heparin, anticoagulant drug that is used to prevent blood clots from forming during and after surgery and to treat various heart, lung, and circulatory disorders in which there is an increased risk of blood clot formation. Discovered in 1922 by American physiologist William Henry Howell, heparin is a naturally occurring mixture of mucopolysaccharides that is present in the human body in tissues of the liver and lungs. Most commercial heparin is obtained from cow lungs or pig intestines. Heparin was originally used to prevent the clotting of blood taken for laboratory tests. Its use as a therapy for patients who already have a blood clot in a vein (venous thrombosis) began in the 1940s; low-dose heparin treatment to prevent blood clots from forming in patients who are at high risk for pulmonary embolisms and other clotting disorders was introduced in the early 1970s.
The biological activity of heparin depends on the presence of antithrombin III, a substance in blood plasma that binds and deactivates serum clotting factors. Heparin is poorly absorbed by the intestine, so it must be given intravenously or subcutaneously. Because of its anticlotting effect, the drug creates a significant risk of excessive bleeding, which may be reversed with protamine, a protein that neutralizes heparin’s anticoagulant effect. Other adverse effects of heparin include thrombocytopenia (reduced number of circulating platelets) and hypersensitivity reactions.