any drug that, when added to blood, prevents it from clotting (coagulation). Anticoagulants achieve their effect by suppressing the synthesis or function of various clotting factors that are normally present in the blood. Such drugs are often used to prevent blood clots (thrombi) from forming in the veins or arteries. Anticoagulants are generally of two types. One type is heparin, which is a mixture of mucopolysaccharides that promote the activity of antithrombin III, a blood plasma protein that inactivates thrombin (an enzyme that promotes clotting). The other type consists of the coumarin derivatives, which interfere in the metabolism of vitamin K in the liver. Since vitamin K is an important element in the synthesis of various clotting factors, interference in the metabolism of that vitamin gives rise to clotting factors that are defective and hence inactive in the blood. Coumarin derivatives differ from heparin primarily in their longer duration of action; of these, the most widely used is warfarin. A major side effect of anticoagulant therapy is hemorrhage. Anticoagulants are also used in drawing and storing blood.