Any substance that prevents blood clotting. Heparin is one which inactivates thrombin and several other clotting factors and is administered IV or subcutaneously. Oral anticoagulants include warfarin, dicumarol (coumadin), which prevent the liver from using Vitamin K clotting factors. Any patient taking these must be checked regularly by his physician and have prothrombin time tests performed periodically for the entire course of his anticoagulant therapy. He should also carry a medical alert card to warn emergency personnel that he is taking this medication.