Heparin is a naturally occurring anticoagulant drug which combines with antithrombin III to inhibit many steps of the coagulation pathway. Clinically, heparin is used in small doses for the prevention of venous thrombosis and pulmonary embolism and in large doses is the treatment of choice in acute venous thromboembolism. It may be given by intermittent intravenous or continuous intravenous injection, usually in doses of approximately 30,000 units/day. The main side effect of heparin is haemorrhage. Haemorrhage appears to be less frequent when therapeutic heparin is given by continuous intravenous infusion than by intermittent intravenous injection. Heparin is also used for the treatment of some acute arterial thromboembolic events.