Blood clots in the deep veins of the legs or pelvis are called deep venous thrombosis (DVT). These blood clots are dangerous because they can break free and travel to the lungs. This problem, called pulmonary embolism (clot in the lung), can be fatal. Doctors typically treat patients with DVT with blood-thinning drugs for 3 to 6 months. Some patients with high risks for repeated clots need even longer treatment. The blood-thinning drugs prevent the formation of new clots, reduce the risk that a clot will travel to the lungs, and prevent repeated clots. Doctors often use a powerful blood-thinning drug called warfarin to prevent repeated clots in patients with DVT. People taking this drug need frequent blood tests to make sure that the dose being given reduces the blood's tendency to clot, but not so much that bleeding occurs spontaneously. Even with careful monitoring, some patients who take warfarin have bleeding complications. The consequences of the bleeding complications aren't well known.