Warfarin is an anticoagulant used to prevent heart attacks, strokes, and blood clots. Learn about side effects, interactions and indications. Warfarin is used to decrease the tendency for thrombosis, or as secondary prophylaxis (prevention of further episodes) in those individuals who have already formed a blood clot (thrombus). Warfarin: learn about side effects, dosage, special precautions, and more on MedlinePlus Warfarin is used to prevent or treat blood clots, including deep venous thrombosis or pulmonary embolism. It is also used for blood clots that may be caused by certain heart conditions, open-heart surgery, or after a heart attack. What Is Warfarin? Warfarin (Coumadin® and Jantoven®) is an anticoagulant medication or “blood thinner” you take to prevent and treat blood clots that can lead to a stroke or heart attack. Warfarin slows down your blood’s ability to clot. Warfarin is used to treat blood clots and/or to prevent new clots from forming in your body. Learn about side effects, drug interactions, dosages, warnings, and more. Find patient medical information for Warfarin on WebMD including its uses, side effects and safety, interactions, pictures, warnings, and user ratings.