What is warfarin? — Warfarin (sample brand names: Coumadin, Warfilone) is a prescription medicine that makes your blood less likely to clot. It is usually taken as a pill but is sometimes given as an injection.

People sometimes call warfarin a “blood thinner.” But it does not actually thin the blood.

Why do I need to take warfarin? — Doctors prescribe warfarin to people who might get dangerous blood clots. This includes people who have:

  • A mechanical heart valve
  • An irregular heart rate (called “atrial fibrillation”)
  • A genetic condition that makes their blood more likely to clot

Doctors also might prescribe warfarin to people who have had a blood clot in the past. This includes people who have had:

  • A stroke
  • A heart attack
  • A blood clot in the lung (called a “pulmonary embolism”)
  • A blood clot in the leg (called a “deep vein thrombosis” or “DVT”)

If you take warfarin, there are certain things you should do. These are described below:

Take your medicine on a schedule — It is very important to take warfarin exactly as your doctor tells you to. If you forget or miss a dose, call your doctor to find out what to do.

Make sure you take the right dose — Warfarin pills come in different strengths. Each strength is usually a different color, with the amount of warfarin (in milligrams) printed on the tablet. If the color or dose of your warfarin tablet looks different than those you have taken before, check with your doctor or pharmacist.

Get your blood tested — When you start taking warfarin, you will need to have your blood tested often since the effects of warfarin can change over time.

Ask your doctor before taking any new medicines — These include prescription or over-the-counter medicines, herbs, and vitamins. All of these medicines might change the effects of warfarin.

Watch what you eat and drink — Eating leafy greens and other vegetables high in vitamin K (table 1) can make warfarin not work as well. Don’t avoid these healthy foods, as they have other vitamins, minerals, and fiber. Instead, try to eat similar amounts of foods that contain high amounts of vitamin K on a regular basis. That way, your vitamin K level will stay about the same, and you won’t need to adjust your warfarin dose.

You should also drink alcohol only once in a while. Limit yourself to 1 to 2 servings per day. Heavy drinking affects how well your body is able to handle warfarin, and might increase your risk of bleeding.

Wear an alert tag — Always wear a bracelet, necklace, or another kind of alert tag that warns people that you take warfarin and why. That way, if you are in an accident and are unable to explain your condition, responders will know how to care for you.

Your alert tag should also have the name and phone number of an emergency contact.

What are the side effects of warfarin? — The most common side effect is bleeding from any part of your body. This even includes inside your body, where you cannot see it.

Call your doctor RIGHT AWAY if you have any symptoms that could mean you are bleeding, such as you:

  • Feel sick to your stomach or throw up blood or something that looks like coffee grounds
  • Have headaches, dizziness, and weakness
  • Have nosebleeds
  • Have dark red or brown urine
  • See blood in your bowel movements, or have dark-colored bowel movements
  • Develop pain and swelling after an injury

You should also call your doctor if you:

  • Bleed from the gums after you brush your teeth
  • Swell or have pain where you got an injection
  • Have heavy menstrual periods or bleeding between periods
  • Have diarrhea, vomit, or are unable to eat for more than 24 hours
  • Have a fever (temperature higher than 100.4º F or 38º C)

Is there anything I can do to lower the risk of bleeding? — Yes. You should:

  • Use a soft bristle toothbrush.
  • Floss with waxed floss (not unwaxed floss).
  • Shave with an electric razor rather than a razor blade.
  • Take care when using sharp objects, such as knives and scissors.
  • Avoid doing things might make you likely to fall, such as walking on slippery surfaces or climbing on a high stool.

For more detailed information about your medicines, ask your doctor or nurse for information from Lexicomp available through UpToDate. The Lexicomp hand-outs explain how to use and store your medicines. They also list possible side effects and warn you if your medicines should not be taken with certain other medicines or foods.