A blood clot inside the deep veins of the legs causes deep venous thrombosis (DVT). Pieces of the clot can break off, travel to the lungs, and cause serious and even fatal complications. Doctors usually give blood thinners to patients with DVT for several months to prevent pieces of clots from breaking loose and to stop new clots from forming. Vitamin K antagonists (warfarin, also known by the brand name Coumadin) are blood thinners that can be taken by mouth, but they take about a week to adequately thin blood and block clotting. Thus, patients with DVT must take other blood thinners during the first several days after a vitamin K antagonist is given. For many years, doctors initiated therapy for DVT with a blood-thinning drug known as unfractionated heparin. They gave this drug through a needle in a vein for a few days while patients started vitamin K antagonist therapy in the hospital. More recently, doctors have been using another preparation of heparin, low-molecular-weight heparin (LMWH), or drugs that selectively block clotting (factor Xa inhibitor). Both LMWH and factor Xa inhibitors can be injected under the skin, can be given at home, and do not require close monitoring of the dose. Several studies show that both drugs are safe and effective for treating DVT, but we do not know whether one is better than the other.