Warfarin side effects: Watch for interactions

Although commonly used to treat blood clots, warfarin (Coumadin, Jantoven) can have dangerous side effects or interactions that can place you at risk of bleeding. Here are precautions to take to avoid warfarin side effects.

By Mayo Clinic Staff

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 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, which can be a complication of 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. 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:

  • High blood pressure (hypertension)
  • A history of stroke
  • Kidney problems
  • Cancer
  • Alcoholism
  • Liver disease

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

Warfarin side effects that require immediate medical attention

  • 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
  • Bruising that develops without an injury you remember
  • Dizziness or weakness

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.

Less serious warfarin side effects to tell your doctor about

  • Bleeding from the gums after you brush your teeth
  • Swelling or pain at an injection site
  • Heavier than normal menstrual bleeding or bleeding between menstrual periods
  • Diarrhea, vomiting or inability to eat for more than 24 hours
  • Fever
Jan. 06, 2015 See more In-depth