What is warfarin?

Warfarin sodium is an anticoagulant medication. "Anti" means against and "coagulant" means causing blood clotting. Warfarin controls the way that blood clots (thickens into a lump) inside your blood vessels. The brand name of warfarin is Coumadin®.

What do anticoagulants like warfarin do?

An anticoagulant helps your body control how fast your blood clots; therefore, it prevents clots from forming inside your arteries, veins, or heart during certain medical conditions.

If you have a blood clot, an anticoagulant may keep the clot from getting larger. It also may prevent a piece of the clot from breaking off and traveling to your lungs, brain, or heart.

The anticoagulant medication does not dissolve the blood clot. With time, however, this clot may dissolve on its own.

Warfarin is made by several different drug manufacturers and is available as a pill in many different shapes and colors. Each color is a different strength, measured in milligrams (mg).

How do I take warfarin?

You will take warfarin every day. The dose usually ranges from 1 mg to 10 mg once a day. The doctor will prescribe one strength and change the dose as needed (your dose may be changed every time you have a laboratory test).

Each tablet has the strength stamped on one side, and is scored so you can break it in half easily to adjust your dose as your doctor instructs. For example: if your doctor prescribes a 5 mg tablet and then changes the dose to 2.5 mg (2½ mg), which is half the strength, you should break one of the 5 mg tablets in half and take the half-tablet. If you have any questions about your dose, talk with your doctor or pharmacist.

Here are some other instructions for taking warfarin:

  • Take the dose as instructed once a day.
  • Take the dose at the same time each day. We recommend 5:00 p.m.
  • The medication can be taken before or after eating.
  • If you forget to take your dose and remember within eight hours, take the dose. If it is past eight hours, wait until the next day and take only the prescribed dose for that day. DO NOT TAKE A DOUBLE DOSE.
  • If you forget two or more days in a row, call your doctor. The dose may need to be changed.
  • When you take the dose, check off the day on your home calendar.
  • Refill your prescription one week before your supply runs out to avoid missing a dose.
  • Continue to take warfarin as long as your doctor prescribes it.

What blood tests will I need while I am on warfarin?

You will need to get regular blood tests to tell how well the medication is working. The test results help the doctor decide the dose of warfarin that will keep a balance between clotting and bleeding. Follow your doctor's instructions for getting blood tests and adjusting your daily warfarin dose.

The blood tests for clotting time are called prothrombin time (Protime, PT) and international normalized ratio (INR). The tests are done at a laboratory, usually once a week to once a month. Your doctor will help you decide which laboratory you will go to for these tests, and how often you should have them.

Some important things to keep in mind about these blood tests:

  • Have your INR checked when it is scheduled.
  • Go to the same laboratory each time. (There can be a difference in results between laboratories.)
  • If you are planning a trip, talk with your doctor about using another laboratory while traveling.

Where should I store warfarin?

Store the medicine at room temperature, away from extreme cold, heat, light, or dampness. Note: Bathroom cabinets usually are not a good place for storing medications because of the dampness.

Always keep medications out of the reach of children.

What precautions should I take while taking warfarin?

It is important that you follow these precautions when taking warfarin.

Other medications and vitamin supplements

Many medications and vitamins can affect how warfarin works, including:

  • Many prescription medications, especially some common antibiotics like azithromycin and ciprofloxacin.
  • Non-prescription medications such as acetaminophen, aspirin, ibuprofen, nonsteroidal anti-inflammatory drugs (NSAIDs), cold and cough medicines, antacids, laxatives, or other medications for pain or discomfort.
  • Vitamin preparations containing Vitamin K (phytonadione) or large amounts of vitamins E or C.

Remember to talk with the doctor or pharmacist before you take any medications or vitamins, whether from the drugstore or from another doctor or dentist. This is very important because you may need some of these medications for another medical condition, and your doctor will regulate them with warfarin.

Do not stop or start any medications without first talking to your doctor.

Pregnancy, surgery, and dental work

It is important to avoid getting pregnant while on warfarin. Use at least two forms of birth control.

If you are a woman taking warfarin and planning to become pregnant, talk with your doctor about the possible risks and ways to reduce those risks. Tell your doctor right away if you become pregnant. Before receiving treatment, tell all of your doctors and dentists you are taking warfarin.

You may need to have a blood test and to stop taking warfarin for a few days before having surgery or a dental procedure. Check with your doctor before any surgical or dental procedure.


Check with your doctor before starting any exercise or sports program. Talk with your doctor if you are planning any major diet changes such as a weight-reducing diet, or if you plan to add any nutritional supplements.


Vitamin K is needed for normal blood clotting. Large amounts of foods that are high in vitamin K can change the way warfarin works. Limit foods high in vitamin K to a ½-cup cooked serving or one 3-ounce raw serving per day. Foods rich in vitamin K include green tea, beef liver, soy oil, tofu, broccoli, Brussels sprouts, cabbage, cauliflower, chick peas, kale, lettuce, turnip greens, seaweed, and spinach.

Alcoholic beverages can also change the way warfarin works. Ask your doctor about the amount of alcoholic beverages you may drink.

The following guidelines will help control the amount of Vitamin K you are getting from the foods you eat. To help the medicine perform well, you should follow these guidelines:

  • Avoid grapefruit and cranberry products.
  • If you eat spinach, turnip greens, other leafy greens, broccoli, Brussels sprouts, kale, parsley (except as a garnish or minor ingredient), natto (a Japanese dish), liver, or green tea, be sure to eat the same amount week to week.
  • Eat all other foods as you normally would.
  • Tell your doctor if you are thinking about changing your current eating habits. Tell your doctor if you are planning to:
    • Eat more or fewer vegetables.
    • Change to a vegetarian style of eating.
    • Follow a special meal plan to lose or gain weight.
  • Changing your eating habits may mean that you will be getting more or less Vitamin K in the foods you eat. If you change your eating habits, your doctor may want to check your blood more frequently to see how the warfarin therapy is working.
  • Tell your doctor if you are currently taking any herbal supplements. Do not take any herbal supplements that may keep your blood from clotting. The following supplements should not be used when you are taking anticoagulant medications before surgery:
    • Garlic
    • Ginger
    • Gingko biloba
    • Ginseng
    • Feverfew
    • Fish oil
    • Turmeric
    • St. Johns Wort
    • Chondroitin sulfate
  • Do not take any vitamin supplements that provide more than 100 percent of the Recommended Daily Allowance (RDA). Tell your doctor if you are currently taking more than the RDA of any vitamins (especially vitamins A, C, or E).
  • Avoid chronic (long-term), heavy drinking of beverages that contain alcohol. (Heavy drinking is more than two ounces of liquor, 10 ounces of wine, or 24 ounces of beer per day.)
  • If you drink tea, black tea (such as orange pekoe tea) is recommended because it is not high in Vitamin K.

If you want more servings of vegetables in your daily meal plan, choose vegetables that are not high in Vitamin K, such as corn, squash, potatoes, onions, carrots, cucumbers, celery, peppers, pumpkin, and tomatoes.

Daily activities

  • Be careful when using razors. Use an electric razor or hair-removing creams to lessen the chance of cuts.
  • Use a soft toothbrush. Brush and floss gently to prevent bleeding from the gums.

Illness and emergencies

  • Keep your doctor's phone number close by in the event of an emergency.
  • Call your doctor if you have any symptoms of illness, such as vomiting, diarrhea, infection, or fever. Illness can change the way warfarin works.
  • Always carry or wear identification that states you are taking warfarin. In an emergency, you may not be able to speak for yourself.
  • Avoid situations where you may get hurt at home or at work. Even minor injuries must be watched for bleeding because warfarin affects clotting.
  • Falls that cause bruising (bleeding under the skin) and cuts from sharp objects are more serious when you are taking warfarin.

Call your doctor if you have any injuries from falls or blows to the body or head.


  • Check with your doctor before you travel. You may need to have a blood test and an adjustment to the warfarin dose before you leave.
  • While you travel, carry your medications with you at all times: do not put medications in checked baggage and do not leave them in the car.

What should I do if I cut myself while on warfarin?

If the cut is small, put pressure over the cut until the bleeding stops (this may take up to 10 minutes). If the bleeding doesn't stop, continue to apply pressure and go to the nearest emergency room.

If the cut is large, apply pressure and get help immediately, either by phone or by going to the nearest emergency room.

What are the side effects of warfarin?

Bleeding is the most common side effect of warfarin and can appear as any of several different symptoms. Call your doctor if you notice any of the following signs of bleeding:

  • Feeling tired or looking pale (sign of anemia)
  • Bleeding from cuts that won't stop after applying pressure for 10 minutes
  • Bleeding from the nose, gums, or ears
  • Menstrual bleeding (period) that is heavier or longer than normal
  • Reddish or rusty-colored urine
  • Bowel movements that look bright red, black, or tarry
  • Bruises that appear without reason or become swollen; purplish spots on your skin
  • Vomiting blood (which may look like coffee grounds)
  • Coughing up blood
  • Unusual bleeding from hemorrhoids
  • Unusual pain or swelling, especially in the joints
  • Unusual headache
  • Stomach or abdominal pain
  • Sudden changes in speech or vision
  • Numbness/tingling on one side of the face or in the arm

If you have any of these symptoms, your doctor may want to do a blood test, change your dose, stop the medication, or give you medicine to stop the bleeding.

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