatives of
the Guarantor and shall inure to the benefit of the Holders of the Preferred
Securities then outstanding. Except in connection with a consolidation, merger
or sale involving the Guarantor that is permitted under Article VIII of the


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<PAGE>   17

Indenture and pursuant to which the successor or assignee agrees in writing to
perform the Guarantor's obligations hereunder, the Guarantor shall not assign
its obligations hereunder.

         SECTION 8.2. Amendments.

         Except with respect to any changes which do not materially adversely
affect the rights of the Holders (in which case no consent of the Holders will
be required), this Guarantee Agreement may only be amended with the prior
approval of the Holders of not less than a Majority in Aggregate Liquidation
Amount of the Preferred Securities and of the Guarantee Trustee. The provisions
of Article VI of the Trust Agreement concerning meetings of the Holders shall
apply to the giving of such approval.

         SECTION 8.3. Notice.

         Any notice, request or other communication required or permitted to be
given hereunder shall be in writing, duly signed by the party giving such
notice, and delivered, telecopied or mailed by first class mail as follows:

         (a) if given to the Guarantor, to the address set forth below or such
other address, facsimile number or to the attention of such other Person as the
Guarantor may give notice to the Holders:

                          Caremark Rx, Inc.
                          3000 Galleria Tower
                          Suite 1000
                          Birmingham, Alabama 35244
                          Attention:  Chief Financial Officer

         (b) if given to the Trust, in care of the Guarantee Trustee, at the
Trust's (and the Guarantee Trustee's) address set forth below or such other
address as the Guarantee Trustee on behalf of the Trust may give notice to the
Holders:

                          Caremark Rx Capital Trust I
                          c/o Caremark Rx, Inc.
                          3000 Galleria Tower
                          Suite 1000
                          Birmingham, Alabama 35244
                          Attention:  Chief Financial Officer



         (c) if given to any Holder, at the address set forth on the books and
records of the Securities Registrar.

         All notices hereunder shall be deemed to have been given when received
in person, telecopied with receipt confirmed, or mailed by first class mail,
postage prepaid, except that if a notice or other document is refused delivery
or cannot be delivered because of a changed address of which no notice was
given, such notice or other document shall be deemed to have been delivered on
the date of such refusal or inability to deliver.


                                       13
<PAGE>   18

         SECTION 8.4. Benefit. This Guarantee Agreement is solely for the
benefit of the Holders (subject to the benefits inuring to the holders of Senior
Indebtedness pursuant to the subordination provisions hereof) and is not
separately transferable from the Preferred Securities.

         SECTION 8.5. Interpretation. In this Guarantee Agreement, unless the
context otherwise requires:

         (a) capitalized terms used in this Guarantee Agreement but not defined
in the preamble hereto have the respective meanings assigned to them in Section
1.1;

         (b) a term defined anywhere in this Guarantee Agreement has the same
meaning throughout;

         (c) all references to "the Guarantee Agreement" or "this Guarantee
Agreement" are to this Guarantee Agreement as modified, supplemented or amended
from time to time;

         (d) all references in this Guarantee Agreement to Articles and Sections
are to Articles and Sections of this Guarantee Agreement unless otherwise
specified;

         (e) a term defined in the Trust Indenture Act has the same meaning when
used in this Guarantee Agreement unless otherwise defined in this Guarantee
Agreement or unless the context otherwise requires;

         (f) a reference to the singular includes the plural and vice versa; and

         (g) the masculine, feminine or neuter genders used herein shall include
the masculine, feminine and neuter genders.

         SECTION 8.6. Governing Law.

         THIS GUARANTEE AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED AND
INTERPRETED IN ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK WITHOUT REGARD
TO THE CONFLICT OF LAW PRINCIPLES THEREOF.

         Section 8.7. Counterparts. This instrument may be executed in any
number of counterparts, each of which so executed shall be deemed to be an
original, but all such counterparts shall together constitute but one and the
same instrument.


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<PAGE>   19

         THIS GUARANTEE AGREEMENT is executed as of the day and year first above
written.

                                CAREMARK RX, INC.


                                By: /s/ James H. Dickerson, Jr.
                                   ---------------------------------------
                                Name:  James H. Dickerson, Jr.
                                Title: Executive Vice President and
                                       Chief Financial Officer

                                WILMINGTON TRUST COMPANY,
                                as Guarantee Trustee

                                By: /s/ Patricia A. Evans
                                   ---------------------------------------
                                Name:  Patricia A. Evans
                                Title: Financial Services Officer


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