warfarin, anticoagulant drug, marketed as Coumadin. Originally developed to treat thromboembolism (see thrombosis), it interferes with the liver’s metabolism of vitamin K, leading to production of defective coagulation factors. Warfarin therapy risks uncontrollable hemorrhage, either spontaneously or from any cut or bruise; it requires frequent checks to maintain the proper level in the blood. In high concentrations, warfarin is used as a rodent poison, causing death by internal bleeding.