The deadline for the payments under the Drug Refund Scheme is being extended until the end of October.
Anyone who purchased drugs costing more than £32 between July 1999 and February 2001 is entitled to a rebate from the Department of Health.
However, members of the public will have to produce receipts to prove their entitlements.
The Irish Pharmaceutical Union said it has been inundated with requests for receipts going back as far as four years ago.
President Richard Collis said it was ludicrous to expect them to deal with every customer by September 30.