Hirudin is a naturally occurring peptide in the salivary glands of medicinal leeches (Hirudo medicinalis) that has a blood anticoagulant property. This is fundamental for the leeches’ alimentary habit of hematophagy, since it keeps the blood flowing after the initial phlebotomy performed by the worm on the host’s skin.A key event in the final stages of blood coagulation is the convertion of fibrinogen into fibrin by the serine protease enzyme thrombin.[4] Thrombin is produced from prothrombin, by the action of an enzyme, prothrombinase, in the final states of coagulation. Fibrin is then cross linked by factor XIII to form a blood clot. The principal inhibitor of thrombin in normal blood circulation is antithrombin III.[3] Similar to antithrombin III, the anticoagulatant activity of hirudin is based on its ability to inhibit the pro-coagulant activity of thrombin.Hirudin is the most potent natural inhibitor of thrombin. Unlike antithrombin III hirudin binds to and inhibits only the activity of thrombin forms with a specific activity against fibrinogen.[3] Therefore, hirudin prevents or dissolves the formation of clots and thrombi (i.e. it has a thrombolytic activity), and has therapeutic value in blood coagulation disorders, in the treatment of skin hematomas and of superficial varicose veins, either as an injectable or a topical application cream. In some aspects, hirudin has advantages over more commonly used anticoagulants and thrombolytics, such as heparin, as it does not interfere with the biological activity of other serum proteins and can also act on complexed thrombin.