Also called anticoagulants, blood thinners are drugs that decrease the ability of the blood to clot, or coagulate. They are used to treat certain blood vessel and heart and lung conditions. They are also given to some people at high risk for blood clots, including those with atrial fibrillation or artificial heart valves. Blood thinners do not dissolve clots but may prevent existing clots from becoming larger and causing more serious problems. They are often prescribed to prevent first or recurrent heart attack or stroke. Common anticoagulant drugs are heparin and warfarin.