If you’ve been prescribed warfarin (Jantoven) to prevent blood clots, you probably already know that this powerful drug can save your life if you are at risk of or previously had blood clots. But it’s also important to remember that taking warfarin can result in serious side effects.

The same action of warfarin that prevents blood clotting can result in bleeding. Warfarin treatment is a careful balance. Certain things can alter levels of the drug in the body, increasing the risk of bleeding. Such things include:

  • Incorrect use of the drug
  • Use of certain medications or supplements
  • Diet changes
  • Illnesses

When is warfarin prescribed?

Warfarin might be prescribed for people who have:

  • A blood clot in or near the heart that could trigger stroke, heart attack or organ damage
  • A blood clot in the lungs (pulmonary embolism)
  • A blood clot elsewhere in the body (venous thrombosis)
  • Certain irregular heart rhythms (arrhythmias), such as atrial fibrillation, which increases blood clot risk
  • A mechanical artificial heart valve, which increases the risk of blood clots

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 on the hand or a nosebleed. More-serious bleeding may be inside the body (internal).

Seek immediate help if you take warfarin and you have any of the following:

  • Severe bleeding, including heavier than usual menstrual bleeding
  • Red or brown urine
  • Black or bloody stool
  • Severe headache or stomach pain
  • Joint pain, discomfort or swelling, especially after an injury
  • Vomiting of blood or material that looks like coffee grounds
  • Coughing up blood
  • Bruising that develops without a remembered injury
  • Dizziness or weakness
  • Vision changes
  • Head injury, even without bleeding

Rarely, warfarin can cause the death of skin tissue (necrosis). This complication occurs within a few days of starting warfarin treatment. Seek immediate medical care if you notice sores, changes in skin color or temperature, or severe pain on your skin.

Talk to your health care provider about these less serious side effects.

  • Bleeding from the gums after toothbrushing
  • Bleeding between menstrual periods
  • Diarrhea, vomiting or inability to eat for more than 24 hours
  • Fever

What can increase the risk of bleeding?

Some studies suggest 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 medication also increases the risk.

A number of people who take warfarin are at a higher risk of bleeding because their genetics make them more sensitive to warfarin. A health care provider may recommend a genetic test to guide decisions about appropriate dosing of the drug.

Medical conditions that increase the risk of bleeding include:

  • Uncontrolled blood pressure
  • A history of stroke
  • Stomach ulcers, gastritis or peptic disease
  • Kidney problems
  • Cancer
  • Alcoholism
  • Liver disease
  • Increased risk of falls

What drugs, supplements and foods interact with warfarin?

Like any other medication, warfarin can interact with drugs, foods, vitamins or herbal supplements. The interaction might lower the effectiveness of warfarin or increase the risk of bleeding.

Common drugs that can interact with warfarin include:

  • Aspirin or aspirin-containing products
  • Acetaminophen (Tylenol, others) or acetaminophen-containing products
  • Antacids or laxatives
  • Many antibiotics
  • Antifungal medications, such as fluconazole (Diflucan)
  • Cold or allergy medicines
  • Ibuprofen (Advil, Motrin IB, others) or naproxen sodium (Aleve, others)
  • Medications that treat irregular heart rhythms, such as amiodarone (Pacerone, Nexterone)

Common supplements that can interact with warfarin 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:

  • Cranberries or cranberry juice
  • Grapefruit
  • Alcohol
  • Garlic
  • Black licorice

What does vitamin K have to do with warfarin?

It’s important to understand the role of vitamin K in warfarin treatment and a healthy diet. Warfarin works by disrupting the role of vitamin K in a complex series of molecular events that cause blood clotting. But vitamin K is a nutrient essential for heart and bone health.

Foods rich in vitamin K are green vegetables, such as lettuce, spinach and broccoli. If you take warfarin, it’s important to have a consistent amount of vitamin K in your diet. If you have little vitamin K in your diet, a sudden spike in the vitamin can increase your risk of bleeding.

Talk to your health care provider about healthy choices to make to get an adequate and consistent supply of vitamin K.

What can you do to lower the risk of bleeding?

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

  • Tell your health care provider about all medications, herbal remedies or supplements you take. Ask if you need testing to see if a new medication affects your warfarin treatment.
  • Tell care providers that you take warfarin before you have medical or dental procedures. Share this information even before minor procedures, such as vaccinations and routine dental cleanings. You might need to decrease or stop your warfarin dose at least five days before surgery. Your doctor might prescribe a shorter acting blood thinner (heparin) while you’re not taking warfarin.
  • Protect yourself from injury. Avoid contact sports or activities with a high risk of head injury. Wear a bicycle helmet. Tell your health care provider if you are unsteady while walking or have a history of falling.
  • Use safer hygiene and grooming products. A soft-bristle toothbrush, waxed dental floss and an electric razor for shaving can help prevent bleeding.
  • Consider wearing a bracelet or carrying a card that says you take warfarin. This identification can be useful if emergency medical providers need to know what medications you take.

What should you do if you forget a dose?

If you miss a dose, take it as soon as you remember. If you don’t remember until the next day, call your health care provider for instructions. If your provider isn’t available, skip the missed dose and resume your normal dosing schedule. Never take a double dose.

If you follow dosing instructions and tell all your health care providers that you take warfarin, you’ll be at a much lower risk of dangerous interactions and side effects. Talk to your health care provider or pharmacist if you have concerns about warfarin.


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