If you've been prescribed warfarin (Coumadin, 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 you may not realize how serious warfarin side effects can be.

Warfarin, especially if taken incorrectly, increases your risk of dangerous bleeding. Warfarin side effects can also include interactions with some foods, prescription medicines and over-the-counter supplements.

If your doctor prescribes warfarin for you, make sure you understand all the potential warfarin side effects and interactions it could have.

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 abnormalities (arrhythmias)
  • A mechanical artificial heart valve that is prone to forming blood clots

When you take warfarin, your blood won't clot as easily. If you accidentally cut yourself while taking warfarin, you may bleed heavily. However, the risk of a major bleeding event is low.

You're more likely to have bleeding problems if you're older than 75 or take other blood-thinning medications that can further increase your bleeding risk.

You're also at higher risk of bleeding problems if you have:

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

Some studies suggest that bleeding problems are more likely to occur during the first month of taking warfarin rather than later in treatment.

  • 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

Rarely, warfarin can cause the death of skin tissue (necrosis). This complication occurs most often three to eight days after you start taking warfarin. If you notice any sores, changes in skin color or temperature, or severe pain on your skin, seek immediate medical care.

  • 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

To reduce your chance of developing warfarin side effects:

  • Tell your doctor about any other medications or supplements you take. Many medications and supplements can have a dangerous interaction with warfarin. If you receive a new prescription from someone other than your usual doctor, ask if you'll need additional blood tests to make sure the new medication isn't affecting your blood clotting.
  • Tell care providers you take warfarin before you have any medical or dental procedures. It's important to 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 discontinue 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.
  • Avoid situations that increase your risk of injury. Contact sports and other activities that could result in head injury should be avoided. 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.
  • Consider a warfarin sensitivity test. A significant number of people who take warfarin are at a higher risk of bleeding because their genes make them more sensitive to warfarin. If a family member experienced side effects from warfarin, talk to your doctor about taking a genetic warfarin sensitivity test. The test can determine if you have the genes that can increase your risk of bleeding.

Like any other medication, warfarin can interact with foods, other drugs, vitamins or herbal supplements. The interaction might lower the effectiveness of warfarin or increase your risk of bleeding. More than 120 drugs and foods that can interact with warfarin have been identified.

Drugs that can interact with warfarin include:

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

Many other medications interact with warfarin. Be sure to tell your doctor or pharmacist that you take warfarin.

Supplements that can interact with warfarin include:

  • Coenzyme Q10 (Ubiquinone)
  • Dong quai
  • Garlic
  • Ginkgo biloba
  • Ginseng
  • Green tea
  • St. John's wort
  • Vitamin E

Many other supplements can interact with warfarin. Be sure to tell your health care provider about any supplements you take.

Foods and drinks that might interact with warfarin include:

  • Cranberries or cranberry juice
  • Alcohol
  • Foods that are high in vitamin K, such as soybean and canola oils, spinach and broccoli
  • Garlic
  • Black licorice

Your doctor might recommend keeping the level of vitamin K in your diet consistent rather than avoiding vitamin K-rich foods altogether.

Never take a double dose of warfarin. Doing so could greatly increase your risk of side effects.

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 start again the next day.

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.

Last Updated: 02-03-2018
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