Warfarin (brand name: Coumadin) is a medication in the class of drugs known as anticoagulants, which were developed to lower the risk of blood clots. People who have atrial fibrillation (Afib or AF) are at risk for blood clots because their heart does not beat normally. When the heartbeat becomes irregular, the heart may not be able to pump blood as it should. As a result, blood can collect in the heart’s left atrial appendage and clots can form. If a clot travels to the brain, it can cause an ischemic stroke. Warfarin has been prescribed for stroke prevention since the 1950s. It is a less expensive option than the newer anticoagulant medications that are available.

If you have Afib, it is very likely that your doctor has prescribed medications to help control your heart’s rate and rhythm as well as an anticoagulant medication to prevent clots. Many people do very well with anticoagulants, but some cannot take an anticoagulant because it increases the risk for internal bleeding. Others struggle to take the medications as prescribed or cannot handle the side effects. If you and your doctor decide that anticoagulants are not right for you, there are other treatment options available. You can learn about these treatment options here.  

This page is meant to familiarize you with warfarin and how it may be used to treat Afib. Note: This information is intended only to provide an overview. It should not take the place of a doctor’s recommendation. Be sure to speak with your doctor about your condition, symptoms and treatment options.

Limitations of Warfarin Therapy

Warfarin is the oldest and perhaps the best known, of the blood-thinner medications. While warfarin has been proven to be effective for preventing blood clots, it does have some drawbacks that you should be aware of.

Warfarin Requires Regular Blood Tests

Like all blood-thinning medications, warfarin increases the risk for severe internal bleeding. This is why people who are taking warfarin are required to have regular INR blood tests to confirm the drug is continuing to work and that it is not causing problems. INR stands for International Normalized Ratio. These tests are necessary to monitor how your body is responding to the medication. INR tests allow your doctor to see how fast your blood clots and determine a healthy dose of warfarin for you.

INR tests must be done frequently; however, your exact testing schedule will depend on your doctor’s recommendation. If you are taking warfarin, you will need to monitor your schedule carefully to ensure that you do not miss a dose of medication and you do not miss an INR test.

If you develop bleeding complications from taking warfarin, your doctor will need to address it immediately. Fortunately, because warfarin has been in use for a long time, there is a proven method for addressing bleeding problems. If there is a problem, your doctor may give you vitamin K, which is the antidote for warfarin. An antidote (in this case, vitamin K) is a substance that reverses or decreases the harmful effects of the previous substance (the warfarin). Vitamin K enables clotting factors in the blood to work properly. Depending on the severity of the bleeding, vitamin K can be given orally or through an IV.

Vitamin K is an antidote for warfarin, not other anticoagulant medications. If needed, your doctor will formulate an exact dosage of vitamin K for you.

People who have Afib and who are taking warfarin should not take Vitamin K for any reason without a doctor’s consultation.

Warfarin & Interactions

Warfarin interacts with many commonly used medications. This is why it is very important that you tell your healthcare team about all of your over-the-counter and prescribed medications, supplements and vitamins. Be sure to share information about everything you take, even if you only take it occasionally. Some of the medications, supplements and even foods that interact with warfarin include (but are not limited to) the following:

Medications That Interact with Warfarin

• Many antibiotics
• Drugs that treat heart rhythm disorders (for example: amiodarone, known as Cordarone or Pacerone)
• Some antifungal medications
• Aspirin (or products that contain aspirin)
• Non-steroidal anti-inflammatory medications, such as ibuprofen (found in Motrin, Advil and many other over-the-counter medications) or Naproxen (found in Aleve)
• Acetaminophen (found in Tylenol and similar products)
• Cold and allergy medications
• Various antacids and laxatives, both prescription and over-the-counter
• Many other medications, both of the prescription and non-prescription varieties

Supplements That Interact with Warfarin

• Garlic
• Ginkgo biloba
• John’s Wort
• Green tea
• Ginseng
• Vitamin E

If you commonly take supplements, make a list of them and bring them to your doctor so that he or she can let you know what is safe to use with warfarin.

Foods & Drinks That Interact with Warfarin

The foods that tend to interact with warfarin are high in Vitamin K. They include (but are not limited to) the following:

• Spinach
• Broccoli
• Soybean and canola oils
• Alcohol
• Cranberries and cranberry juice
• Garlic
• Black licorice

If you have Afib, your doctor will advise you to eat a heart-healthy diet, but if you are taking warfarin, he or she will have extra precautions regarding what you eat and drink.

Reminder: Before you start taking any new medication, supplement or vitamin, it is a good idea to tell your health provider and pharmacist that you are taking warfarin.

Pregnancy & Warfarin

If you are taking warfarin, have a discussion with your physician about birth control. You must avoid becoming pregnant while on warfarin, as it is known to cause birth defects. If you are taking warfarin and wish to start a family, talk with your doctor about the risks of stopping therapy and possible substitute blood-thinning medications. You must avoid breast-feeding while on warfarin since the medication can be excreted in your milk and could harm your baby.

Signs of Problems with Taking Warfarin

If you are taking warfarin, you should be alert to any changes in your body that could signal a bleeding problem; a negative interaction with a medication, food or supplement; or other issue.

The following symptoms should be considered to be an emergency. This means you should contact your doctor immediately.

• Severe bleeding (in women, this may include a heavier-than-normal menstrual period)
• Urine that is red or brown in color
• Stools that appear bloody or black in color
• Joint pain as well as swelling and discomfort in joints
• Bruising of your skin when you have not taken a fall or bumped yourself
• Vomiting blood (or of anything that resembles coffee grounds)
• Severe weakness or dizziness
• Severe headache or stomachache

Other symptoms that may be less severe but that indicate bleeding problems include bleeding of your gums when you brush your teeth, unexplained fever, diarrhea or vomiting that lasts more than 24 hours.

Talk with Your Doctor

If you have been recently diagnosed with atrial fibrillation, it is likely you have a number of questions to ask your doctor about your condition. To get you started, we invite you to download Questions to Ask Your Doctor About Atrial Fibrillation.  

If you and your doctor are considering anticoagulant therapy, you may have specific questions about how it will affect you. Some common questions are provided in SecondsCount’s article on What You Need to Know About Anticoagulant Medications.