An anticoagulant is a drug that interferes with blood clotting, used to treat or prevent thrombosis and clot embolism. The two main types of anticoagulants are heparin, which is injected and has an immediate but short-lived effect, and the commarins (including warfarin) which are taken by mouth and are longer-lasting. They affect different parts of the clotting mechanism, commarins depleting factors in the liver.