Warfarin for Atrial Fibrillation


Generic Name Brand Name
warfarin Coumadin

How It Works

Warfarin helps prevent the formation of blood clots by increasing the time it takes a blood clot to form. This also prevents an existing clot from getting larger.

Why It Is Used

Warfarin is used to lower the risk of stroke in people who have atrial fibrillation. Your doctor may recommend warfarin based on your risk factors and on whether you can take warfarin safely. Anything that increases your risk for a disease or problem is called a risk factor. The more risk factors you have, the greater your risk of stroke.

Warfarin can reduce the risk of stroke in anyone who has atrial fibrillation. Even if your heart rhythm is under control, you will still take warfarin. Some people go in and out of atrial fibrillation without even knowing it. Taking warfarin can lower your chances of having a blood clot or a stroke.

Your doctor may have you take an anticoagulant for a few weeks after cardioversion for atrial fibrillation.

How Well It Works

Warfarin lowers the risk of stroke in people who have atrial fibrillation. But how much your risk will be lowered depends on how high your risk was to start with. Not everyone with atrial fibrillation has the same risk of stroke. It's a good idea to talk with your doctor about your risk.

You will want to weigh the benefits of reducing your risk of stroke against the risks of taking warfarin. Warfarin works well to prevent stroke. But warfarin also raises the risk of bleeding. Each year about 1 to 3 out of 100 people who take warfarin will have a problem with severe bleeding, and about 97 to 99 will not.footnote 1 But this is an average risk. Your own risk may be higher or lower than average, based on your age and your own health. For example, your risk may be higher if you have kidney or liver disease.

Side Effects

All medicines have side effects. But many people don't feel the side effects, or they are able to deal with them. Ask your pharmacist about the side effects of each medicine you take. Side effects are also listed in the information that comes with your medicine.

Here are some important things to think about:

  • Usually the benefits of the medicine are more important than any minor side effects.
  • Side effects may go away after you take the medicine for a while.
  • If side effects still bother you and you wonder if you should keep taking the medicine, call your doctor. He or she may be able to lower your dose or change your medicine. Do not suddenly quit taking your medicine unless your doctor tells you to.

Allergic reaction

Call 911 or other emergency services right away if you have:

  • Trouble breathing.
  • Swelling of your face, lips, tongue, or throat.

Call your doctor if you have:

  • Hives.


Call 911 or other emergency services right away if you have:

  • A sudden, severe headache that is different from past headaches. (It may be a sign of bleeding in the brain.)

Call your doctor now or seek immediate medical care if you have:

  • Any abnormal bleeding, such as:
    • Nosebleeds.
    • Vaginal bleeding that is different (heavier, more frequent, at a different time of the month) than what you are used to.
    • Bloody or black stools, or rectal bleeding.
    • Bloody or pink urine.

If you are injured, apply pressure to stop the bleeding. Realize that it will take longer than you are used to for the bleeding to stop. If you can't get the bleeding to stop, call your doctor.

Other side effects of warfarin include:

  • Skin rash.

See Drug Reference for a full list of side effects. (Drug Reference is not available in all systems.)

What To Think About

When you take warfarin, you need to take extra steps to avoid bleeding problems.

  • Get regular blood tests.
  • Prevent falls and injuries.
  • Eat a steady diet, and pay attention to foods that contain vitamin K.
  • Tell your doctors about all other medicines and vitamins that you take.

For more information, see:

Warfarin: Taking Your Medicine Safely.

Know what to do if you miss a dose of anticoagulant.

Taking medicine

Medicine is one of the many tools your doctor has to treat a health problem. Taking medicine as your doctor suggests will improve your health and may prevent future problems. If you don't take your medicines properly, you may be putting your health (and perhaps your life) at risk.

There are many reasons why people have trouble taking their medicine. But in most cases, there is something you can do. For suggestions on how to work around common problems, see the topic Taking Medicines as Prescribed.

Advice for women

Do not take warfarin if you are pregnant. Warfarin can cause miscarriage or birth defects. If you are taking warfarin, talk to your doctor about how you can prevent pregnancy.

If you think you might be pregnant: Call your doctor. If you are pregnant, you will take heparin during your pregnancy.

If you plan on getting pregnant: Talk with your doctor. You and your doctor will decide which medicine you will take—warfarin or heparin—while trying to get pregnant.


Follow-up care is a key part of your treatment and safety. Be sure to make and go to all appointments, and call your doctor if you are having problems. It's also a good idea to know your test results and keep a list of the medicines you take.

Complete the new medication information form (PDF)(What is a PDF document?) to help you understand this medication.



  1. Ruff CT, et al. (2014). Comparison of the efficacy and safety of new oral anticoagulants with warfarin in patients with atrial fibrillation: A meta-analysis of randomised trials. The Lancet, 383(9921): 955–962. DOI: 10.1016/S0140-6736(13)62343-0. Accessed: April 15, 2014.


ByHealthwise Staff
Primary Medical ReviewerRakesh K. Pai, MD, FACC - Cardiology, Electrophysiology
E. Gregory Thompson, MD - Internal Medicine
Specialist Medical ReviewerJohn M. Miller, MD, FACC - Cardiology, Electrophysiology

Current as ofFebruary 20, 2015