Why is there an interaction between coumadin and bactrim (sulfamethoxazole and trimethoprim)?

Increases free drug: Much of Coumadin is bound to protein. Many drugs, including bactrim, (sulfamethoxazole and trimethoprim) dislodge it from protein and increase the free (active) drug in the circulation. This usually results in a sharp spike in the inr and can lead to bleeding. If you must take batrim, you may need to skip a dose or two of coumadin. Discuss with your doctor.