Warfarin side effects: Watch for dangerous interactions

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Warfarin side effects: Watch for dangerous interactions

If you've been prescribed warfarin (Coumadin) to prevent blood clots, you probably already know that this powerful drug can save your life if you're at risk of or have 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.

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, such as in some heart rhythm abnormalities (arrhythmias)
  • A mechanical artificial heart valve that is prone to forming blood clots

What warfarin side effects should you watch for?

When you take warfarin, your blood won't clot as easily. If you accidentally cut yourself while taking warfarin, you may bleed heavily. 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.

Warfarin side effects that require immediate medical attention

  • Severe bleeding
  • Black stool or bleeding from the rectum
  • Skin conditions such as hives, a rash or itching
  • Swelling of the face, throat, mouth, legs, feet or hands
  • Bruising that comes about without an injury you remember
  • Chest pain or pressure
  • Nausea or vomiting
  • Fever or flu-like symptoms
  • Joint or muscle aches
  • Diarrhea
  • Difficulty moving
  • Numbness of tingling in any part of your body
  • Painful erection lasting four hours or longer

Although rare, warfarin can also cause skin tissue death (necrosis) and gangrene requiring amputation. This complication most often happens 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, notify your doctor immediately.

Less serious warfarin side effects to tell your doctor about

  • Gas
  • Feeling cold
  • Fatigue
  • Pale skin
  • Changes in the way foods taste
  • Hair loss

What precautions can you take against warfarin side effects?

There are some key steps you can take to reduce your chance of developing warfarin side effects.

  • Tell your doctor about any other medications or supplements you take. Some medications and supplements can have a dangerous interaction with warfarin.
  • Tell your health care provider you take warfarin before you have any medical or dental procedures. Tell providers that you take warfarin even before minor procedures, such as vaccinations and routine dental cleanings. If you're going to have surgery, discuss what dosing changes might be necessary at least 10 days before the procedure. Your doctor may decrease your dose of warfarin or discontinue warfarin altogether before the procedure. Talk to the doctor who prescribed warfarin to find out if it's safe to discontinue for a few days. If you are at high risk of blood clots, you may need a shorter acting blood thinner, heparin, to keep a lower risk of blood clots while you're not taking warfarin.
  • Avoid situations that increase your risk of injury. Avoid contact sports or activities that could result in head injury. Make sure you tell your doctor if you are unsteady while walking or have a history of falling.
  • Consider wearing a bracelet or carrying a card that identifies that you take warfarin. This can be useful if emergency medical personnel need to know what medications you take.
  • Consider a warfarin sensitivity test. About a third of the people who take warfarin are at a higher risk of bleeding because their genes make them more sensitive to warfarin. If you're worried that you might be more sensitive to warfarin because a family member experienced side effects, talk to your doctor about taking a warfarin sensitivity test. The test can tell if you have the genes that can increase your risk of bleeding.
  • If you receive new prescriptions from someone other than your usual medical provider, make sure to remind them that you are taking warfarin. Ask if you'll need additional blood tests from your regular care provider to make sure your blood clotting hasn't been affected by the new medications you're taking.

What drugs and supplements can interact with warfarin?

Warfarin, like any other medication, can interact with foods, other drugs, vitamins or herbal supplements. Be ready to provide your doctor with a complete list of everything you're taking.

Drugs that can interact with warfarin include:

  • Aspirin or aspirin-containing products
  • Ibuprofen (Advil, Motrin, others) or naproxen (Aleve)
  • Acetaminophen (Tylenol, others) or acetaminophen-containing products, especially when the dose of acetaminophen exceeds 1,500 milligrams a day
  • Many antibiotics
  • Heparin
  • Cold or allergy medicines
  • Birth control pills
  • Medications that treat abnormal heart rhythms, such as amiodarone

This isn't a complete list. Many other medications interact with warfarin. You should always consider that a new medication could interact with warfarin until your doctor, nurse, or pharmacist tells you otherwise.

Supplements that can interact with warfarin include:

  • Fish oil and omega-3 supplements
  • Vitamin K
  • Alfalfa
  • Bromelains, a type of supplement that comes from pineapples (pineapple itself doesn't interact with warfarin)
  • Coenzyme Q10 (ubidecarenone)
  • Cranberry extracts
  • Danshen
  • Dong quai
  • Evening primrose oil
  • Garlic
  • Ginkgo
  • Ginseng
  • Glucosamine
  • St. John's wort
  • Willow bark
  • Wintergreen

What foods might interact with warfarin?

If you sometimes eat foods that are high in vitamin K — such as soybean and canola oils, spinach or broccoli — tell your doctor. Vitamin K can affect how your blood clots and how warfarin protects you.

Garlic and black licorice also may interact with warfarin, so talk to your doctor if you eat a lot of these foods.

Eating cranberries or drinking cranberry juice can affect the way warfarin works and can increase your risk of bleeding. While an occasional glass of cranberry juice is unlikely to injure you, you should avoid drinking it regularly.

Drinking alcohol can also affect the way warfarin works. Talk to your doctor about drinking alcohol while taking warfarin. While having an occasional drink shouldn't create a problem, excessive drinking or binge drinking can greatly increase your bleeding risk.

What should you do if you forget a dose?

If you forget to take a dose of warfarin within 12 hours of when you were supposed to take it, take it as soon as you remember and get back on your daily dosing schedule. If you forget to take a dose and it has been more than 12 hours, wait until the next dose is due. Unlike some other medications, doubling your warfarin dose could greatly increase your risk of side effects.

Always talk to your doctor if you're unsure if you took your warfarin as directed. 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 warfarin side effects. Talk to your doctor, nurse or pharmacist if you have any concerns about warfarin. They can be a valuable source of information.

Last Updated: 2010-01-15
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