Heparin is an anticoagulant, i.e., a substance that slows the formation of blood clots (see blood clotting). It works by inhibiting the action of the enzyme thrombin in the final stage of blood coagulation.

Heparin is produced by the liver, lungs, and other tissues in the body, and by leukocytes (white blood cells); it can also made in the laboratory. It may be injected into muscle or blood to prevent or break up blood clots, and is particularly useful as an immediate treatment for deep vein thrombosis and similar conditions. The most important side-effect, as might be expected, is bleeding.