Anti-coagulant medications, such as coumadin and heparin, are used to prevent blood clots that can obstruct veins and arteries. These medications, also called blood thinners, inhibit clot formation by blocking the action of clotting factors or platelets. Anti-coagulant medications are used to treat a number of conditions. They are often given to reduce the risk of stroke or heart attack. They are also used during open heart or bypass surgery to reduce the change of blood clots forming during surgery. Low doses of these medications are frequently given to patients who must stay in bed for long periods of time.