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What is warfarin?

Warfarin (also called Coumadin®) is an anticoagulant. “Anti” means against and “coagulant” refers to blood clotting. An anticoagulant helps to reduce clots from forming in the blood. Warfarin is used to treat harmful blood clots (thrombosis) in veins or arteries or to prevent blood clots from coming back. It is also used to prevent blood clots from forming after certain surgeries or periods of inactivity or with artificial heart valves.

Warfarin is available as tablets in several strengths. All are taken by mouth. Below is a list of the tablet strengths and their colors:

  • 1-mg pink tablet
  • 2-mg lavender tablet
  • 2.5-mg green tablet
  • 3-mg tan tablet
  • 4-mg blue tablet
  • 5-mg peach tablet
  • 6-mg teal tablet
  • 7.5-mg yellow tablet
  • 10-mg white tablet

Important blood tests when taking warfarin

  • To help your doctor find the dosage of warfarin you need, a small amount of your blood will be taken. This sample will be tested to find your prothrombin time (PT) and International Normalized Ratio (INR). The PT/INR blood test checks to see how fast your blood clots. Your dose of warfarin will be adjusted to keep your PT/INR in a target range for you.
  • A large number of factors can affect how your body responds to warfarin, such as changes in diet, physical state (fever or illness), medicines, herbal remedies, and travel. Blood samples will be taken every 3 days when you first start taking warfarin or if you experience any of the factors above.
  • Once the PT/INR is stable, these blood tests must be done every 2 to 4 weeks. The doctor will use the results to decide if you should stay on the same dose of warfarin or if the dose needs to be changed.
  • It is very important that the effect of warfarin stays in the desired range. If the effect is too big, you could have severe bleeding. If the effect is too small, you will have a greater chance of a blood clot forming.

Possible side effects of warfarin

Bleeding is the most common side effect seen with warfarin. Very minor bleeding may occur including an increase in small bruises or slight gum bleeding when brushing your teeth even if your PT/INR are in the target range. These are other possible side effects:

  • Unusual bruising
  • Frequent nose bleeds
  • Coughing or vomiting up blood
  • Red spots on skin
  • Heavy menstrual bleeding
  • Red or dark brown urine
  • Black, tarry stools
  • Headaches, dizziness, weakness
  • Pain, swelling, or heat in an arm or leg

These are the most common signs or symptoms of bleeding problems, but there may be others. Please report all side effects right away to your doctor or nurse.

In case of a severe side effect or reaction, call the doctor, nurse, or pharmacist at 595-3300. If you are outside the Memphis area, dial toll-free 1-866-2STJUDE (1-866-278-5833), and press 0 once the call is connected.

How to give warfarin

  • Give warfarin at the same time every day with or without food. Give with food if the drug causes an upset stomach.
  • If you cannot swallow tablets, you can crush the tablets and dissolve them in a small amount of water or cherry syrup right before giving the dose.
  • Since warfarin tablets are available in many strengths, it is important for you to read the directions on the medicine bottle(s) closely.
  • Try not to miss doses of warfarin. If you forget to take a pill, take the missed dose as soon as you can on the same day. Do not double your dose the next day to “catch up.” Call your doctor or nurse for guidelines. Weekly pill boxes are available upon request from pharmacy to help you keep track of doses.
  • Do not take aspirin, medicines that contain aspirin, ibuprofen or naproxen while you are taking warfarin unless your doctor tells you to do so. These drugs can increase the effect of warfarin and lead to bleeding.
  • Vitamins, herbals, and other medicines can increase or decrease the effects of warfarin. Always give a complete list of medicines you are taking to your doctor or pharmacist. Check with your doctor or pharmacist before taking any new vitamins, herbals, or other medicines.
  • Warfarin tablets can be stored at room temperature. Keep this and all medicines out of the reach of children and pets.

Diet information when taking warfarin

  • Many foods you eat have vitamin K in them. Vitamin K helps your blood make clots. Warfarin works by decreasing the activity of vitamin K.
  • Large changes in the amount of vitamin K you eat may affect the way warfarin works. It is important for you to keep your diet steady so the amount of vitamin K you eat is steady. Do not make any major changes in your daily diet. Foods that are high in vitamin K are usually green leafy vegetables (such as broccoli, collard greens, lettuce and spinach), green tea, liver and some vegetable oils. Refer to “Do You Know… Anticoagulants and Vitamin K.”
  • Tell your doctor or nurse if you change your diet because you are sick.
  • Avoid drinking large amounts of cranberry juice or eating cranberry products, which can increase the effect of warfarin and lead to bleeding.
  • Avoid drinking alcohol.
  • When you are started on warfarin, you will meet with a dietitian who will provide a complete list of diet guidelines.

Special instructions for warfarin

  • Check your skin closely for easy bruising or red spots, which may mean that you are bleeding. If you notice any bleeding, tell your doctor right away so your warfarin dose can be lowered.
  • You should not take warfarin if you are pregnant or are planning to become pregnant.
  • If you need to have surgery or dental work, tell your surgeon or dentist that you are taking warfarin.
  • Be careful to avoid injury while you are taking warfarin. The risk of heavy bleeding is increased even for routine activities like shaving and brushing or flossing your teeth.
  • While you are taking warfarin, tell your doctor right away if you have problems breathing, chest pain, dizziness, shortness of breath, swelling in legs or arms, severe stomach pain, vision problems, pain when walking, or pain and warmth in your arms or legs.
  • In case of emergency, carry a card that says you are taking warfarin.


This document is not intended to take the place of the care and attention of your personal physician or other professional medical services. Our aim is to promote active participation in your care and treatment by providing information and education. Questions about individual health concerns or specific treatment options should be discussed with your physician.

St. Jude complies with health care-related federal civil rights laws and does not discriminate on the basis of race, color, national origin, age, disability, or sex.

ATTENTION: If you speak another language, assistance services, free of charge, are available to you. Call 1-866-278-5833 (TTY: 1-901-595-1040).

ATENCIÓN: si habla español, tiene a su disposición servicios gratuitos de asistencia lingüística. Llame al 1-866-278-5833 (TTY: 1-901-595-1040).

تنبيه: إذا كنت تتحدث باللغة العربية فيمكنك الاستعانة بخدمات المساعدة اللغوية المتوفرة لك مجانا. .يرجى الاتصال بالرقم. 5833-278-866-1  (الهاتف النصي: 1040-595-901-1).