The anticoagulant effects of warfarin, acenocoumarol, and phenprocoumon are increased by
co-trimoxazole (sulfamethoxazole with trimethoprim). Bleeding may occur if the anticoagulant dosage is not reduced appropriately. There is also evidence that sulfaphenazole, sulfafurazole (
sulfisoxazole) and sulfamethizole may interact like
co-trimoxazole.
The
incidence of the interaction with
co-trimoxazole appears to be high. If bleeding is to be avoided the INR should be well monitored and the warfarin, acenocoumarol, or phenprocoumon dosage should be reduced. Phenindione is said not to interact. The other interactions are poorly documented. However, it would seem prudent to follow the precautions suggested for
co-trimoxazole if any sulfonamide is given with an oral anticoagulant.