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Warfarin is a drug that prevents harmful clots from forming in the blood. Warfarin is sometimes called an anticoagulant or a “blood thinner.” It does not actually thin the blood, and it does not dissolve clots that already have formed. Warfarin prevents new clots from forming, or existing clots from becoming larger and causing more serious problems. Warfarin is used to prevent or to treat harmful clotting related to certain blood vessel, heart, and lung conditions.

Dosage and Administration

Your doctor or pharmacist selects the correct dose of Warfarin by doing a blood test called the international normalized ratio (INR), and a prothrombin time (PT or pro-time). These tests show how long it takes your blood to clot. If your clotting time changes, the dose of Warfarin will be changed also. When you first start taking Warfarin, your INR or PT will be checked often. Once the dose is set, the number of tests are decreased. It is important to check your INR or PT on a regular basis. This helps assure the proper Warfarin dose based on your needs. Too large a dose may cause bleeding. Too small a dose may allow blood clots to form. Take this medicine only as directed by your doctor. Do not take more or less of it, do not take it more often, and do not take it for a longer time than your doctor ordered. Take Warfarin at the same time each day. The evening is usually best, but check with your doctor or pharmacist.

Missed Dose

If you miss a dose, take it as soon as possible. Then go back to your regular schedule. If you do not remember until the next day, do not take the missed dose at all and do not double the next one. Instead, go back to your regular dosing schedule. Always tell your doctor or pharmacist about a missed dose. Keep a record (such as a “Warfarin calendar”) of each dose as you take it, to avoid mistakes. Bring this Warfarin calendar to all of your doctor or clinic appointments.

Food Interactions

The foods you eat can alter the way this medicine works in your body. The effects of Warfarin depend on the amount of Vitamin K in your body. You should maintain your regular diet to promote consistent effects of your medicine. Any abrupt changes, such as crash diets, food fads or nutrition supplements may alter the normal amount of Vitamin K in your body. Avoid excessive amounts of foods that are high in Vitamin K. You do not need to eliminate all foods containing Vitamin K from your diet. The following are some general guidelines; keep in mind, a consistent diet is best. If you have any questions, contact your doctor, dietitian or pharmacist.

If you change your use of the following foods, contact your doctor or clinic. Avoid eating the following because they are very high in Vitamin K:

  • Kale
  • Herbal teas made from coumarin sources such as: tonka beans, melilot (sweet clover), and sweet woodruff

The following foods are also high in Vitamin K. You need to be consistent in your intake of these foods:

  • Broccoli
  • Brussel sprouts
  • Chick peas (Garbanzo beans)
  • Spinach
  • Greens (turnip, collard, and mustard)
  • Cabbage, raw
  • Lettuce

You should also be consistent with your intake of these other food sources of Vitamin K:

  • Lentils
  • Chick peas (Garbanzo beans)
  • Soybeans
  • Liver (beef, pork, and chicken)
  • Soybean products and soybean oil


Drinking too much alcohol may change the way this medicine works in your body. Patients taking oral anticoagulants should avoid alcohol. Please talk with your doctor if you have questions about alcohol intake.

Drug Interactions

Some over-the-counter (OTC) medicines and herbal remedies can increase or lessen the effect of Warfarin. It is important to consult your doctor or pharmacist before you use any of these. Also, talk to your doctor of pharmacist before taking any vitamin supplements that have Vitamin K. Large amounts of Vitamins A and E have been shown to interfere with Warfarin therapy. Herbal supplements may also interact, especially gingko biloba. This supplement should be avoided to prevent excess bleeding.

Aspirin and medicines for pain and inflammation may increase the effect of Warfarin. However, there are some conditions that are best treated with both Warfarin and aspirin. Do not take aspirin and Warfarin unless your doctor has advised you to do this. When buying a pain reliever, always read the label ingredients and ask your doctor, nurse or pharmacist for advice. Aspirin, for example, is often a hidden ingredient in many cold, sinus, and pain remedies. A few common over-the-counter medicines to avoid are:

Aspirin-containing Medications


Medications for Pain and Inflammation

Ibuprofen (Advil®, Nuprin®, etc.)
Ketoprofen (Orudis KT®)
Naproxen (Aleve®)

It is not possible to have a complete list because of the large number of such medicines. For this reason, you must consult your doctor or pharmacist before taking any cold products or herbal medicines, pain medicine, vitamin, analgesic, laxative or antacid, and tell them that you are also taking Warfarin.

Special Instructions

  • If you cut yourself, apply pressure to the area for five minutes to make sure the bleeding has stopped.
  • Do not begin taking this medicine during pregnancy, and do not become pregnant while taking it. This medicine can cause birth defects. Consult your doctor for added information.
  • If you are to be away for any length of time, inform your doctor.
  • Tell all of your doctors (including dentist) and pharmacists that you are taking Warfarin.
  • It is important that you carry identification, such as the Medic-Alert bracelet, stating that you are taking this medicine so that proper treatment can be given in case of an emergency.
  • Avoid activities and sports that may cause you to be injured. Report to your doctor any falls, blows to the body or head, or other injuries.
  • Place a non-slip bath mat in the tub to prevent accidents.
  • Be careful to avoid cutting yourself. This includes taking special care in brushing your teeth and in shaving. Use a soft toothbrush and use dental floss gently. Also, it is best to use an electric shaver rather than a razor blade.
  • Keep all drugs out of the reach of children.
  • Store medicine away from heat and direct light.
  • Do not store in the bathroom, near the kitchen sink, or in other damp places. Heat or moisture may cause the medicine to break down.
  • Do not keep outdated medicine that is no longer needed. Be sure that any discarded medication is out of the reach of children.

Notify your doctor right away if you notice any of the following:

  • Prolonged bleeding
  • Stomach pain
  • Nosebleed, bloody gums, bloody sputum, blood in urine or stool
  • Black stool (if not taking iron supplements)
  • Unusual bruising or change in skin color, dizziness, or unusually heavy menstrual bleeding
  • Unexplained swelling
  • Shortness of breath, chest pain, pain in the joints, difficulty breathing or swallowing.

Please call your doctor, nurse or pharmacist with any questions you may have about Warfarin and how to take it. They will be happy to help.

Last UpdateSeptember 21, 2011