Coumadin (Warfarin) is an anticoagulant used to treat or prevent harmful blood clots that may occur in the veins (venous thrombosis) or lungs (pulmonary embolism), or with a type of abnormal heartbeat (such as atrial fibrillation), or following a heart valve replacement. This medicine is also used to reduce the risk of stroke and other problems in patients who have had a heart attack.