Warfarin side effects: Watch for interactions

Although commonly used to treat blood clots, warfarin (Jantoven) can have dangerous side effects and put you at risk of heavy bleeding. Know the warning signs and how to prevent side effects.

By Mayo Clinic Staff

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 warfarin can result in serious side effects.

The same action of warfarin that prevents blood clotting can result in bleeding. Warfarin treatment requires careful monitoring. Certain foods, medications, diet changes and illnesses can interfere with warfarin and increase your risk of bleeding.

If your doctor prescribes warfarin for you, make sure you understand what the potential side effects are and how to help prevent them.

When is warfarin prescribed?

You might be given warfarin if you have:

  • A blood clot in or near your heart that could trigger stroke, heart attack or organ damage
  • A blood clot in your lungs (pulmonary embolism)
  • A blood clot elsewhere in your body (venous thrombosis)
  • A high risk of blood clots forming in the heart, which can be a complication of some heart rhythm problems (arrhythmias)
  • A mechanical artificial heart valve that is more likely to forming blood clots

What are the side effects of warfarin?

The main side effect of warfarin is bleeding. While the risk of major bleeding is low, you need to be aware of potential problems. For example, you might have trouble stopping the bleeding from a cut on your hand or a nosebleed. More-serious bleeding may occur inside the body (internal).

Seek immediate medical help if you have any of the following:

  • Severe bleeding, including heavier than normal 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 an injury you remember
  • Dizziness or weakness
  • Vision changes
  • Head injury, even if you’re not 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 any sores, changes in skin color or temperature, or severe pain on your skin.

Talk to your doctor about these less serious side effects:

  • Bleeding from the gums after you brush 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 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 your risk.

Some people who take warfarin are at a higher risk of bleeding because their genetics make them more sensitive to the medication. Your doctor may recommend a genetic test to determine the best dose for you.

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 certain foods, drugs, vitamins or herbal supplements. An interaction might lower the effectiveness of warfarin or increase your 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, Naprelan, others)
  • Medications that treat abnormal heart rhythms, such as amiodarone (Pacerone, Nexterone)

Common supplements that can interact with warfarin include:

  • Coenzyme Q10 (ubiquinone)
  • 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, including lettuce, spinach and broccoli. If you are taking warfarin, the amount of vitamin K in your diet may affect how well the medicine works. If you take warfarin, it’s important to have a consistent amount of vitamin K in your diet. Too little vitamin K can increase your risk of bleeding. Too much vitamin K can decrease warfarin’s anti-clotting ability.

Talk to your doctor about healthy choices you can 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 doctor 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 your doctor and dentist that you take warfarin before you have any medical or dental procedures. Share this information even before minor procedures, such as vaccinations and routine dental cleanings. If you’re going to have surgery, you may need to decrease or stop your warfarin dose at least five days before the procedure. 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 doctor 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 doctor for instructions. If your doctor isn’t available, skip the missed dose and resume your normal dosing schedule. Never take a double dose.

If you follow your doctor’s 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 doctor, nurse or pharmacist if you have any concerns about warfarin.

May 14, 2021

See more In-depth