Anticoagulants are a class of drugs that work to prevent blood coagulation (clotting). Such substances occur naturally in leeches and blood-sucking insects. A group of pharmaceuticals called anticoagulants can be used as an injection 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.