Exhibit 4.1

                                    AMENDMENT  TO RIGHTS  AGREEMENT  dated March
                           25,  1998,  to the Rights  Agreement  dated August 6,
                           1991,  as amended  (the "Rights  Agreement"),  by and
                           between Pharmaceutical Resources,  Inc., a New Jersey
                           corporation ("the "Company"), and First City Transfer
                           Company (the "Rights Agent").

     WHEREAS,  the  Board of  Directors  of the  Company,  on  August  6,  1991,
authorized and adopted a share purchase  rights plan (the "Plan") to protect the
Company's  shareholders  against  unsolicited  and  hostile  attempts to acquire
control of the Company and, in connection therewith,  executed and delivered the
Rights  Agreement to  effectuate  the terms of the Plan;  

     WHEREAS, the Board of Directors of the Company, on March 25, 1998, approved
and adopted an amendment to the Plan as described  herein in  contemplation of a
certain negotiated transaction;

     WHEREAS,  the Board of Directors of the Company authorized and directed the
proper  officers  of the  Company  as well as the Rights  Agent to  execute  and
deliver  this  Amendment  to the Rights  Agreement  in order to  effectuate  the
foregoing amendments to the Plan; and 

     WHEREAS, all capitalized terms used herein and not otherwise defined herein
shall have the meanings set forth in the Rights Agreement.  

     NOW,  THEREFORE,  in  consideration  of the  mutual  premises,  the  Rights
Agreement is hereby amended as follows:

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     Section 1.  Effective  Dates.  (a) The  amendment set forth in Section 2(a)
hereof shall be effective  upon the execution and delivery of the Stock Purchase
Agreement,  dated March 25, 1998,  between the Company and Lipha Americas,  Inc.
(the "Stock Purchase Agreement"). 

     (b) The  amendments  set forth in Sections  2(b),  (c),  (e) and (f) hereof
shall be  effective  upon the  consummation  of the  Stock  Purchase  Agreement.


     Section 2. Certain Definitions. (a) The definition of "Acquiring Person" as
set forth in the first  sentence  in Section  1(a) shall be amended by  deleting
"or" before "(v)" and  inserting in its stead "," and by inserting the following
at the end of the  first  sentence  thereof  before  the ".":  

     "and  (vi)  Lipha  Americas,  Inc.  ("Lipha"),  Merck  KGaA  ("Merck")  and
     Genpharm,  Inc.  ("Genpharm")  under a stock purchase agreement and options
     approved by the Board of Directors of the Company,  as amended from time to
     time,  between the Company and Lipha,  Merck and Genpharm,  as the case may
     be, so long as any  acquisition  or tender or exchange  offer is  permitted
     under such stock purchase agreement or options"

     (b) The definition of "Acquiring Person" as set forth in the first sentence
of  Section  1(a) shall be amended by  amending  clause (v) in its  entirety  as
follows: 

     "(v) [intentionally omitted],"

     (c) The definition of "Permitted Offer" in Section 1(n) shall be amended in
its entirety as follows:  

     "(n) "Permitted  Offer" shall mean the following  tender offers made in the
     manner  prescribed  by Section  14(d) of the Exchange Act and the rules and
     regulations promulgated thereunder:  (i) a tender offer for all outstanding
     Common Shares;  provided,  however, that such tender offer occurs at a time
     when  Continuing  Directors are in office and a majority of the  Continuing
     Directors  has  determined  that the offer is fair to, and otherwise in the
     best interests of, the Company and

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         its  stockholders,  and  (ii) a  tender  offer  for  Common  Shares  as
         permitted  by a stock  purchase  agreement  approved  by the  Board  of
         Directors  of the Company,  as amended  from time to time,  between the
         Company and Lipha."

     (d) The proviso in the  definition  of  "Person"  in Section  1(o) shall be
amended in its entirety with the following: 

     "; provided,  however, that "Person" shall exclude Lipha and its associates
     and  affiliates  until such time as any  acquisition of or tender offer for
     Common Shares by Lipha or any of its affiliates or associates  shall not be
     permitted  under  a stock  purchase  agreement  approved  by the  Board  of
     Directors of the Company, as amended from time to time, between the Company
     and Lipha"

     (e) The  proviso  in the  definition  of  "Section  11(a)(ii)(A)  Event" in
Section 1(v) shall be amended in its entirety as follows: 

          "; provided,  however,  in no event shall any of the actions described
          in  Section  11(a)(11)(A)  by  Lipha  or  any  of  its  affiliates  or
          associates  constitute a Section 11(a)(ii)(A) Event to the extent that
          any  such  actions  shall  not be  prohibited  under a stock  purchase
          agreement  approved  by the  Board of  Directors  of the  Company,  as
          amended  from time to time,  between  the  Company  and Lipha" 

     (f) The proviso in the  definition  of  "Section 13 Event" in Section  1(w)
shall be amended in its entirety as follows:  

          "; provided,  however,  in no event shall any of the actions described
          in  Section  13(a)  by Lipha or any of its  affiliates  or  associates
          constitute  a Section  13 Event to the  extent  that any such  actions
          shall not be prohibited under a stock purchase  agreement  approved by
          the Board of Directors  of the Company,  as amended from time to time,
          between the Company and Lipha"  

     Section 3.  Authority for  Amendment.  This Amendment is being executed and
delivered as of the date hereof by the Company and the Rights Agent  pursuant to
and in accordance  with Section 27 of the Rights  Agreement.  By executing  this
Amendment,  the Company hereby certifies to the Rights Agent that this Amendment
is in compliance with Section 27 of the Rights Agreement. Except as

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otherwise  amended  hereby,  all the  provisions of the Rights  Agreement  shall
remain in full force and effect. This Amendment shall be deemed to be a part of,
and shall be construed as part of, the Rights Agreement.

     IN WITNESS HEREOF, the parties hereby have caused this Amendment to be duly
executed as of the date first above written.

                         PHARMACEUTICAL RESOURCES, INC.



                         By /s/ Kenneth I. Sawyer
                         -----------------------------
                         Title:  President


                         FIRST CITY TRANSFER COMPANY


                         By /s/ Kathleen M. Zaleske
                         -----------------------------
                         Title:  Kathleen M. Zaleske
                         Assistant Vice President



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