Co-trimoxazole and Warfarin: Case Report of an Interaction

Abstract

A case of apparent co-trimoxazole (sulfamethoxazole 400 mg and trimethoprim 80 mg per tablet) potentiation of warfarin's anticoagulant effect in a 76-year-old female with pulmonary embolism is reported. After two 15-mg/day doses of warfarin, the patient’s prothrombin time/control (PT/C) ratio was 1.8. An additional 10 mg of warfarin then was given concurrently with two co-trimoxazole tablets; PT/C increased to 5.5 and, despite discontinuation of warfarin, remained elevated during the six days of co-trimoxazole therapy. Phytonadione 10 mg was administered i.m. on the day co-trimoxazole was discontinued and the PT/C fell rapidly. When warfarin was administered again, PT/C rose sharply but not to the same extent or for the same duration as it did when co-trimoxazole was given simultaneously. A computer- simulated warfarin dose-response curve further demonstrated the influence of a variable other than warfarin dose on the patient's anticoagulant response. The patient also was receiving other medications concurrently with warfarin. The factors that might affect warfarin's anticoagulant response are discussed. A substantial interaction between warfarin and co-trimoxazole probably occurs. Close monitoring of patients receiving these two drugs concurrently is recommended.