Drugs that delay blood clotting (e.g., coumarin, heparin and warfarin). Patients will often be given an anticoagulant for 3-6 months after problems with blood clots in the legs (deep vein thrombosis) or lungs (pulmonary embolism).They are also used in patients who are at risk of developing blood clots, for example in patients with irregular heart beats (atrial fibrillation).