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[hepar + -in]
A parenteral anticoagulant drug with a faster onset than warfarin or its derivatives. It is composed of polysaccharides that inhibit coagulation by forming an antithrombin that prevents conversion of prothrombin to thrombin and by preventing liberation of thromboplastin from platelets. Because heparin is poorly absorbed from the gastrointestinal tract, it is usually administered intravenously or subcutaneously as a sodium or calcium salt.

Heparin is used as an anticoagulant to prevent and treat thrombosis and embolism. It is an important agent in the management of acute coronary syndromes, e.g., unstable angina pectoris or acute myocardial infarction. Because heparin compounds are too large to cross the placental barrier, they are the preferred anticoagulants in pregnant women. The antagonist for an overdose is protamine sulfate. The most common side effect of heparin is abnormal bleeding.

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