Harvard Heart Letter

Understanding blood thinners

These medications don’t actually “thin” your blood, but they do help prevent dangerous clots.

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, says cardiologist Dr. Gregory Piazza, associate professor of medicine at Harvard Medical School.

“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,” he says. Many people, including doctors, refer to the drugs used to achieve this feat as blood thinners. But they don’t actually make the blood less viscous, says Dr. Piazza. Rather, they discourage clots by interfering with one of the two key components involved in clot formation: fibrin and platelets (see “Anatomy of a blood clot”). As such, they’re also known as anti-clotting drugs.

Anatomy of a blood clot

Blood clots consist of two key components:

  • fibrin (a strand-shaped protein that forms a mesh that traps red blood cells)
  • platelets (tiny cell fragments that clump together).

Together, they stabilize clots, keeping them from falling apart.

Clots that form in the veins are made mostly of fibrin. Such a clot can form in the deep veins of the legs, arms, 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.

Clot disruption

These drugs fall into two main categories (see “Common anti-clotting drugs”). Anticoagulants inhibit the creation of fibrin, whereas antiplatelets 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. This rapid, irregular heart rhythm can cause blood to pool in the heart’s upper chambers (atria), raising the risk of clots. Warfarin, the old standby drug, is increasingly being replaced by newer drugs in this class.

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, Dr. Piazza explains. Typically, people receive aspirin and a prescription antiplatelet drug for at least six months to a year after receiving a stent. People with peripheral artery disease (narrowed arteries in the legs) may also take antiplatelet drugs.

In some cases, people take anticoagulants and antiplatelets together. As you might expect, this combination therapy increases the risk of bleeding problems. But the truth is that even taking a single anti-clotting drug for more than a few weeks can cause potentially serious bleeding. “So it’s important to pay attention to even minor bleeding,” Dr. Piazza says.

Common anti-clotting drugs

Anticoagulant drugs

These include warfarin (Coumadin) and the following direct oral anticoagulants (DOACs):

  • apixaban (Eliquis)
  • betrixaban (Bevyxxa)
  • dabigatran (Pradaxa)
  • edoxaban (Savaysa)
  • rivaroxaban (Xarelto)

Antiplatelet drugs

  • aspirin
  • clopidogrel (Plavix)
  • prasugrel (Effient)
  • ticagrelor (Brilinta)

Bleeding problems: Minor and major

What are the warning signs? Some people notice bleeding gums after toothbrushing or flossing, or they have nosebleeds that take longer than usual to stop. Frequent or large bruises (especially on the trunk of the body) are another potentially worrisome sign. Be sure to mention these symptoms to your doctor.

See a doctor right away if you have more serious symptoms. These include

  • tea-colored, pink, or red urine (possible bleeding in the urinary tract)
  • stools that are black and tarry or red (possible gastrointestinal bleeding)
  • a sudden, very bad headache (possible bleeding in the brain).

You should also get medical attention if you fall or get hit hard, even if there’s no visible blood, says Dr. Piazza. Finally, be aware that common over-the-counter pain relievers such as ibuprofen (Advil, Motrin), and naproxen (Aleve) can interact with anti-clotting drugs. Check with your doctor if you take these drugs on a routine basis.

Image: wildpixel/Getty Images

Learn more about our
health content.