Warfarin is an anticoagulant drug you swallow to prevent and treat blood clots. You take it if you’re at risk of getting harmful blood clots in your body. Blood clots can keep blood from reaching your organs, which is how a heart attack or stroke happens. Your dose of warfarin will change based on results from frequent blood tests.


What is warfarin?

Warfarin is an anticoagulant medication you take to prevent and treat blood clots that can hurt you. You may know warfarin by its brand names, Coumadin® and Jantoven®. Warfarin slows down how quickly your blood clots. This helps keep unwanted clots from forming inside your blood vessels or heart when you have certain medical conditions or don’t walk or move around for a long time.

If you have a blood clot, warfarin keeps 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.

Warfarin helps prevent new clots from forming, but it doesn’t dissolve blood clots. With time, a clot may dissolve on its own. A clot that doesn’t dissolve gets in the way of blood flowing through a blood vessel.

What is warfarin used for?

Warfarin uses include the prevention and/or treatment of:

You may take warfarin if you’ve had the above conditions before or if you’re at risk now because:

How common is warfarin?

About 2 million people in the United States take warfarin.


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Procedure Details

How does warfarin work?

Warfarin interferes with part of your body’s clot-making process. It stops an enzyme from activating vitamin K. Some of your body’s clotting factors need vitamin K to do their part in the process of making a clot.

What is the warfarin dosage?

The dose usually ranges from 1 mg (milligram) to 10 mg. Your healthcare provider will prescribe a specific dosage strength. But you may take a different dose based on the results of frequent blood tests.

You can buy warfarin as a tablet in many different shapes, sizes and colors. Each tablet color represents a different strength in milligrams. All manufacturers follow the same color code for the different strengths of their tablets, but the size and shape of the tablets may differ from one company to the next.

Each tablet has its strength stamped on one side. It also has an indent across the tablet so you can break it in half if your provider adjusts your dose. For example: if your provider prescribes a 5 mg tablet and then changes your dose to 2.5 mg, you should break one of the 5 mg tablets in half and take the half tablet. If you have any questions about your dose of warfarin, contact your provider.

What blood tests will I need while I’m on warfarin?

A provider will need to check your blood with a prothrombin time (PT or protime) test. They can use these results to calculate the international normalized ratio (INR), which they use to standardize the results across different labs and testing centers. The PT and INR help a provider monitor how well warfarin is working to prevent blood clots. The results tell them if they need to adjust your dose.

You can get the blood tests at a lab or anticoagulation clinic, in a medical office or at home. You usually need to do the PT test one or more times a week to once a month if you’ve had stable results. Follow your provider’s instructions for how often you need to get blood tests and when to adjust your daily warfarin dose.

Other medications may increase or decrease warfarin’s anticoagulant effect. You might need to get blood tests more often when you stop, start or increase the dose of medications that are likely to affect the way warfarin works. You may need a different dose of warfarin, too. Talk to your healthcare provider who manages your warfarin if you have any changes in your other prescription medications. Even over-the-counter (OTC) medicines may interfere with warfarin.

Other information about your blood tests includes:

  • Illness, health status, diet or medication changes can affect how well your blood clots. Tell your healthcare provider about changes in your health, medications (prescription and over-the-counter) or lifestyle so they can adjust your dose.
  • If you’re planning a trip, talk with your provider about using another lab for your blood tests while traveling.


What do I need to know about taking warfarin?

Other instructions for taking warfarin include:

  • Take your dose of warfarin once a day at the same time of day.
  • Take warfarin before or after eating.
  • If you forget to take your dose and remember within eight hours of the time you were supposed to take your dose, take it. If it’s past eight hours, wait until the next day and take only the prescribed dose for that day. Don’t take a double dose.
  • If you forget to take your warfarin two or more days in a row, call your healthcare provider. They may need to change your dose. (Never change the dose yourself without calling your provider first.)

How long will I need warfarin?

You’ll need warfarin as long as you’re at risk of a blood clot that can harm you. Keep taking warfarin as long as your provider prescribes it. Never stop taking warfarin on your own without talking to your provider.

Risks / Benefits

What are the potential benefits of warfarin?

Benefits of warfarin include:

  • Providers have decades of experience using it to treat and prevent blood clots.
  • It has a low cost compared to newer alternatives.
  • It comes in a tablet form you can take at home.
  • You only need to take it once a day.
  • Providers can reverse its effects if necessary.


What are the side effects of warfarin?

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

  • Feeling colder, weaker or more tired than usual, or looking pale (symptoms of anemia).
  • Bleeding from cuts that won’t stop after applying pressure for 10 minutes.
  • Bleeding from the nose, gums or ears that won’t stop after 10 minutes.
  • Having a fever or illness that gets worse.
  • Having menstrual bleeding (period) that’s heavier or lasts longer than normal.
  • Having reddish or rusty-colored urine (pee).
  • Passing poop that looks bright red, black or tarry.
  • Having bruises that appear without reason or become swollen or painful.
  • Having purplish spots on your skin.
  • Coughing up blood.
  • Vomiting blood (which may look like coffee grounds).
  • Having unusual bleeding from hemorrhoids.
  • Having unusual pain or swelling, especially in your joints.
  • Having an unusual headache.
  • Having pain in your belly.
  • Experiencing sudden changes in your speech or vision.
  • Feeling numbness/tingling on one side of your face or in your arm.
  • Having dizziness.
  • Having difficulty breathing.

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

Recovery and Outlook

How long will it take warfarin to work?

Warfarin starts to work in 24 to 72 hours. Your blood may clot less easily 36 to 72 hours after you start taking warfarin. While your provider is waiting for warfarin to kick in, they’ll sometimes give you a different anticoagulant, usually injectable, that works right away.

Is there anything I can do to make taking warfarin easier on me?

With the risk of bleeding, it’s a good idea to take care with devices or tools that can hurt you. It’s also important to take your daily dose without skipping a day.

  • Be careful when using razors. Use an electric razor or hair-removing creams to lessen the chance of cuts.
  • Avoid climbing to tall heights that could lead to a fall.
  • Use a soft toothbrush. Brush and floss gently to keep your gums from bleeding.
  • Consider using a pillbox to help you remember to take your medication or checking off every day on a calendar after you take your dose.
  • Refill your prescription one week before your supply runs out to avoid missing a dose.

When To Call the Doctor

When should I call my healthcare provider?

Contact your provider if:

  • You have any symptoms of illness, such as vomiting, diarrhea, infection or fever. Illness can change the way warfarin works.
  • You have any injuries from falls or blows to your body or head.

When should I get emergency treatment?

You should get emergency treatment if:

  • Bleeding from a small cut doesn’t stop in 10 minutes. You should keep applying constant pressure over the cut on your way to the nearest emergency room (ER).
  • You have a large cut. Apply constant pressure and get help immediately. Call 911 or your local emergency number or have someone drive you to the nearest emergency room.

Additional Common Questions

What should be avoided when taking warfarin?

While taking warfarin, you should avoid:

  • Storing warfarin in places with too much heat or moisture, like a bathroom.
  • Getting pregnant.
  • Eating grapefruit, pomegranate and cranberry products.
  • Drinking too much alcohol or any at all.
  • Making big changes to your diet without talking to your provider.
  • Having surgery or dental work without telling your providers you take warfarin. (You may need to have a blood test and stop taking warfarin for a few days before having surgery or a dental procedure.)
  • Stopping warfarin without talking to your provider first.
  • Starting any exercise or sports program without checking with your provider.
  • Using herbal teas and supplements without running them by your provider.

What do I need to understand about vitamin K while taking warfarin?

You need vitamin K for normal blood clotting. But large changes in the amount of vitamin K in your diet can change the way warfarin works. If you eat foods high in vitamin K, it’s important to eat the same amount of these foods from week to week.

Tell your provider if you’re planning to:

  • Eat more or fewer vegetables.
  • Change to a vegetarian diet.
  • Follow a special meal plan to lose or gain weight.

Changing your eating habits may mean that you’ll be getting more or less vitamin K in the foods you eat. If you change your eating habits, your provider may want to check your blood more frequently to see how warfarin is working.

These vegetables have high amounts of vitamin K:

  • Broccoli.
  • Brussels sprouts.
  • Cabbage.
  • Spinach.
  • Kale.
  • Asparagus.
  • Collard, mustard and turnip greens.
  • Lettuce (Boston, Bibb, head, red leaf and romaine).
  • Endive.

If you want more servings of vegetables in your daily meal plan, choose vegetables that aren’t high in vitamin K, like:

  • Corn.
  • Squash.
  • Potatoes.
  • Onions.
  • Carrots.
  • Cucumbers.
  • Celery.
  • Peppers.
  • Pumpkin.
  • Tomatoes.

A note from Cleveland Clinic

Healthcare providers have prescribed warfarin for decades, but it’s a finicky medicine. Foods and other medicines are just two of the things that can affect how well it works from day to day. That’s why it’s so important to keep up with the blood tests that check how well it’s working. Since people bleed more easily while taking warfarin, it’s a good idea to use an electric razor and a toothbrush with soft bristles. Being careful with meal prep can keep you safer, too.

Medically Reviewed

Last reviewed on 06/09/2023.

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