Anticoagulant drugs, whether classical ones such as warfarin and heparin or newer ones such as dabigatran and apixaban, can be effective for treating and preventing deep vein thrombosis, myocardial infarction, ischemic stroke and pulmonary embolisms. Targeting factor XIa, a serine protease in the coagulation pathway, may inhibit thrombosis without increasing bleeding risk. Numerous inhibitors of factor XIa have been developed and several have entered clinical trials, but most have shown problems of poor efficacy, inadequate selectivity or drug-drug interactions.