Blood clots that form in the veins (deep venous thrombosis) are a common and often dangerous problem. The veins in the legs are the usual site of these blood clots. Pieces of the clot can break off and travel through the veins to lodge in the lungs and put strain on the heart or even cause death (pulmonary embolism). Clots that remain in the leg veins can cause leg swelling for the rest of a person's life. People vary in their tendency to develop blood clots. Blood clots are rare in healthy, active people. They are more common in several groups: people with chronic, disabling disease, such as cancer or heart failure; people who can't move around because of recent surgery or a fractured bone; and people who have a blood clotting disorder that is inherited and is due to a specific mutation in a gene. People who have had at least 1 occurrence of blood clots are at the highest risk, and they frequently have to take medication (such as heparin or warfarin) to reduce the tendency of their blood to clot. These medications can cause severe bleeding, so only people at the highest risk take them. People at low to moderate risk might benefit from a weaker, safer medication—like aspirin—to reduce clotting.