Medicines used to prevent and treat blood clots. Sometimes known as “blood thinners,” anticoagulants are given for a variety of conditions that increase the risk that a blood clot will form. The two most common anticoagulants are warfarin (Coumadin) and heparin.

One of the blood’s most important properties is its ability to coagulate, or clot. Clotting helps prevent excessive blood loss after an injury. In some cases, however, blood clots when it is not supposed to, causing problems. A blood clot impeding blood flow in a large vein is known as deep vein thrombosis. If there is a blood clot in the heart, it can reduce blood flow to parts of the heart, causing a heart attack. A blood clot that lodges in the brain can cause a stroke, and one that lodges in the lungs can cause a potentially life-threatening condition known as pulmonary embolism. Blood clots can form in all these places in the body. They can also form in a vein and migrate elsewhere. Pulmonary embolism is most commonly caused by a clot that forms in a deep vein in the leg and then makes its way to the lungs.

An anticoagulant can treat these conditions and is often used to prevent them from occurring (or recurring) in people at risk of blood clots. For example, anticoagulants are used in people with atrial fibrillation, in which the upper chambers of the heart, called the atria, beat irregularly and out of sync with the lower chambers. Because the heart does not pump blood efficiently, blood can pool and clot there. The blood clot can then migrate, potentially leading to a stroke. People who have an artificial heart valve are also at increased risk for blood clots and often take anticoagulants.

In addition, people with antiphospholipid syndrome (APS), which occurs more often in people with systemic lupus erythematosus (SLE or lupus), have a greater risk of blood clots and are usually prescribed an anticoagulant. Lupus is most common among women of childbearing age, and so APS is also more likely to affect this population. Because warfarin can cause birth defects, pregnant women with APS are treated with heparin.

Anticoagulants are distinct from another type of blood thinner called antiplatelet drugs, which include aspirin and clopidogrel (Plavix), although both types of drugs have an anti-clotting effect. Whereas anticoagulants block chemicals in the blood responsible for clotting, antiplatelet drugs prevent the clumping together of cells in the blood called platelets. Warfarin or heparin should not be taken along with antiplatelet drugs except under the guidance of a physician because using both drugs at the same time can increase the risk of excess bleeding. Many other drugs, including nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen (Advil), naproxen (Aleve, Naprosyn), and celecoxib (Celebrex) may also bring about unwanted side effects when used at the same time as an anticoagulant. Anyone taking either warfarin or heparin should be sure to tell the doctor about all the drugs, both prescription and over-the-counter, he or she is using.

A nutrient called vitamin K is known to decrease warfarin’s effectiveness. People taking warfarin should keep their intake of vitamin K as consistent as possible, which usually means eating less of it. Foods rich in vitamin K include kale, spinach, Brussels sprouts, parsley, collard greens, and chard. People taking warfarin should also limit their intake of cranberry juice and alcohol, which may increase the effects of the drug and cause bleeding.

Because anticoagulants reduce the ability of the blood to form clots, they put people at increased risk of bleeding. If you take an anticoagulant, you should be very careful to prevent injury. Call your doctor immediately if you have a cut that doesn’t stop bleeding, menstrual bleeding that is much heavier than usual, or signs that may indicate internal bleeding, such as the following:

  • Red or brown urine
  • Bowel movements that are red or look like tar
  • Brown or bright red vomit
  • Severe headache or stomachache
  • Unusual bruising

Robert S. Dinsmoor is a medical writer and editor based in Massachusetts.

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