Anticoagulants are a class of drugs that work to prevent the coagulation (clotting) of blood. Such substances occur naturally in leeches and blood-sucking insects. A group of pharmaceuticals called anticoagulants can be used as an injection into human beings as a medication for thrombotic disorders. Oral anticoagulants are also available. Some anticoagulants are used in medical equipment, such as test tubes, blood transfusion bags, and renal dialysis equipment.