Anticoagulants are prescribed to prevent the formation of blood clots in high-risk patients or to limit the growth of existing clots. Larger clots are more prone to falling apart and forming emboli – pieces of clots that break off and travel to other areas of the body. If these traveling clots become lodged in a blood vessel, the resulting embolism could be fatal. Anticoagulants are commonly prescribed for people with the following conditions, or who have undergone certain medical procedures. These include:

• Deep vein thrombosis. One or more blood clots in veins embedded deeply in the muscle mass of the legs or lower abdomen. Patients with this condition are at great risk for developing pulmonary emboli.
  
  
• Pulmonary embolism. A blockage in the blood vessels of the lungs, which is caused by a blood clot that has broken away from its original location and traveled through the bloodstream to the lungs, often as the result of deep vein thrombosis.
  
  
• Coronary artery disease with unstable angina. This is a condition in which hardening and narrowing of the arteries (atherosclerosis) causes unpredictable episodes of chest pain, pressure or discomfort. In contrast, stable angina is usually triggered by a predictable event, such as exercise.
  
  
• Certain types of heart attacks. Anticoagulants are a primary treatment of certain types of heart attacks, which are life-threatening events that result in permanent heart damage or death.
  
  
• Atrial fibrillation. A rapid, irregular heart rhythm (arrhythmia) caused by faulty electrical signals from the upper chambers of the heart (atria). This arrhythmia causes blood clots to form in the left atrium. If these blood clots travel out of the heart and to the brain, they could cause a stroke.
  
  
• Cerebral thrombosis. The formation of an obstructing blood clot (thrombus) in a blood vessel within the brain. This increases the risk of a stroke or mini-stroke (transient ischemic attack).
  
  
• Valvular heart disease. Any dysfunction or abnormality of one or more of the heart’s four valves. Anticoagulants are prescribed for patients who have both valvular heart disease and atrial fibrillation. They are also prescribed for every patient with a mechanical valve.
  
  
• Cardiomyopathy. Anticoagulants are sometimes used to treat this disease of the heart muscle.
  
  
• Balloon angioplasty. A procedure in which the physician inflates a balloon at the tip of a catheter to press plaque back against the artery wall to widen the vessel and make room for blood to flow. An angioplasty is used to treat both heart attacks and angina, and anticoagulants are often prescribed for patients who have recently undergone this procedure.
  
  
• Open-heart surgery. An operation in which the chest and (sometimes) the heart are opened surgically while the entire blood flow is diverted through a heart-lung machine.