Warfarin side effects: Watch for interactions

This medicine, commonly used to treat or prevent blood clots, can increase the risk of heavy bleeding. Learn how to prevent side effects.

By Mayo Clinic Staff

Warfarin (Jantoven) is a medicine to prevent blood clots. It may save your life if you’ve had blood clots or are at risk of them. But warfarin can sometimes cause serious side effects, such as heavy bleeding. It’s important to have regular health checkups if you take the medicine.

The same action of warfarin that stops blood from clotting can cause bleeding. Warfarin treatment is a careful balance. Diet changes, being sick, and using some medicines, herbs or supplements can change the levels of the medicine in the body.

When is warfarin prescribed?

Your healthcare team might prescribe warfarin if you have:

  • A blood clot in or near the heart that could cause a stroke, heart attack or organ damage.
  • A blood clot in the lungs, also called a pulmonary embolism.
  • A blood clot in other places in the body.
  • An irregular heartbeat called atrial fibrillation (AFib). AFib increases the risk of blood clots that can lead to strokes.
  • An artificial heart valve.

What are the side effects of warfarin?

The main side effect of warfarin is bleeding. If you’re taking warfarin, you may have trouble stopping the bleeding from a cut or a nosebleed. More-serious bleeding may happen inside the body. Bleeding inside the body is called internal bleeding.

Get medical help right away if you take warfarin and you have any of the following:

  • Bleeding from a cut or the nose or gums that lasts more than 5 or 10 minutes when applying pressure.
  • Vaginal bleeding, including menstrual bleeding that’s heavier than usual.
  • Coughing up blood.
  • Dizziness or weakness.
  • Severe headache.
  • Head injury or fall, even if there are no signs of bleeding.
  • Red or brown urine.
  • Severe stomach pain.
  • Vomiting of blood or material that looks like coffee grounds.
  • Stools that are black or bloody.
  • Unusual bruising.
  • Pain or swelling in the joints, especially after an injury.
  • Vision changes.

Rarely, warfarin can cause the death of skin tissue, also called necrosis. This complication occurs within a few days of starting warfarin treatment. If you take warfarin, get immediate medical care if you have these skin changes:

  • Sores on the skin.
  • Changes in the color or temperature of the skin.
  • Severe pain on your skin.

Talk to your healthcare team if you take warfarin and have these symptoms:

  • Bleeding from a cut or the nose or gums that lasts less than 5 minutes. For example, if your gums bleed after brushing your teeth.
  • Bleeding between menstrual periods.
  • Diarrhea, vomiting or inability to eat for more than 24 hours.
  • Fever.

What can increase the risk of bleeding?

Some people who take warfarin have a higher risk of bleeding because their genes make them more sensitive to medicine.

Health conditions and some lifestyle choices also may increase the risk of bleeding. Examples include:

  • A history of stroke.
  • Cancer.
  • Drinking too much alcohol.
  • Kidney disease.
  • Liver disease.
  • Stomach ulcers, swelling and irritation of the stomach, or peptic disease.
  • Uncontrolled blood pressure.

People who have an increased risk of falls also have a higher risk of bleeding.

Some studies say that the risk of bleeding is generally higher in the first three months of warfarin treatment. Older adults are generally at greater risk of bleeding. Taking other blood-thinning medicine also increases the risk.

What medicines, supplements and foods interact with warfarin?

Some medicines, foods, vitamins, herbs and supplements can change the levels of warfarin in the body. This can make warfarin more or less effective.

Some medicines that can interact with warfarin are:

  • Aspirin.
  • Acetaminophen (Tylenol).
  • Antacids or laxatives.
  • Covid-19 medicine called nirmatrelvir/ritonavir (Paxlovid).
  • Many antibiotics.
  • Medicines to treat fungal infections, including fluconazole (Diflucan).
  • Ibuprofen (Advil, Motrin IB, others) or naproxen sodium (Aleve, others).
  • Medicines to treat irregular heart rhythms such as amiodarone (Pacerone, Nexterone).
  • Seizure medicines, including phenytoin (Dilantin, Phenytek), phenobarbital (Sezaby), carbamazepine (Carbatrol, Tegretol, Equetro).

Some herbs or supplements may interact with warfarin. These include:

  • Dong quai
  • Garlic
  • Ginkgo biloba
  • Ginseng
  • Green tea
  • St. John’s wort
  • Vitamin E

Common foods and drinks that might interact with warfarin include:

  • Alcohol.
  • Black licorice.
  • Cranberries or cranberry juice.
  • Garlic.
  • Green tea.
  • Grapefruit.

What does vitamin K have to do with warfarin?

Vitamin K is an important nutrient for heart and bone health. It helps blood clot. But warfarin blocks the action of vitamin K. If you take warfarin, eating foods rich in vitamin K can make the medicine less effective.

Foods rich in vitamin K are green vegetables, such as lettuce, spinach and broccoli. If you take warfarin, it’s important to have a steady amount of vitamin K in your diet. Do not change the amounts of vitamin K-rich foods or drinks you have from day to day or week to week. A sudden change in vitamin K levels may affect how much warfarin you need.

Ask your healthcare team or nutritionist how much vitamin K you should get in your diet.

What can you do to lower the risk of bleeding?

To reduce the risk of a bleeding from warfarin treatment, follow these tips:

  • Tell your care team about all the medicines you take. Include those bought without a prescription. Also include herbs and supplements. Before starting a new medicine, check with the care professional who manages your warfarin treatment.
  • Always let your doctor, dentist and other healthcare professionals know that you take warfarin. Do this even when getting vaccines and routine dental cleanings. You might need to stop or lower your warfarin dose before surgery. Sometimes a shorter-acting blood thinner is given by injection. Never stop or change your warfarin treatment without talking to a member of your care team.
  • Protect yourself from falls and injury. Ask your care team if contact sports are safe for you. Always wear a bicycle helmet. Tell your care team if you are unsteady while walking or have a history of falling.
  • Choose dental and grooming products carefully. Use a soft-bristle toothbrush and waxed dental floss. They may help lower the risk of bleeding from the gums. Choose an electric razor for shaving.
  • Wear a medical ID bracelet. This identification tells healthcare professionals that you take warfarin. An option is to carry a medical ID card.

What should you do if you forget a dose?

If you take warfarin every day and miss one dose of the medicine, take it as soon as you remember. If you missed two doses, call your healthcare team for instructions. Never take a double dose.

Taking medicine as directed lowers the risk of side effects and interactions. Talk to your healthcare team or pharmacist if you have concerns about warfarin.


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