Senior Health

2/22/2022 | By Howard LeWine, M.D.

Dr. Howard LeWine of Harvard Medical School explains what blood thinners do and the difference in several common such medications.


Why do some people take aspirin to thin their blood and others take Coumadin or Eliquis?


For many people with cardiovascular disease, drugs that discourage blood clots are potential lifesavers. If a blood clot lodges in an artery or vein, it can choke off the blood supply to the heart, brain, leg, or lung, with possibly dire consequences.

Not all clots are bad, of course. Clots help stanch bleeding if you’re injured. The trick with clot-preventing drugs – commonly referred to as blood thinners – is finding just the right balance between helpful and harmful clotting.

The idea is to prevent unnecessary clots inside blood vessels while still allowing the blood to clot normally if, for example, you cut yourself making dinner. Many people refer to the drugs used to achieve this feat as blood thinners. But they don’t actually make the blood less viscous.

What blood thinners do – and how

Rather, they discourage clots by interfering with one of the two key components involved in clot formation: fibrin and platelets. Fibrin is a strand-shaped protein that forms a mesh that traps red blood cells. Platelets are tiny cell fragments that clump together. As such, they’re also known as anti-clotting drugs.

Clots that form in the veins are made mostly of fibrin. Such a clot can form in a deep vein of the leg, arm, or abdomen and is known as a deep-vein thrombosis, or DVT. Clots in the arteries, on the other hand, tend to contain more platelets. These tend to form in the arteries feeding the heart and brain, as well as the legs.

Anti-clotting medications fall into two main categories. Anticoagulants, such as warfarin (Coumadin) and apixaban (Eliquis), inhibit the creation of fibrin. Antiplatelet drugs, such as aspirin, prevent platelets from sticking together. Both classes of drugs prevent clots from forming and growing.

Anticoagulants treat blood clots in the legs (deep-vein thrombosis) and the lungs (pulmonary embolism). They’re also prescribed to people with atrial fibrillation to prevent a stroke. This rapid, irregular heart rhythm can cause blood to pool in the heart’s upper chambers (atria), raising the risk of clots. Clots in the atria can break free and travel to the brain.

Antiplatelet drugs are used to prevent heart attacks and strokes and to treat people who receive stents, the tiny metal mesh tubes placed in clogged blood vessels. Platelets tend to stick inside stents and cause them to close up, so the drugs help counteract that tendency. People with peripheral artery disease (narrowed arteries in the legs) also typically take aspirin or another antiplatelet drug.

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Howard LeWine, M.D.

Howard LeWine, M.D., is an internist at Brigham and Women’s Hospital in Boston and assistant professor at Harvard Medical School. For additional consumer health information, please visit