Acute venous thromboembolism is a condition in which blood clots form in veins. The clots most often occur in leg veins, causing pain and swelling. Pieces of these clots can break off and travel to the lung, a serious condition called pulmonary embolism. To treat acute venous thromboembolism, doctors prescribe anticoagulant drugs that make the blood take longer to clot. Treatment usually starts with the drug heparin, followed by treatment with warfarin. Heparin begins to work quickly but must be given by injection. Warfarin is taken by mouth, so it is easy for a patient to use at home. However, it can take several days after starting warfarin before the drug reaches a therapeutic level. A therapeutic level means that the blood is anticoagulated enough to treat the thromboembolism, but not too much. Too much warfarin can lead to bleeding complications. Doctors use a test called the international normalized ratio (INR) to see whether a person is taking the correct amount of warfarin. The correct dose varies greatly from patient to patient. The best starting dose is not clear, but many doctors start with 5 mg of warfarin. A higher starting dose might achieve a therapeutic level more quickly.