<PAGE>
                                                                   Exhibit 10(a)

                       FIRST AMENDMENT TO CREDIT AGREEMENT

     THIS FIRST AMENDMENT TO CREDIT AGREEMENT, dated as of September 30, 2005
(this "Amendment"), is among Perrigo Company (the "U.S. Borrower"), the Foreign
Subsidiary Borrowers party hereto, the Lenders party hereto, JPMorgan Chase
Bank, N.A., as Administrative Agent for the Lenders (in such capacity, the
"Administrative Agent"), Bank Leumi USA, as Syndication Agent, and Bank of
America, N.A., Standard Federal Bank N.A. and National City Bank of the Midwest,
as Documentation Agents.

                                     RECITAL

     The Borrowers, the Lenders, the Administrative Agent, the Syndication Agent
and the Documentation Agents are parties to a Credit Agreement dated as of March
16, 2005 (the "Credit Agreement"). The Borrowers desire to amend the Credit
Agreement as set forth herein and the Lenders are willing to do so in accordance
with the terms hereof.

                                      TERMS

     In consideration of the premises and of the mutual agreements herein
contained, the parties agree as follows:

                                   ARTICLE 1.
                                   AMENDMENTS.

     The Credit Agreement is amended as follows:

     1.1 The following definitions are added to Section 1.01 in appropriate
alphabetical order:

     "Israeli Borrower" means, collectively, Agis and/or Agis Commercial
Agencies (1989) Ltd. and all other Foreign Subsidiary Borrowers organized under
the laws of the State of Israel, provided that, regarding each Israeli Borrower,
there shall be no limitation or restriction under Israeli law and regulations of
the Bank of Israel or of other regulatory entities, to the granting of the
Swingline Shekel Loans by the Swingline Shekel Lender to it, and further
provided that, regarding each such other Foreign Subsidiary Borrower, its
financial condition shall, in the opinion of the Swingline Shekel Lender,
support and justify the Swingline Shekel Loans requested by it in the amounts
requested by it..

     "Swingline Shekel Lender" means Bank Leumi, or any of its Affiliates
designated as the Swingline Shekel Lender hereunder by Bank Leumi in writing to
the Administrative Agent, in its capacity as the Swingline Lender of Swingline
Shekel Loans. Bank Leumi hereby designates Bank Leumi le-Israel B.M., an
Affiliate of Bank Leumi and the parent company of Bank Leumi, as the initial
Swingline Shekel Lender.

     "Swingline Shekel Loan" means a Swingline Loan denominated in Shekels or a
Swingline Loan otherwise made to an Israeli Borrower.

     1.2 The definitions Agis, Business Day, Foreign Currency, Local Time and
Swingline Lender in Section 1.01 are restated as follows:
<PAGE>
     "Agis" means Perrigo Israel Pharmaceuticals Ltd., formerly known as Agis
Industries (1983) Ltd., an Israeli public company.

     "Business Day" means any day that is not a Saturday, Sunday or other day on
which commercial banks in New York City or Chicago are authorized or required by
law to remain closed; provided that, (i) when used in connection with a
Eurocurrency Loan, the term "Business Day" shall also exclude any day on which
banks are not open for dealings in deposits in the currency in which such
Eurocurrency Loan is denominated in the London interbank market, (ii) when used
in connection with an Ancillary Loan, the term "Business Day" shall mean any day
defined as a Business Day in the relevant Ancillary Facility Document, and (iii)
when used in connection with a Swingline Shekel Loan, the term "Business Day"
shall mean any day on which banks generally are open in Israel for the conduct
of substantially all of their commercial lending activities.

     "Foreign Currency" means (a) with respect to an Ancillary Loan, any
currency acceptable to the Administrative Agent that is freely available, freely
transferable and freely convertible into Dollars, including the Shekel and the
Peso, and agreed to by the Ancillary Lenders making such Ancillary Loan, (b)
with respect to any Revolving Loan, Euros, Sterling and any other currency
acceptable to the Administrative Agent that is freely available, freely
transferable and freely convertible into Dollars and in which dealings in
deposits are carried on in the London interbank market, (c) with respect to any
Letter of Credit, any currency acceptable to the Administrative Agent that is
freely available, freely transferable and freely convertible into Dollars, and
agreed to by the Issuing Bank issuing such Letter of Credit, (d) with respect to
any Swingline Foreign Currency Loan other than Swingline Shekel Loans, any
currency acceptable to the Administrative Agent that is freely available, freely
transferable and freely convertible into Dollars and agreed to by JPMorgan or
any of its Affiliates in its capacity as a Swingline Lender and (e) with respect
to any Swingline Shekel Loans, Shekels (so long as Shekels are freely available,
freely transferable and freely convertible into Dollars) or any other currency
acceptable to the Administrative Agent that is freely available, freely
transferable and freely convertible into Dollars and agreed to by the Swingline
Shekel Lender.

     "Local Time" means (a) with respect to a Loan or Borrowing denominated in
Dollars, Chicago time, (b) with respect to a Loan or Borrowing denominated in
any Foreign Currency (other than an Ancillary Loan or Swingline Shekel Loan),
London time, (c) with respect to a Swingline Shekel Loan, Israeli time, and (d)
with respect to an Ancillary Loan, such time as designated as the local time in
the relevant Ancillary Facility Documents.

     "Swingline Lender" means each of JPMorgan, in its capacity as lender of
Swingline Loans hereunder, and the Swingline Shekel Lender, and their respective
successors in such capacity. Each Swingline Lender may, in its discretion,
arrange for one or more Swingline Loans to be made by Affiliates of such
Swingline Lender, in which case the term "Swingline Lender" shall include any
such Affiliate with respect to Swingline Loans made by such Affiliate.
References herein to the Swingline Lender shall be deemed references to the
Swingline Lender that made the relevant Swingline Loan.

     1.3 Reference in clause (b) of the definition of Interest Period to "which
shall not be later than thirty days thereafter" shall be amended by adding the
following words thereafter "unless, in the case of Swingline Shekel Loans,
otherwise agreed between the Swingline Shekel Lender and the Israeli Borrower in
respect of any particular Swingline Shekel Loan or Loans, and unless, in the
case of any


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other Swingline Loan, otherwise agreed between the applicable Swingline Lender
thereof and the Borrower thereof in respect of such Swingline Loans,".

     1.4 Section 2.01(a)(iii) is restated as follows:

     (iii) the Dollar Equivalent of the aggregate amount of all Revolving Loans
and Swingline Loans denominated in Foreign Currencies exceeding $55,000,000.

     1.5 Section 2.04 is restated as follows:

     SECTION 2.04. Swingline Loans. (a) Subject to the terms and conditions set
forth herein, a Swingline Lender may make Swingline Loans to any Borrower from
time to time during the Availability Period, in an aggregate principal amount at
any time outstanding that will not result in (i) the Dollar Equivalent of the
aggregate principal amount of outstanding Swingline Loans, other than Swingline
Shekel Loans, exceeding $25,000,000, (ii) the Dollar Equivalent of the aggregate
principal amount of outstanding Swingline Shekel Loans exceeding $15,000,000,
(iii) the (A) Aggregate Revolving Credit Exposure exceeding (B) the Aggregate
Revolving Commitments minus the Aggregate Ancillary Commitments or (iv) the
Aggregate Total Revolving Exposure exceeding the Aggregate Revolving
Commitments. Notwithstanding the foregoing, the Swingline Shekel Lender may only
make Swingline Shekel Loans and shall not make any other Swingline Loans, and
any other Swingline Lender may not make Swingline Shekel Loans. Within the
foregoing limits and subject to the terms and conditions set forth herein, such
Borrowers may borrow, prepay and reborrow Swingline Loans.

     (b) To request a Swingline Borrowing:

     (i) in the case of a Swingline Loan denominated in Dollars to the U.S.
Borrower, the applicable Borrower shall notify the Applicable Agent of such
request by telephone (confirmed in a writing acceptable to the Applicable Agent
if requested by the Applicable Agent), not later than 2:00 p.m. (or such other
time agreed to by the U. S. Borrower and the applicable Swingline Lender),
Chicago time, on the day of such proposed Swingline Loan,

     (ii) in the case of a Swingline Shekel Loan, the Israeli Borrower shall
notify the Swingline Shekel Lender directly by telephone or in writing (in
accordance with the standard borrowing procedures and written terms and
conditions signed by Agis in favor the Swingline Shekel Lender prior to the date
hereof) by 2:00 p.m. (or such later time agreed to by the Israeli Borrower and
the Swingline Shekel Lender), Local Time, on the day of a proposed Swingline
Shekel Loan, and

     (iii) in the case of any other Swingline Loan, the applicable Borrower
shall notify the Applicable Agent of such request by telephone (confirmed in a
writing acceptable to the Applicable Agent if requested by the Applicable
Agent), not later than 10:00 a.m. (or such other time agreed to by the
applicable Borrower and such Swingline Lender), Local time, on the day of such
proposed Swingline Loan.

Each such notice shall be irrevocable and shall specify (A) the requested date
(which shall be a Business Day), (B) whether such Swingline Loan is to be
denominated in Dollars or a Foreign Currency, (C) the amount of the requested
Swingline Borrowing, and (D) in the case of a Swingline Borrowing denominated in
a Foreign Currency, the Interest Period requested to be applicable thereto,
which shall be


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a period contemplated by clause (b) of the definition of the term "Interest
Period". The Applicable Agent shall promptly advise JPMorgan or the Affiliate
designated by JPMorgan for such Swingline Borrowing of any such notice received.
The applicable Swingline Lender and the applicable Borrower shall agree upon the
interest rate applicable to such Swingline Loan, provided that if such agreement
cannot be reached prior to 2:00 p.m., Chicago time, on the day of a proposed
Swingline Loan in the case of Swingline Loans denominated in Dollars to the U.S.
Borrower, prior to 2:00 p.m., Local Time, on the day of a proposed Swingline
Shekel Loan and prior to 10:00 a.m., Local Time, on the day of any other
proposed Swingline Loan, or, in each of the foregoing cases, such other time
agreed to by the applicable Swingline Lender and applicable Borrower, then such
Swingline Loan shall not be made. In addition to any other requirements for
obtaining a Swingline Loan, the applicable Borrower shall comply with all
applicable legal and regulatory requirements.

Any funding of a Swingline Loan by a Swingline Lender shall be made on the
proposed date thereof by 3:00 p.m., Local Time, to the account of the Applicable
Agent or, in case of a Swingline Shekel Loan, to the account of the Swingline
Shekel Lender by 3:00 p.m. (or such later time agreed to by the Israeli Borrower
and the Swingline Shekel Lender), Local Time. The Applicable Agent will make
such Swingline Loan available to the applicable Borrower by promptly crediting
the amounts so received, in like funds, to the account of the applicable
Borrower with the Applicable Agent (or, in the case of a Swingline Borrowing
made to finance the reimbursement of an LC Disbursement as provided in Section
2.05(e), by remittance to the applicable Issuing Bank); provided that, in the
case of a Swingline Shekel Loan, the Swingline Shekel Lender will make such
Swingline Loan available to the applicable Israeli Borrower by promptly
crediting the amounts so received, in like funds, to the account of the
applicable Israeli Borrower with the Swingline Shekel Lender. The Administrative
Agent shall determine the procedures to be followed by the Swingline Lenders to
ensure that the Dollar Equivalent of the aggregate principal amount of the
Swingline Loans does not exceed the amount permitted by Section 2.04(a) at the
time any Swingline Loan is made and to ensure that the amount of Advances made
does not exceed the amounts permitted by Section 2.01(a), and each Swingline
Lender and the other parties hereto agrees to abide by such procedures. Without
limiting such procedures, for purposes of determining the amount of any
Borrowing that is permitted to be made under Section 2.01(a) or 2.04(a), the
Administrative Agent may at any time assume that the Dollar Equivalent of the
aggregate amount of Swingline Shekel Loans is equal to $15,000,000 unless the
request for such Borrowing delivered to the Administrative Agent specifies the
Dollar Equivalent of the aggregate amount of Swingline Shekel Loans on the day
of such request and on the day such Borrowing is to be made, and the
Administrative Agent shall be entitled to rely thereon in determining the
permissible amount of such Borrowing. If the Swingline Loans at any time exceed
any of the amounts permitted by Section 2.01(a) or 2.04(a), the relevant
Borrower or Borrowers shall promptly prepay the relevant Swingline Loans by the
amount of such excess.

     (c) Each Swingline Lender may by written notice given to the Applicable
Agent not later than 1:00 p.m., Chicago time (or 11:00 a.m. London time in the
case of any Swingline Loan denominated in any Foreign Currency or made to any
Foreign Subsidiary Borrower), on any Business Day require the Lenders to acquire
participations on such Business Day in all or a portion of the outstanding
Swingline Loans, provided that the Swingline Shekel Lender may do so only upon
and during the continuance of an Event of Default. Such notice shall specify the
aggregate amount of such Swingline Loans in which the Lenders will participate,
and such Swingline Loans, if denominated in Foreign Currency, shall be converted
to Dollars and shall bear interest at the Alternate Base Rate. Promptly upon
receipt of such notice, the Applicable Agent will give notice thereof to each
Lender, specifying in such notice such Lender's Applicable Adjusted


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Percentage of such Swingline Loan or Loans. Each Lender hereby absolutely and
unconditionally agrees, upon receipt of notice as provided above, to pay to the
Applicable Agent, for the account of the applicable Swingline Lender, such
Lender's Applicable Adjusted Percentage of such Swingline Loan or Loans. Each
Lender acknowledges and agrees that its respective obligation to acquire
participations in Swingline Loans pursuant to this paragraph is absolute and
unconditional and shall not be affected by any circumstance whatsoever,
including the occurrence and continuance of a Default or reduction or
termination of the Commitments, and that each such payment shall be made without
any offset, abatement, withholding or reduction whatsoever. Each Lender shall
comply with its obligation under this paragraph by wire transfer of immediately
available funds, in the same manner as provided in Section 2.06 with respect to
Loans made by such Lender (and Section 2.06 shall apply, mutatis mutandis, to
the payment obligations of the Lenders), and the Applicable Agent shall promptly
pay to the applicable Swingline Lender the amounts so received by it from the
Lenders. The Applicable Agent shall notify the applicable Borrower of any
participations in any Swingline Loan acquired pursuant to this paragraph (c),
and thereafter payments in respect of such Swingline Loan shall be made to the
Applicable Agent and not to the applicable Swingline Lender. Any amounts
received by a Swingline Lender from the applicable Borrower (or other party on
behalf of such Borrower) in respect of a Swingline Loan after receipt by such
Swingline Lender of the proceeds of a sale of participations therein shall be
promptly remitted to the Applicable Agent; any such amounts received by the
Applicable Agent shall be promptly remitted by the Applicable Agent to the
Lenders that shall have made their payments pursuant to this paragraph and to
such Swingline Lender, as their interests may appear; provided that any such
payment so remitted shall be repaid to such Swingline Lender or to the
Applicable Agent, as applicable, if and to the extent such payment is required
to be refunded to the applicable Borrower for any reason. The purchase of
participations in a Swingline Loan pursuant to this paragraph shall not relieve
the applicable Borrower of any default in the payment thereof and the applicable
Borrower shall reimburse each Lender for any amounts that may be due under
Section 2.14, 2.16. 2.20 or any other term of this Agreement.

     1.6 The last sentence of Section 2.09(a) is restated as follows:

Each Foreign Subsidiary Borrower hereby unconditionally promises to pay (x) to
the Applicable Agent for the account of each Lender the then unpaid principal
amount of each Revolving Loan and Ancillary Loan to such Foreign Subsidiary
Borrower on the Maturity Date and (y) to the applicable Swingline Lender the
then unpaid principal amount of each Swingline Loan owing by it on the earlier
of the Maturity Date and the date agreed to between the such Foreign Subsidiary
Borrower and such Swingline Lender.

     1.7 The last paragraph of Section 6.01 is restated as follows:

Notwithstanding the foregoing, the aggregate amount of all Indebtedness of all
Material Non-Guarantor Subsidiaries, other than Indebtedness permitted under
Section 6.01(a), (b) or (e) above, shall not exceed $15,000,000.

     1.8 Clause (iii) of Section 2.10(b) is restated as follows: "(iii) in the
case of prepayment of a Swingline Shekel Loan, not later than 2:00 p.m., (or
such later time agreed to by the Israeli Borrower and the Swingline Shekel
Lender) Local time, and in the case of prepayment of any other Swingline Loan,
not later than 12:00 noon, Local time, on the date of prepayment, or such other
time agreed to by the applicable Borrower and Swingline Lender".


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<PAGE>
                                   ARTICLE 2.
                                REPRESENTATIONS.

     Each Borrower represents and warrants to the Lenders and Administrative
Agent that:

     2.1 The execution, delivery and performance of this Amendment are within
its powers and have been duly authorized by it. This Amendment is the legal,
valid and binding obligation of it, enforceable against it in accordance with
the terms thereof.

     2.2 After giving effect to the amendments and waiver herein contained, the
representations and warranties contained in the Credit Agreement and the
representations and warranties contained in the other Loan Documents are true in
all material respects on and as of the date hereof with the same force and
effect as if made on and as of the date hereof, and no Default or Event of
Default exists or has occurred and is continuing on the date hereof.

                                   ARTICLE 3.
                             CONDITIONS PRECEDENT.

     This Amendment shall be effective as of the date hereof when this Amendment
shall be executed by the Borrowers, the Required Lenders, the Administrative
Agent and the Syndication Agent and the Consent and Agreement attached hereto is
signed by the Guarantors.

                                   ARTICLE 4.
                                     WAIVER.

     The Lenders hereby waive compliance with Section 5.01(c) of the Credit
Agreement for the fiscal quarter ending March 31, 2005 and any Default or Event
of Default that may have occurred as a result of any non-compliance with such
Section 5.01(c) for the fiscal quarter ending March 31, 2005, but do not waive
compliance with such Section 5.01(c) for any other fiscal quarter and do not
waive any other Default or Event of Default.

                                   ARTICLE 5.
                                 MISCELLANEOUS.

     5.1 References in the Credit Agreement or in any other Loan Document to the
Credit Agreement shall be deemed to be references to the Credit Agreement as
amended hereby and as further amended from time to time.


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     5.2 Except as expressly amended hereby, each Borrower agrees that the Loan
Documents are ratified and confirmed and shall remain in full force and effect
and that it has no set off, counterclaim, defense or other claim or dispute with
respect to any of the foregoing. The terms used but not defined herein shall
have the respective meanings ascribed thereto in the Credit Agreement.

     5.3 This Amendment may be signed upon any number of counterparts with the
same effect as if the signatures thereto and hereto were upon the same
instrument, and telecopied signatures shall be enforceable as originals.

                       [Signatures on the following pages]


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     IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be
duly executed by their respective authorized officers as of the day and year
first above written.

                                          PERRIGO COMPANY


                                          By /s/ James R. Ondersma
                                             -----------------------------------
                                             Name: James R. Ondersma
                                             Title: Treasurer


                                          JPMORGAN CHASE BANK, N.A., as a Lender
                                          and as Administrative Agent


                                          By /s/ Christopher C. Cavaiani
                                             -----------------------------------
                                             Name: Christopher C. Cavaiani
                                             Title: Vice President


                                          BANK LEUMI USA, as a Lender and as
                                          Syndication Agent


                                          By /s/ Dr. Avram Keusch
                                             -----------------------------------
                                             Name: Dr. Avram Keusch
                                             Title: Vice President


                                          BANK OF AMERICA, N.A., as a Lender and
                                          as Documentation Agent


                                          By /s/ B. Kenneth Burton, Jr.
                                             -----------------------------------
                                             Name: B. Kenneth Burton, Jr.
                                             Title: Vice President


                                          LASALLE BANK MIDWEST N.A., formerly
                                          known as STANDARD FEDERAL BANK N.A.,
                                          as a Lender and as Documentation Agent


                                          By /s/ Rachel Glupker
                                             -----------------------------------
                                             Name: Rachel Glupker
                                             Title: Assistant Vice President


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<PAGE>
                                          NATIONAL CITY BANK OF THE MIDWEST, as
                                          a Lender and as Documentation Agent


                                          By /s/ Jason T. Byrd
                                             -----------------------------------
                                             Name: Jason T. Byrd
                                             Title: Vice President


                                          FIFTH THIRD BANK


                                          By /s/ Randal S. Wolffis
                                             -----------------------------------
                                             Name: Randal S. Wolffis
                                             Title: Vice President


                                          HARRIS N.A., successor by merger to
                                          HARRIS TRUST AND SAVINGS BANK


                                          By /s/ Patrick J. McDonnell
                                             -----------------------------------
                                             Name: Patrick J. McConnell
                                             Title: Managing Director


                                          COMERICA BANK


                                          By /s/ Jeffrey J. Judge
                                             -----------------------------------
                                             Name: Jeffrey J. Judge
                                             Title: Vice President


                                          THE NORTHERN TRUST COMPANY


                                          By /s/ Mark Taylor
                                             -----------------------------------
                                             Name: Mark Taylor
                                             Title: Vice President


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<PAGE>
                              CONSENT AND AGREEMENT

     As of the date and year first above written, each of the undersigned
hereby: (a) fully consents to the terms and provisions of the above Amendment
and the consummation of the transactions contemplated thereby; (b) agrees that
the Guaranty to which it is a party and each other Loan Document to which it is
a party are hereby ratified and confirmed and shall remain in full force and
effect, acknowledges and agrees that it has no setoff, counterclaim, defense or
other claim or dispute with respect the Guaranty to which it is a party and each
other Loan Document to which it is a party; and (c) represents and warrants to
the Administrative Agent and the Lenders that the execution, delivery and
performance of this Consent and Agreement are within its powers, have been duly
authorized and are not in contravention of any statute, law or regulation or of
any terms of its organizational documents or of any material agreement or
undertaking to which it is a party or by which it is bound, and this Consent and
Agreement is the legal, valid and binding obligations of it, enforceable against
it in accordance with the terms hereof and thereof. Terms used but not defined
herein shall have the respective meanings ascribed thereto in the Credit
Agreement.

                                          L. PERRIGO COMPANY


                                          By /s/ James R. Ondersma
                                             -----------------------------------
                                             Name: James R. Ondersma
                                             Title: Treasurer


                                          PERRIGO COMPANY OF SOUTH CAROLINA,
                                          INC.


                                          By /s/ James R. Ondersma
                                             -----------------------------------
                                             Name: James R. Ondersma
                                             Title: Treasurer


                                          PERRIGO PHARMACEUTICALS COMPANY


                                          By /s/ James R. Ondersma
                                             -----------------------------------
                                             Name: James R. Ondersma
                                             Title: Treasurer


                                          PERRIGO INTERNATIONAL, INC.


                                          By /s/ James R. Ondersma
                                             -----------------------------------
                                             Name: James R. Ondersma
                                             Title: Treasurer


                                          PERRIGO INTERNATIONAL HOLDINGS, INC.


                                          By /s/ James R. Ondersma
                                             -----------------------------------
                                             Name: James R. Ondersma
                                             Title: Treasurer


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