<PAGE>   1
                                                                     EXHIBIT 4.5

                               THIRD AMENDMENT TO
                AMENDED TO RESTATED LOAN AND SECURITY AGREEMENT

         THIS THIRD AMENDMENT TO AMENDED AND RESTATED LOAN AND SECURITY
AGREEMENT (the "Amendment") dated as of April 14, 2000, is between FIRST
HORIZON PHARMACEUTICAL CORPORATION, formerly known as Horizon Pharmaceutical
Corporation (the "Borrower") and LASALLE BANK NATIONAL ASSOCIATION, a national
banking association ("Bank").

                                   RECITALS:

         WHEREAS, the Borrower and the Bank have previously entered into that
certain Amended and Restated Loan and Security Agreement, dated December 22,
1998, as amended by that certain First Amendment to Amended and Restated Loan
and Security Agreement dated as of May 10, 1999, and as amended by that certain
Second Amendment to Amended and Restated Loan and Security Agreement dated as
of January 6, 2000 (the "Agreement"); and

         WHEREAS, the Borrower requests, and the Bank is agreeable to provide a
new Bridge Loan in the amount of $13,000,000, to extend the maturity date of
the Revolving Loan, and to make such additional modifications as set forth
herein.

         NOW, THEREFORE, in consideration of the premises and of the mutual
agreements hereinafter set forth, it is agreed by the parties hereto as
follows:

                  RECITALS. The Recitals set forth above are hereby incorporated
herein and made a part hereof.

                  DEFINITIONS. All capitalized terms used herein without
definition shall have the respective meanings set forth in the Agreement.

                  AMENDMENTS TO THE AGREEMENT.

                           Fixed Charge Coverage Ratio.  The definition of
Fixed Charge Coverage Ratio set forth in Section 1.1 of the Agreement is hereby
amended as restated in its entirety as follows:

                           " 'Fixed Charge Coverage Ratio' shall mean the ratio
                  of EBITDA less unfunded capital expenditures for the period
                  measured to scheduled principal and interest payments for the
                  period measured."

                           Loan.  The definition of Loans set forth in Section
1.1 of the Agreement is hereby amended and restated in its entirety as follows:

                           " 'Loans' shall mean, collectively, all Revolving
                  Loans, Term Loans and the Bridge Loan made by the Bank to the
                  Borrower and all Letters of Credit


<PAGE>   2

                  issued by the Bank for the benefit of Borrower under and
                  pursuant to this Agreement.

                           Revolving Loan Maturity Date.  The definition of
Revolving Loan Maturity Date set forth in Section 1.1 of the Agreement is
hereby amended and restated in its entirety as follows:

                           " 'Revolving Loan Maturity Date' shall mean May 2,
                  2001, unless extended by the Bank pursuant to any
                  modification, extension or renewal note executed by the
                  Borrower and accepted by the Bank in its sole and absolute
                  discretion in substitution for the Revolving Note."

                           Term Loan Maturity Date.  The definition of Term
Loan Maturity Date set forth in Section 1.1 of the Agreement is hereby amended
and restated in its entirety as follows:

                           " 'Term Loan Maturity Date' shall mean May 2, 2001,
                  unless extended by the Bank pursuant to any modification,
                  extension or renewal note executed by the Borrower and
                  accepted by the Bank in its sole and absolute discretion in
                  substitution for the Term Note."

                           The following definitions are hereby added to
Section 1.1 of the Agreement:

                           "Bridge Interest Rate" shall mean the Borrower's
                  option of (i) the Prime Rate per annum, or (ii) LIBOR plus
                  150 basis points, except that if the Bridge Loan is not
                  repaid in full within six months from the date hereof, the
                  LIBOR interest rate will increase by an additional 50 basis
                  points.

                           "Bridge Loan" shall mean the Loan made by Bank to
                  the Borrower in the form of a Bridge Loan under and pursuant
                  to this Agreement, as set for in Section 2.6 of this
                  Agreement.

                           "Bridge Loan Commitment" shall mean Thirteen Million
                  and 00/100 Dollars ($13,000,000.00).

                           "Bridge Loan Maturity Date" shall mean the earlier
                  of a Public Offering or one year from the date hereof.

                           "Bridge Note" shall mean the note, the form of which
                  is attached hereto as Exhibit C.

                           "Public Offering" shall mean any offering of debt
                  issued by or securities of the Borrower offered pursuant to
                  registration thereof under the Securities Act of 1993 and the
                  Securities Exchange Act of 1934.

                           Bridge Loan. The following is hereby added to the
Agreement:


<PAGE>   3

                  2.6      Bridge Loan.

                  1)           Bridge Loan Commitment. Subject to the terms and
                  conditions of this Agreement and the other Loan Documents,
                  and in reliance upon the representations and warranties of
                  the Borrower set forth herein and in the other Loan
                  Documents, the Bank agrees to make a Bridge Loan in the
                  amount of the Bridge Loan Commitment. Up to $9,500,000 of the
                  proceeds of the Bridge Loan shall be used solely by Borrower
                  to purchase rights to and know how to manufacture the drug
                  "Ponstel" from Warner-Lambert and Parke Davis & Company and
                  up to $3,500,000 of the proceeds of the Bridge Loan shall be
                  used solely by Borrower to purchase rights to and know how to
                  manufacture the drug "Cognex" from Warner-Lambert and Parke
                  Davis & Company. The Bridge Loan may be prepaid in whole or
                  in part at any time without penalty, but shall be due in full
                  on the Bridge Loan Maturity Date, unless the credit extended
                  under the Bridge Loan is otherwise terminated or extended as
                  provided in this Agreement.

                  2)           Term Loan Interest and Payments. Except as
                  otherwise provided in this Section 2.2(b), the principal
                  amount of the Bridge Loan outstanding from time to time shall
                  bear interest at the Bridge Interest Rate. Accrued and unpaid
                  interest on that portion of the unpaid principal balance of
                  the Bridge Loan outstanding from time to time which is a
                  Prime Loan, shall be due and payable monthly, in arrears,
                  commencing May 1, 2000 and continuing on the first day of
                  each calendar month thereafter, and on the Bridge Loan
                  Maturity Date. Accrued and unpaid interest on that portion of
                  the unpaid principal balance of the Bridge Loan outstanding
                  from time to time which is a LIBOR Loan shall be payable on
                  (i) first day of each calendar month, (ii) on the date of any
                  principal repayment of a LIBOR Loan, and (iii) on the Bridge
                  Loan Maturity Date. Any amount of principal or interest on
                  the Bridge Loan which is not paid when due, whether at stated
                  maturity, by acceleration or otherwise, shall bear interest
                  payable on demand at the Default Rate.

                  3)           Mandatory Prepayment. If the Borrower completes
                  a Public Offering, all of the proceeds, after deducting
                  therefrom the expenses of the Public Offering, shall be used
                  by Borrower to immediately prepay the Bridge Loan. If the
                  Borrower does not complete a Public Offering within six
                  months from the date hereof, or if the Public Offering is not
                  sufficient to repay the Bridge Loan, Borrower shall commence
                  making monthly principal payments of $150,000 on October 30,
                  2000 and continuing on the last day of each month thereafter
                  through the Term Maturity Date.

                  4)       Voluntary Prepayments. If the Borrower makes any
                  voluntary principal prepayment to Bank (except for principal
                  payments which may be required to be paid under the Revolving
                  Loan), such principal payments shall be applied first to
                  principal due on the Bridge Note. Thereafter, any voluntary
                  prepayments may be applied in such order as Borrower directs.


<PAGE>   4

                               Net Worth plus Subordinated Debt. Section 10.1
                  of the Agreement is hereby amended to delete the reference to
                  the amount of "Three Million Three Hundred Thousand Dollars
                  ($3,300,000.00) and to insert a reference to the amount of
                  "Three Million Three Hundred Thousand Dollars
                  ($3,300,000.00), plus 75% of net income earned in each fiscal
                  year subsequent to the fiscal year ending December 31, 1999".

                               Leverage. Section 10.2 of the Agreement is hereby
                  amended and restated as follows:

                  Until the net proceeds of a Public Offering are delivered to
         Bank, as of the end of each of its fiscal quarters, the Borrower shall
         maintain a ratio of Liabilities to Net Worth plus Subordinated Debt
         according to the following schedule:

<TABLE>
<CAPTION>
                           Quarter Ended               Maximum Leverage
                           -------------               ----------------
                           <S>                         <C>
                           March 31, 2000                 3:50:1:00
                           June 30, 2000                  7.00:1.00
                           September 30, 2000             6.50:1.00
                           December 31, 2000              5.50:1.00
</TABLE>

                  Immediately after the proceeds of a Public Offering are
         delivered to Bank, the Borrower shall maintain a ratio of Liabilities
         to Net Worth of not greater than 2.25 to 1.00.

                               EBITDA. Section 10.3 of the Agreement is amended
                  and restated as follows:

                           As of the end of its fiscal quarters, the Borrower
                  shall maintain a minimum EBITDA according to the following
                  schedule:

<TABLE>
<CAPTION>
                           Quarter Ended                       Minimum EBITDA
                           -------------                       --------------
                           <S>                                 <C>
                           March 31, 2000                        $  100,000
                           June 30, 2000                         $  800,000
                           September 30, 2000                    $2,000,000
                           December 31, 2000 and thereafter      $3,800,000
</TABLE>

                               Fixed Charge Coverage Ratio.  Section 10.4 of
                  the Agreement is amended and restated as follows:

                           As of the end of each of its fiscal quarters for the
                  trailing twelve month period then ending, the Borrower shall
                  maintain a Fixed Charge Coverage Ratio according to the
                  following schedule:

<TABLE>
<CAPTION>
                           Quarter Ended                          Minimum Ratio
                           -------------                          -------------
                           <S>                                    <C>
                           March 31, 2000                            .75:1:00
                           June 30, 2000                             .75:1.00
</TABLE>


<PAGE>   5

<TABLE>

                           <S>                                      <C>
                           September 30, 2000                        .90:1.00
                           December 31, 2000 and thereafter         1.25:1.00
</TABLE>

                               The following is hereby added as Section 11.13 of
                  the Agreement:

                               11.13  Breach of Security Agreement.a default
                           under the terms of that certain Security Agreement
                           of even date herewith between ____________, as
                           Trustee ("Pledgor") and the Bank wherein "Pledgor"
                           grants a security interest to Bank in that certain
                           Investment Account Number 2638254 at Northern Trust
                           Company, which default is not cured within five (5)
                           days after written notice thereof from the Bank to
                           Pledgor.

                  WARRANTIES. To induce the Bank to enter into this Amendment,
the Borrower warrants that:

                               Authorization. The Borrower is duly authorized to
                  execute and deliver this Amendment and is and will continue
                  to be duly authorized to borrow monies under the Agreement,
                  as amended hereby, and to perform its obligations under the
                  Agreement, as amended hereby.

                               No Conflicts. The execution and delivery of this
                  Amendment and the performance by the Borrower of its
                  obligations under the Agreement, as amended hereby, do not
                  and will not conflict with any provision of law or of the
                  charter or by-laws of the Borrower or of any agreement
                  binding upon the Borrower.

                               Validity and Binding Effect. The Agreement, as
                  amended hereby, is a legal, valid and binding obligation of
                  the Borrower, enforceable against the Borrower in accordance
                  with its terms, except as enforceability may be limited by
                  bankruptcy, insolvency or other similar laws of general
                  application affecting the enforcement of creditors' rights or
                  by general principles of equity limiting the availability of
                  equitable remedies.

                               No Default. As of the closing date hereof, no
                  Event of Default under Section 11 of the Agreement, as
                  amended by this Amendment, or event or condition which, with
                  the giving of notice or the passage of time, shall constitute
                  an Event of Default, has occurred or is continuing.

                               Warranties. As of the closing date hereof, the
                  representations and warranties in Section 7 of the Agreement
                  are true and correct as though made on such date, except for
                  such changes as are specifically permitted under the
                  Agreement.

                  CONDITIONS PRECEDENT TO INITIAL $9,500,000 ADVANCE UNDER THE
BRIDGE NOTE. This Amendment shall become effective as of the date above first
written


<PAGE>   6

and the Bank shall advance to Borrower the amount of $9,500,000 under the
Bridge Note after receipt by the Bank of the following:

         5)       a fully executed Amendment;

         6)       an executed Revolving Note of the Borrower payable to the Bank
         in the form attached hereto as Exhibit A;

         7)       an executed Term Note of the Borrower payable to the Bank in
         the form attached hereto as Exhibit B;

         8)       an executed Bridge Note of the Borrower payable to the Bank
         in the form attached hereto as Exhibit C;

         9)       Secretary's certificate relating to corporate resolutions
         authorizing the Amendment and the $13,000,000 Bridge Note;

         10)      a Security Agreement in the form attached hereto as Exhibit D
         executed by Pledgor, pledging her interest in an investment account at
         Northern Trust Company to secure the Bridge Loan;

         11)      a Custodial Account Control Agreement among Editha Kapoor,
         Northern Trust Company and LaSalle Bank National Association;

         12)      an opinion of counsel for Pledgor in form and substance
         satisfactory to Bank;

         13)      an executed copy of the Purchase Agreement wherein Borrower
         agrees to purchase "Ponstel";

         14)      a consent from Warner Lambert to the transfer of Ponstel as
         contemplated by this Amendment and Intellectual Property Security
         Agreement;

         15)      a copy of the Bill of Sale from Warner Lambert;

         16)      a copy of the Warner Lambert Assignment letters to the Food
         and Drug Administration;

         17)      an Intellectual Property Security Agreement for Ponstel;

         18)      a UCC-1 Financing Statement relating to the Intellectual
         Property Security Agreement for "Ponstel";

         19)      a commitment fee in the amount of Nineteen Thousand Five
         Hundred Dollars ($19,500.00);


<PAGE>   7

         20)      A UCC-3 financing statement reflecting the name change of
         Borrower; and

         21)      such other documents that the Bank may reasonably require.

         CONDITIONS PRECEDENT TO USE OF THE REMAINING $3,500,000 AVAILABLE
UNDER THE BRIDGE NOTE. The Bank shall advance the remaining $3,500,000
available under the Bridge Note after receipt by the Bank of the following:

         22)      a written request to the Bank specifying the basis upon which
         the $3,500,000 payment is due to Warner-Lambert Company under the
         terms of Section 2.1(a)(ii) of the Purchase Agreement referred to in
         section 5(i) above;

                  If the reason for funding is that Borrower is purchasing
         Cognex, then said funds shall be available only upon receipt by the
         Bank of the following:

         23)      evidence satisfactory to Bank that the Federal Trade
         Commission has approved the sale of Cognex to Borrower;

         24)      an Intellectual Property Security Agreement for Cognex;

         25)      a UCC-1 financing statement relating to the Intellectual
         Property Security Agreement for "Cognex"; and

         26)      a copy of the Bill of Sale from Warner Lambert;

         27)      a copy of the Warner Lambert Assignment letters to the Food
         and Drug Administration;

         28)      such other documents that the Bank may reasonably require.

         GENERAL.

                               Law. This Amendment shall be construed in
         accordance with and governed by the laws of the State of Illinois.

                               Successors. This Amendment shall be binding upon
         the Borrower and the Bank and their respective successors and assigns,
         and shall inure to the benefit of the Borrower and the Bank and its
         successors and assigns.

                               Confirmation of Agreement. Except as amended
         hereby, the Agreement shall remain in full force and effect and is
         hereby ratified and confirmed in all respects.


                  (REMAINDER OF PAGE LEFT INTENTIONALLY BLANK)
<PAGE>   8


                           IN WITNESS WHEREOF, the parties hereto have executed
                  this Amendment as of the date first above written.

                                            First Horizon Pharmaceutical
                                            Corporation

                                            By:
                                               --------------------------------
                                            Print Name:
                                                       ------------------------
                                            Title:
                                                  -----------------------------


                                            LaSalle Bank National Association



                                            By:
                                               --------------------------------
                                            Print Name:
                                                       ------------------------
                                            Title:
                                                  -----------------------------