Sometimes adults get blood clots in their veins, a condition known as deep venous thrombosis (DVT). The blood clots usually form in leg veins. Blood clots in the deep veins of the legs 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. The blood-thinning drugs help dissolve clots, reduce the risk that a clot will travel to the lungs, and prevent repeat clots. Some patients may have high risks for repeat clots and may benefit from longer treatment.