<PAGE>
 
                                 EXHIBIT 10.18


                             CONSULTING AGREEMENT
                             --------------------

     THIS CONSULTING AGREEMENT is dated October 31, 1997 and is effective as of 
May 19, 1997 between AmeriSource Corporation, a Delaware corporation (the 
"Company") and John F. McNamara ("Mr. McNamara") (hereinafter, as amended or 
modified and in effect called the Agreement").

                                  BACKGROUND
                                  ----------

     Mr. McNamara has been Chairman, President and Chief Executive Officer of 
the Company since November 1988. On May 19, 1997, Mr. McNamara retired from all 
of his positions with the Company and its affiliates, other than his position as
a director on the board of the Company and the board of AmeriSource Health 
Corporation ("AmeriSource Health"). The Company has requested that Mr. McNamara
make himself available for consulting work for the Company and, subject to the 
terms and conditions of this Agreement, Mr. McNamara has agreed to do so. 
Mr. McNamara and the Company also wish to set forth their understandings 
regarding all of Mr. McNamara's other arrangements with the Company.

                                     TERMS
                                     -----

     INTENDING TO BE LEGALLY BOUND HEREBY, the parties agree as follows:

     1.   ENGAGEMENT. The Company hereby engages Mr. McNamara as a consultant 
          ----------
and Mr. McNamara agrees to accept such engagement pursuant to the terms of this 
Agreement. Mr. McNamara will perform such services in the capacity of Consultant
of the Company as may be assigned to him by the Board of Directors. Mr. McNamara
will devote such of his business skill, time and effort to his consulting 
hereunder as shall be reasonably necessary to discharge his obligations 
hereunder; provided, however, that Mr. McNamara shall not be required to devote 
           --------  -------
more than sixteen (16) hours per month in any single calendar month to the 
performance of services hereunder. Mr. McNamara shall perform such services at a
location mutually acceptable to the Company and Mr. McNamara. The Company will 
not provide Mr. McNamara office space to provide the contemplated consulting 
services.

     2.   TERM. Mr. McNamara shall perform the services required hereunder 
          ----
beginning May 19, 1997 and continuing until June 1, 2000 (the "Consulting 
Term"), unless this Agreement is terminated earlier due to death or disability 
or as provided in Section 6 hereof.
<PAGE>
 
          3.   COMPENSATION.
               ------------

               3.1  For his consulting services hereunder during the Consulting 
Term, Mr. McNamara shall receive the benefits payable under Section 4 hereof.

               3.2  In recognition of past services to the Company, and 
conditioned upon Mr. McNamara's complete compliance with the terms of this 
Agreement and Mr. McNamara's confirmation in writing on October 15, 1997 to the 
Company that this Agreement is in full force and effect, the Company shall pay 
to Mr. McNamara $2,500,000 on October 15, 1997. This payment shall be made 
notwithstanding the death or disability of Mr. McNamara prior to or subsequent 
to October 15, 1997.

               3.3  As an inducement to, and as consideration for, Mr. 
McNamara's obligations set forth in Sections 11 and 12 hereof, and conditioned 
upon Mr. McNamara's complete compliance with the terms of this Agreement, the 
Company agrees to pay Mr. McNamara (or his estate, heirs, or personal 
representative) $500,000 per year, payable every two weeks in arrears, for the 
period beginning May 17, 1997 and ending May 18, 2000. These payments shall 
continue notwithstanding the death or disability of Mr. McNamara prior to May 
18, 2000.

          4.   BENEFITS.           
               --------

               4.1  Mr. McNamara and, as applicable, Mr. McNamara's family, 
shall be entitled to participate during the Consulting Term in the Company's 
medical plan listed on Exhibit 1 hereto (as such plans may be amended, modified 
or replaced during the term of this Agreement for the most senior executives of 
the Company) in accordance with (S)4980B ("COBRA") of the Internal Revenue Code
of 1986. The premiums for such coverage shall be paid for by the Company. In
addition, during the Consulting Term the Company shall provide to Mr. McNamara
and, as applicable, Mr. McNamara's family, (a) coverage under the Company's life
insurance and disability plans listed on Exhibit 1, as such plans may be
amended, modified or replaced during the term if this Agreement for the most
senior executives of the Company, or (b) equivalent coverage.

               4.2  For the term of this Agreement, the Company (a) agrees to 
provide to Mr. McNamara split dollar insurance coverage with a death benefit of 
approximately $1,000,000 pursuant to the Flexible Premium Adjustable Life 
Policy, no. 1A2300590-0 previously entered into by the Company, and (b) agrees 
to pay annual premiums of up to $40,000 per year pursuant to such contract.

                                      -2-
<PAGE>
 
          5.   TERMINATION. The Company may terminate this Agreement and the 
               -----------
Consulting Term upon the occurrence of any of the following events:

               5.1  The death of Mr. McNamara;

               5.2  The disability of Mr. McNamara;

               5.3  The Company, by a vote of a majority of the directors of the
Company, determines in its sole discretion that after the date hereof Mr. 
McNamara has or is (i) violating any terms or provisions of this Agreement; 
and/or (ii) holding himself out in any manner as a representative or agent of 
the Company, its parents, subsidiaries, divisions, affiliates and/or their 
officers, directors, shareholders, employees or agents (other than as a director
and as a consultant); and/or (iii) making any statements or committing any acts 
that are in any way injurious or detrimental to the Company, its parent, 
subsidiaries, divisions, affiliates and/or their officers, directors, 
significant shareholders, employees, agents, suppliers or customers;

               5.4  Mr. McNamara having committed fraud, misappropriation or 
embezzlement against the Company; or

               5.5  Mr. McNamara's having committed any felony.

          For purposes of this Agreement, "Disability" shall mean that Mr. 
McNamara suffers from an illness or other physical or mental impairment which, 
in the judgment of the Company, prevents Mr. McNamara from performing his duties
hereunder for a period of one hundred eighty (180) consecutive days.

          6.   EFFECT OF TERMINATION.
               ---------------------

               6.1  Upon termination by the Company of this Agreement, (i) the 
Company's obligations under this Agreement shall terminate, except (A) 
continuation of the benefits under Section 4 hereof to the date of termination 
and payment of approved expenses to the date of termination, (B) for the 
provisions of Section 9, (C), in the case of a termination pursuant to Sections 
5.1, for the provisions of Sections 3.2 and 3.3. (D) in the case of a 
termination pursuant to Section 5.2, for the provisions of Sections 3.2, 3.3 and
4.1 and (E) all unexercised options under the Stock Option Plans shall terminate
and be forfeit, and (ii) Mr.McNamara's obligations under Section 1 of this 
Agreement shall terminate. Mr. McNamara's other obligations under this Agreement
shall not terminate but shall continue to be in full force and effect in
accordance with the terms of this Agreement.

                                      -3-
<PAGE>
 
               6.2  Upon the termination to Mr. McNamara's services with the 
Company for any reason Mr. McNamara shall promptly deliver to the Company all 
correspondence, notes, notebooks, reports, programs, proposals, price lists, 
customers lists and any documents concerning the Company's customers or its 
business and, without limiting the foregoing, will promptly deliver to the 
Company any and all other documents or materials containing or constituting 
Confidential Information.

          7.   STOCK OPTIONS. Exhibit 2 hereto lists the stock options granted 
               -------------
to Mr. McNamara under the AmeriSource Health's 1995 Stock Option Plan and the
AmeriSource Health's 1996 Stock Option Plan (collectively, the "Stock Option
Plans"). Mr. McNamara acknowledges that the stock options listed as "unvested"
on Exhibit 2 have terminated and are of no further force and effect.

          8.   EXPENSES. In the event Mr. McNamara is requested by the Company 
               --------
to travel on the Company's behalf and Mr. McNamara agrees to such travel, the 
Company will reimburse Mr. McNamara for reasonable expenses incurred in such 
travel, including travel costs, hotel expenses and meal expenses. Mr. McNamara 
shall not be reimbursed for any other expenses incurred for performing work 
under this Agreement, other than telephone, postage, copying and overnight 
courier charges, unless such expenses have been authorized in writing by the 
Company. The Company shall pay reimbursable expenses monthly upon receipt of a 
satisfactory bill and travel expense report, with appropriate receipts, in 
accordance with the Company's policies and procedures.

          9.   CONFIRMATION OF EXISTING PLANS. The Company confirms to Mr. 
               ------------------------------
McNamara that it will fulfill its obligations to Mr. McNamara under the 
AmeriSource Corporation Participating Companies Pension Plan (the "Pension 
Plan"), the Company's Supplemental Retirement Plan (the "SERP"), the Company's 
401(k) Employee Investment Plan and the Company's Deferred Compensation Plan in 
accordance with the provisions of those plans and applicable law. The Company 
agrees to amend the SERP to increase the payments that Mr. McNamara would 
otherwise be entitled to under the SERP (i) to give Mr. McNamara service credit 
under the SERP as if he had continued as an employee until October 15, 1997, and
(ii) to reflect the increase in payments he otherwise would have received under 
the Pension Plan as if he had continued as an employee until October 15, 1997.

                                     -4-
<PAGE>
 
          10.  CONFIDENTIAL INFORMATION.
               ------------------------

               10.1 Mr.McNamara acknowledges that by reason of his past 
employment and future consulting services with the Company he has and will 
hereafter, from time to time during the Consulting Term, become exposed to 
and/or become knowledgeable about proposals, business plans, practices, systems,
programs, processes, methods, techniques, research, records, supplier sources, 
customer lists, prices, profits and volume of sales, prospective customers and 
other forms of business information which are kept secret and confidential by 
the Company (the "Confidential Information"). Confidential Information shall not
include any information generally known to the public from a source other than 
Mr. McNamara. Mr. McNamara recognizes and acknowledges that: (i) the 
Confidential Information is the property of the Company; (ii) the use, 
misappropriation or disclosure of the Confidential Information would cause 
irreparable injury to the Company; and (iii) it is essential to the protection 
of the Company's good will and to the maintenance of the Company's competitive  
position that the Confidential Information be kept secret and that Mr. McNamara 
not disclose the Confidential Information to others or use the Confidential 
Information to Mr. McNamara's own advantage or the advantage of others.

               10.2 Mr. McNamara agrees to hold and safeguard the Confidential 
Information in trust for the Company, its successors and assigns and agrees that
he shall not, without the prior written consent of the Company, disclose or make
available to anyone for use outside the Company at any time, either during the 
Consulting Term with the Company or subsequent to the termination of his 
services with the Company for any reason, any of the Confidential Information, 
whether or not developed by Mr. McNamara, except (i) as required in the 
performance of Mr. McNamara's duties to the Company and then, only to the extent
previously approved in writing by the Company or (ii) to the extent required by 
law or a court order (and then only after giving the Company prompt notice and 
cooperating with the Company in its efforts to obtain a protective order).

          11.  COMPETITION: SOLICITATION. In consideration of the mutual terms
               -------------------------
and agreements set forth in this Agreement, Mr. McNamara hereby agrees that (a) 
during the Consulting Term, (b) while Mr. McNamara is receiving any payments or 
benefits from the Company, and (c) for a period of one year after the expiration
or termination of Mr. McNamara's Consulting Term, he will not, unless authorized
in writing to do so by the Company, directly or indirectly own, manage, operate,
join, control or participate in the ownership, management, operation or control 
of, or be employed or otherwise connected in any substantial manner with any 
business

                                      -5-
                
<PAGE>
 
which directly or indirectly competes to a material extent with any line of 
business of the Company or its subsidiaries; provided, that nothing in this 
                                             --------
paragraph shall prohibit Mr. McNamara from acquiring up to 2% of any class of 
outstanding equity securities of any corporation whose equity securities are 
regularly traded on a national securities exchange or in the "over-the-counter 
market."  Mr. McNamara agrees that for a period ending one year after Mr.
McNamara's Consulting Term is terminated or expires hereunder, Mr. McNamara will
not (i) recruit any employee of the Company or solicit or induce, or attempt to 
solicit or induce, any employee of the Company to terminate his or her
employment with, or otherwise cease his or her relationship with, the Company,
or (ii) solicit, divert or take away, or attempt to solicit, divert or to take
away, the business or patronage of any of the clients, customers or accounts, or
prospective clients, customers or accounts, of the Company.

          12.  ADDITIONAL CONVENANTS BY MR. MCNAMARA.
               -------------------------------------

               12.1 During the Consulting Term and thereafter, Mr. McNamara will
not make any statements or commit any acts that are in any way injurious or 
detrimental to the Company or any of its affiliates, employees, directors, 
significant shareholders' images, businesses, operations, employees or relations
with employees, suppliers or customers.

               12.2 Mr. McNamara shall comply with all securities laws relating 
to his ownership of shares of Class A Common Stock and his ownership of options 
to acquire Class A Common Stock of AmeriSource Health, including any 
prohibitions related to trading while in the possession of inside information 
and shall comply with the policies and procedures of AmeriSource Health related 
to the trading of securities that apply to the most senior executives of the 
Company that were provided to him prior to May 19, 1997 or have been sent to him
in the manner set forth in Section 17.2 hereof.

               12.3 Mr. McNamara shall continue to comply with the terms and 
conditions of the Confidentiality Agreement executed by him on November 18, 1996
attached as Exhibit 4 hereto.

               12.4 Capitalized terms used in this Section 12 that are not 
otherwise defined in this Agreement are defined in Exhibit 3 hereto. During the 
period from the date hereof through June 1, 2000, Mr. McNamara agrees that he 
will not, and will cause each of his Affiliates not to, directly or indirectly, 
individually or as part of a "partnership," "limited partnership," 
"syndicate" or other "group" (as those terms are defined in Section 13(d)(3) of 
the Exchange Act as in effect on the date hereof), or otherwise acting in 
concert with others, unless in any such case specifically

                                      -6-
<PAGE>
 
invited in writing to do so by the Board of Directors of the Company as 
specifically expressed in a resolution adopted by a majority of the directors of
the Company:

                    (1)  other than pursuant to the exercise of stock options, 
acquire, offer or propose to acquire, or agree to acquire, whether by purchase, 
tender or exchange offer, gift or otherwise (any such act, to "acquire"), 
beneficial ownership of any additional Voting Securities or any rights to 
acquire (whether currently, upon lapse of time, following the satisfaction of 
any conditions, upon the occurrence of any event or any combination of the 
foregoing) any Voting Securities, except pursuant to a stock split, stock 
dividend, rights offering, recapitalization, reclassification or similar 
transaction made available to holders of any Voting Securities generally;

                    (2)  make, or in any way participate in any "solicitation" 
of "proxies" to vote (as such terms are defined or used in Regulation 14A under 
the Exchange Act as in effect on the date hereof), solicit any consent or 
communicate with or seek to advise, encourage or influence any person (other 
than members of his immediate family or trusts solely for the benefit of 
immediate family members) or entity (whether or not relating to the election or 
removal of directors) with respect to the voting of any Voting Securities or
become a "participant" in any "election contest" (as such terms are defined or 
used in Rule 14a-11 under the Exchange Act as in effect on the date hereof) with
respect to AmeriSource Health or the Company, or execute any written consent in 
lieu of a meeting of holders of Voting Securities or any class thereof;

                    (3)  form, join, participate in or encourage the formation 
of, any "person" or "group" within the meaning of Section 13(d)(3) of the 
Exchange Act, as in effect as of the date hereof, with respect to any Voting 
Securities;

                    (4)  deposit any Voting Securities into a voting trust or 
subject any such Voting Securities to a voting agreement or any other 
arrangement or agreement with respect to the voting thereof;

                    (5)  initiate, propose or otherwise solicit shareholders for
the approval of one or more shareholder proposals with respect to AmeriSource 
Health or the Company whether made pursuant to Rule 14a-8 under the Exchange Act
(or any successor rule) or otherwise, or induce or attempt to induce any other 
person to initiate any such shareholder proposal;

                    (6)  petition or request the Board of Directors of 
Amerisource Health or the Company to amend the Articles of

                                      -7-
<PAGE>
 
Incorporation or Bylaws of AmeriSource Health or the Company, or induce or 
attempt to induce any other person to initiate any such petition or request;

                    (7)  call or seek to have called any meeting of the 
shareholders of the Company;

                    (8)  otherwise act, alone or in concert with others, or 
solicit, propose, seek to effect, encourage or negotiate with or provide any 
information to any other person with respect to, or make any statement 
regarding, any form of business combination transaction (including without 
limitation any merger, tender offer, exchange offer or other Takeover Proposal) 
with the Company or any Affiliate thereof, or any restructuring,
recapitalization, liquidation or similar transaction with respect to the Company
or any Affiliate thereof, or solicit, make or propose or encourage or negotiate
with any other person with respect to, or announce an intent to make, any tender
offer or exchange offer for any Voting Securities or disclose an intent purpose,
plan or proposal with respect to the Company or any Voting Securities
inconsistent with the provisions of this Agreement, including an intent,
purpose, plan or proposal that is conditioned on or would require the Company to
waive the benefit of or amend any provision of this Agreement, or assist,
participate in, facilitate, instigate, encourage or solicit any effort or
attempt by any person to do or seek to do any of the actions specified in this
Section 12.4;

                    (9)  enter into any transaction, agreement, arrangement or 
understanding that, as a result of the consummation of such transaction or the 
fulfillment of such agreement, arrangement or understanding, would substantially
diminish or otherwise eliminate the benefits intended to be afforded by this 
Agreement (other than Mr. McNamara accepting full-time employment not prohibited
by Section 11 of this Agreement);

                    (10) disclose any intention, plan or arrangement
inconsistent with the provisions of this Section 12.4; or

                    (11) instigate, solicit, facilitate or encourage any effort 
or attempt by, or render advice to make any recommendation or proposal to, any 
third party to do or seek to do any of the actions specified in this Section 
12.4.

               12.5 If at any time during the term of this Agreement, Mr. 
McNamara or any of his Affiliates are approached by any party concerning a 
Takeover Proposal or any other business combination transaction involving the 
Company, its assets,

                                      -8-
<PAGE>
 
businesses or securities, Mr. McNamara will promptly inform the Company of the 
nature of such contact, the parties involved and any other information 
concerning such transaction which the Company may reasonably request.

          13.  GENERAL RELEASE.
               ---------------

               13.1 In consideration of the mutual benefits contained in this
Agreement and intending to be legally bound, Mr. McNamara hereby releases and
discharges the Company, and each of its affiliates, parents, subsidiaries,
successors, and predecessors, and all of their employees, agents,
representatives, shareholders, officers and directors (hereinafter collectively
referred to as, the "Company Released Parties") from any and all claims and/or
causes of action, known or unknown, asserted or unasserted, that he may have or
could claim to have against any of the Company Released Parties up to and
including the date of this Agreement, including, but not limited to, all claims
arising from or during Mr. McNamara's past employment (including, but not limit
to, any and all claims with respect to compensation, benefits, right to
continued employment or other terms of employment), or as a result of all claims
arising under federal, state or local laws prohibiting employment discrimination
based upon age, race, sex, religion, handicap, national origin or any other
protected characteristic, including, but not limited to, any and all claims
arising under the Age Discrimination in Employment Act, Title VII of the Civil
Rights Act of 1964, and/or claims growing out of any legal restrictions,
expressed or implied, on any Company Released Party's right to control or
terminate the employment of its employees.

               13.2 By initialing the space provided immediately below, Mr.
McNamara acknowledges that he has carefully read and fully understands the
provisions of this Section 13 -"GENERAL RELEASE". Mr. McNamara further
acknowledges that he is agreeing to this general release knowingly and
voluntarily and without duress, coercion or undue influence. Mr. McNamara
further agrees that should he file a lawsuit in court or commence any similar
legal proceeding that is found to be barred in whole or part by this general
release, he will be in breach of this Agreement and he will pay the legal fees
incurred by or on behalf of the applicable Released Party in defending those
claims found to be barred.

                                        Agreed and Initialed:

                                        /s/ John F. McNamara 
                                        --------------------
                                        John F. McNamara 

               13.3 In consideration of the mutual benefits contained in this 
Agreement and intending to be legally bound, the

                                      -9-
<PAGE>
 
Company hereby releases and discharges Mr. McNamara, his successors and
representatives (hereinafter collectively referred to as, the "McNamara Released
Parties") from any and all claims and/or causes of action, known or unknown,
asserted or unasserted, that it may have or could claim to have against any of
the McNamara Released Parties up to and including the date of this Agreement,
including but not limited to, all claims arising from or during Mr. McNamara's
past employment.

          14.  EQUITABLE RELIEF. The Company and Mr. McNamara confirm that the 
               ----------------
restrictions contained in Sections 10, 11, 12 and 13 hereof are, in view of the 
nature of the business of the Company, reasonable and necessary to protect the 
legitimate interests of the Company and that any violation of any provision of 
Sections 10, 11, 12 or 13 will result in irreparable injury to the Company. Mr 
McNamara hereby agrees that, in the event of any breach or threatened breach of 
the terms or conditions of this Agreement by Mr. McNamara, the Company's 
remedies at law will be inadequate and, in any such event, the Company shall be
entitled to commence an action for preliminary and permanent injunctive relief 
and other equitable relief in any court of competent jurisdiction.

          15.  ARBITRATION. Except with respect to actions for preliminary and 
               -----------
permanent injunctive relief and other equitable relief under Section 14, any 
controversy or claim arising out of or relating to this Agreement, or any breach
thereof, shall be settled by arbitration in accordance with the rules of the 
American Arbitration Association, and judgment upon such award rendered by the
arbitrator(s) may be entered in any court having jurisdiction thereof. The 
arbitration shall be held in Philadelphia, Pennsylvania, unless another location
shall be mutually agreed to by the parties at the time of the arbitration. In
any dispute between the parties as to which Mr. McNamara is sustained on the
claim(s) by or against him, the Company shall pay all legal fees incurred by Mr.
McNamara in connection with the dispute over such claim(s). If more than one
claim is involved in any dispute and if Mr. McNamara is sustained as to one or
more of such claims, but not as to all of such claims there shall be a
reasonable allocation of applicable legal expenses. The Company will reimburse
Mr. McNamara for those legal expenses determined by the arbitrator(s) or by the
consent of the parties to be allocable to the claim or claims as to which Mr.
McNamara is upheld.

          16.  INDEPENDENT CONTRACTOR. Mr.McNamara shall be an independent 
               ----------------------
contractor and not an employee of the Company. Nothing contained herein shall 
serve to constitute a relationship of partnership or joint venture between the 
parties. Mr. McNamara shall not be an agent of the Company and shall have no 
power to bind or to otherwise obligate the Company in any manner whatsoever

                                     -10-
<PAGE>
 
nor shall Mr. McNamara be authorized to enter into agreements or any other 
contractual relationships on behalf of the Company, without the express prior 
written consent of the Board of Directors of the Company and Mr. McNamara shall 
not hold himself out as having any such authority.

     17.  ADDITIONAL PROVISIONS.
          ---------------------

          17.1 Mr. McNamara's undertakings hereunder will be binding regardless 
of (i) the duration of the Consulting Term with the Company; or (ii) the reasons
for or manner of termination of this Agreement. Mr. McNamara's rights under this
Agreement shall not, in any voluntary or involuntary manner (other than the laws
of descent and distribution), be assignable and may not be pledged or 
hypothecated.
          
          17.2 All notices hereunder shall be given in writing by personal 
delivery by telecopy or by overnight mail addressed to the Company at its 
principal place of business and to Mr. McNamara at his residence address as then
listed in the Company's records.

          17.3 This Agreement shall be governed by and construed in accordance 
with the laws of the Commonwealth of Pennsylvania. Mr. McNamara and the Company 
further irrevocably consent to the exclusive jurisdiction of any Pennsylvania 
state court located in Chester County, Pennsylvania or Federal court located in
the Eastern District of Pennsylvania over any suit, action or proceeding arising
out of or relating to this Agreement and hereby waive, to the fullest extent 
permitted by law, any objection that they may now or hereafter have to such 
jurisdiction or to the laying of venue of any such suit, action or proceeding 
brought in such a court and any claim that such suit, action or proceeding has 
been brought in an inconvenient forum.
     
          17.4 The waiver by the Company of a breach of any provision of this
Agreement by Mr. McNamara shall not operate or be construed as a waiver of any
other or subsequent breach by Mr. McNamara of such or any other provision. No
delay or omission by the Company in exercising any right, remedy or power
hereunder or existing at law or in equity shall be construed as a waiver
thereof, and any such right, remedy or power may be exercised by the Company
from the time to time and as often as may be deemed expedient or necessary by
the Company in its sole discretion.

          17.5 If any term or provision of this Agreement or the application 
thereof to any person or circumstances shall, to any extent, be held invalid or 
unenforceable by a court of competent jurisdiction, the remainder of this 
Agreement or the application of any such term or provision to persons or 
circumstances other than

                                     -11-
<PAGE>
 
those as to which it is held invalid or unenforceable, shall not be affected 
thereby, and each term and provision of this Agreement shall be valid and 
enforceable to the fullest extent permitted by law. If any of the provisions 
contained in this Agreement shall for any reason be held to be excessively broad
as to duration, scope, activity or subject, it shall be construed by limiting 
and reducing it, so as to be valid and enforceable to the extent compatible with
the applicable law or the determination by a court of competent jurisdiction.

               17.6 This Agreement may be executed in any number of 
counterparts, each of which shall be deemed to be an original, but all of which 
together shall constitute one and the same instrument. The section headings of 
this Agreement are for convenience of reference only.

               17.7 This instrument constitutes the entire agreement with 
respect to the subject matter hereof between the parties hereto and replaces and
supersedes as of the date hereof any and all prior oral or written agreements 
and understandings between the parties hereto. This Agreement may only be 
modified by an agreement in writing executed by both Mr. McNamara and the 
Company.

          18.  REVOCATION RIGHTS. MR. MCNAMARA ACKNOWLEDGES THAT THIS IS A VERY 
               -----------------
IMPORTANT DOCUMENT AND THAT HE SHOULD CAREFULLY REVIEW AND UNDERSTAND THE TERMS 
AND EFFECT OF THIS AGREEMENT BEFORE SIGNING IT. MR. MCNAMARA FURTHER
ACKNOWLEDGES THAT BY SIGNING THIS AGREEMENT HE IS AGREEING, AMONG OTHER THINGS,
TO COMPLETELY RELEASE THE RELEASED PARTIES FROM ALL LIABILITY TO HIM OTHER THAN
THE COMPANY'S OBLIGATIONS SET FORTH IN THIS AGREEMENT AND THAT HE SHOULD CONSULT
WITH AN ATTORNEY BEFORE SIGNING THIS AGREEMENT. MR. MCNAMARA SHALL HAVE 21 DAYS
FROM THE DATE OF ITS DISTRIBUTION TO CONSIDER THIS AGREEMENT. IF MR. MCNAMARA
HAS NOT DELIVERED A SIGNED COPY OF THIS AGREEMENT TO COMPANY BY THAT TIME,
COMPANY WILL ASSUME THAT HE HAS ELECTED NOT TO SIGN THIS AGREEMENT. IF MR.
MCNAMARA CHOOSES TO SIGN THIS AGREEMENT, HE WILL HAVE AN ADDITIONAL SEVEN (7)
DAYS FOLLOWING THE DATE OF HIS SIGNATURE TO REVOKE THIS AGREEMENT. NEITHER THIS
AGREEMENT NOR ANY PROVISION HEREOF, INCLUDING, BUT NOT LIMITED TO, COMPANY'S
OBLIGATIONS TO MAKE ANY PAYMENTS TO MR. MCNAMARA, SHALL BECOME EFFECTIVE OR
ENFORCEABLE UNTIL THIS SEVEN-DAY REVOCATION PERIOD HAS EXPIRED AND UPON THE
EXPIRATION OF SUCH SEVEN-DAY PERIOD, COMPANY SHALL PAY ALL AMOUNTS RETROACTIVELY
DUE TO MR. MCNAMARA FROM THE DATE FIRST WRITTEN ABOVE.

                                     -12-
<PAGE>
 
     IN WITNESS WHEREOF, the parties hereto have executed this Agreement as of
the date first above written.

                                             AMERISOURCE CORPORATION 

                                             By:[SIGNATURE ILLEGIBLE]
                                                --------------------------  
                                                  Title:

                                             /s/ John F. McNamara 
                                             -----------------------------
                                             John F. McNamara 10/13/97     

                                     -13-
<PAGE>
 
     IN WITNESS WHEREOF, the parties hereto have executed this Agreement as of
the date first above written.

                                             AMERISOURCE CORPORATION 

                                             By:[SIGNATURE ILLEGIBLE]
                                                --------------------------  
                                                  Title:

                                             _____________________________
                                             John F. McNamara 

                                     -13-
<PAGE>
 
                                   Exhibit 1
                                   ---------

1.   AmeriSource Corporation Managed Medical Plan (CIGNA HealthCare, Claims 
     Administrator

2.   AmeriSource Corporation Group Long Term Disability Plan (Liberty Life
     Assurance Company of Boston)

3.   AmeriSource Corporation Group Life Insurance (Liberty Life Assurance
     Company of Boston)
<PAGE>
 
                                   Exhibit 2
                                   ---------


<TABLE> 
<CAPTION> 
Date       Exercise  Plan        Shares    Vested     Unvested
                     ----                  ------     --------
Granted    Price                 Granted 
-------    -----                 -------
<S>        <C>       <C>         <C>       <C>        <C> 
4/03/95    $21.00   1995 Stock   100,000   50,000     50,000 
                    Option Plan  

3/01/96    $28.00   1995 Stock   40,000    10,000     30,000 
                    Option Plan  

11/12/96   $48.625  1996 Stock   40,000    -0-        40,000 
                    Option Plan  
</TABLE> 
                    
                    
                                      -1-
<PAGE>
 
                                   Exhibit 3
                                   ---------

          DEFINITIONS.   For purposes of Section 12 of this Agreement, the 
          -----------
following terms shall have the following meanings:

                    (1)  Affiliate.  An "Affiliate" of a Person shall have the 
                         ---------
meaning set forth in Rule 12b-2 of the Exchange Act as in effect on the date 
hereof and, in addition, shall include "Associates" (as defined in Rule 12b-2 of
the Exchange Act as in effect on the date hereof) of such Person and its 
Affiliates.

                    (2)  Beneficial Owner.  A Person shall be deemed to 
                         ----------------
"beneficially own," or to have "beneficial ownership" of, any Voting Securities
in accordance with the term "beneficial ownership" as defined in Rule 13d-3 
under the Exchange Act as in effect on the date hereof and, in addition, such 
terms shall include securities which such Person has the right to acquire 
(irrespective of whether such right is exercisable immediately or only after the
passage of time) pursuant to any agreement, arrangement or understanding or upon
the exercise of conversion rights, exchange rights, warrants or options, or 
otherwise.

                    (3)  Exchange Act.  "Exchange Act" shall mean the Securities
                         ------------
Exchange Act of 1934, as amended.

                    (4)  Person.  "Person" shall mean any individual, group (as 
                         ------
such term is defined in Section 13(d)(3) of the Exchange Act as in effect on the
date hereof), corporation, partnership, firm, government or agency or political
subdivision thereof, or other entity of whatever nature.

                    (5)  Takeover Proposal.  "Takeover Proposal" shall mean any 
                         -----------------
solicited or unsolicited tender or exchange offer, proposal for a merger, share
exchange or other business combination involving AmeriSource Health or the
Company or any of its material subsidiaries or any proposal or offer to acquire
in any manner 20% or more of the outstanding Common Stock or Voting Securities
of the Company or AmeriSource Health, or a substantial equity interest in any of
the Company's material subsidiaries or a substantial portion of the assets of
AmeriSource Health, the Company or any of its material subsidiaries.

                    (6)  Voting Securities.  "Voting Securities" shall mean (x) 
                         -----------------
any securities entitled, or which may be entitled, to vote generally in the 
election of directors of the Company or AmeriSource Health, (y) any securities 
convertible or exercisable into or exchangeable for such securities (whether or 
not the right to convert, exercise or exchange is subject to the passage of time

                                      -2-
<PAGE>
 
or contingencies or both), or (z) any direct or indirect rights or options to 
acquire any such securities.

                                      -3-