Patients with cancer sometimes develop blood clots in the deep veins of the leg, a condition known as deep venous thrombosis. In rare cases, blood clots in the deep veins of the leg become so extensive, involving both large and small blood vessels, that they reduce blood and oxygen supply to tissues. This can cause gangrene (rotting of the tissues). The drug warfarin is typically used to treat deep venous thrombosis. Warfarin thins the blood by interfering with normal blood clotting. It helps the body's normal systems for blood-clot removal to work more effectively. Why some patients with clots in their leg veins develop gangrene while they are taking warfarin has been a mystery.