Potentiation of warfarin anticoagulation by sulfisoxazole

The prothrombin time (PT) of a patient undergoing warfarin sodium anticoagulation became elevated when sulfisoxazole was given concurrently. The warfarin dose index is used to demonstrate that this PT prolongation was the result of a warfarin-sulfisoxazole interaction. The mechanism of this interaction may involve displacement of warfarin from serum albumin. Sulfisoxazole should be used cautiously, if at all, in patients taking warfarin.