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Overview: Warfarin is used to help prevent and treat blood clots, as well as complications from blood clots. It also helps lower the risk of complications after a heart attack. The most common side effect of warfarin is bleeding, which can sometimes be serious. It is best to limit alcohol while taking warfarin. Both alcohol and warfarin can increase your risk for bleeding. It is also important to have a consistent diet, especially when it comes to dark, green leafy vegetables. Talk to your health care provider about your dietary habits.

Uses

What is warfarin used for?

Warfarin is commonly used for the following.

  • To prevent and treat blood clots
  • To prevent and treat blood clot complications due to abnormal heart rhythm (atrial fibrillation) or heart valve replacement
  • To lower the risk of death, another heart attack, blood clots, or stroke after having a heart attack

Warfarin may also be used for other conditions as determined by your health care provider.

How does warfarin work (mechanism of action)?

Vitamin K helps your blood clot. Warfarin blocks vitamin K, which helps prevent blood clots from forming.

How is warfarin supplied (dosage forms)?

Brands and other names

  • Coumadin
  • Jantoven

Dosage forms and strengths

  • 1 mg, 2 mg, 2.5 mg, 3 mg, 4 mg, 5 mg, 6 mg, 7.5 mg, and 10 mg oral tablets

How should I store warfarin?

Warfarin should be stored at room temperature, between 68 F and 77 F (20 C and 25 C). It can be exposed to temperatures between 59 F and 86 F (15 C and 30 C) for shorter periods of time, such as when transporting it. Store in a cool, dry place. Protect from light and moisture. 

Side Effects

What are the most common side effects of warfarin?

The most common side effect of warfarin is bleeding (see below).

There may be other side effects of warfarin that are not listed here. Contact your health care provider if you think you are having a side effect of a medicine. In the U.S., you can report side effects to the FDA at www.fda.gov/medwatch or by calling 800-FDA-1088 (800-332-1088). In Canada, you can report side effects to Health Canada at www.health.gc.ca/medeffect or by calling 866-234-2345.

What are the serious side effects of warfarin?

While less common, the most serious side effects of warfarin are described below, along with what to do if they happen.

Severe Allergic Reactions. Warfarin may cause allergic reactions, which can be serious. Stop taking warfarin and get help right away if you have any of the following symptoms of a serious allergic reaction.

  • Breathing problems or wheezing
  • Racing heart
  • Fever or general ill feeling
  • Swollen lymph nodes
  • Swelling of the face, lips, mouth, tongue, or throat
  • Trouble swallowing or throat tightness
  • Itching, skin rash, or pale red bumps on the skin called hives
  • Nausea or vomiting
  • Dizziness, feeling lightheaded, or fainting
  • Stomach cramps
  • Joint pain

Bleeding. Warfarin can cause major bleeding, especially within the first month of taking warfarin. Talk to your health care provider about how often to measure your international normalized ratio (INR). Your INR tells you how well your blood clots. Call your health care provider right away if you have any of the following symptoms of bleeding.

  • Vomit with blood that looks like coffee grounds
  • Coughing up blood or blood clots
  • Black or tarry poop
  • Easy bruising or bleeding
  • Pink or brown pee
  • Bleeding gums
  • Bleeding cuts that take a long time to stop
  • Nosebleeds

Death of Skin Tissue. Warfarin can cause your skin tissue to die. Warfarin can also cause a condition called gangrene, which can happen when blood clots form and block blood flow to an area of your body. Call your health care provider right away if you have pain, color, or temperature changes in any area of your body.

Calcium Buildup. Warfarin can cause a condition called calciphylaxis, which can cause calcium to build up in your blood vessels and block blood flow. Keep all appointments with your health care provider while taking warfarin. Call your health care provider right away if you have any of the following symptoms. 

  • Skin pain
  • Purple or red marks on the skin
  • Blisters or sores on the skin
  • Vision problems

Clots. Warfarin may increase your risk of a clot that is made of fat. These clots can get stuck in small blood vessels in your body and block blood flow to organs such as the liver, pancreas, or spleen. These clots can also happen in your toes, which can cause pain in your toes or make your toes look darker or purple. Call your health care provider right away if you experience this.

Warnings & Precautions

Who should not use warfarin?

Allergies to Ingredients. People who are allergic to any of the following should not take warfarin.

  • Coumadin
  • Jantoven
  • Warfarin
  • Any of the ingredients in the specific product dispensed

Your pharmacist can tell you all of the ingredients in the specific warfarin products they stock.

Pregnancy. Most people should not take warfarin while pregnant. If you become pregnant, contact your health care provider right away. See the Pregnancy section for more details.

Pregnancy Complications. Do not take warfarin if you are planning to have an abortion or have eclampsia or preeclampsia

Bleeding Problems. Do not take warfarin if you have low levels of certain blood cells, have bleeding problems, or bleed easily. 

Medical Procedures With Bleeding Risk. Do not take warfarin if you have any upcoming medical procedures that require needles (such as a spinal puncture) or may cause bleeding. This can also include procedures or surgeries where you are given medicine in your spine or in a specific part of your body, to help you fall asleep. Tell your health care provider if you have any upcoming medical procedures or surgeries.

Sudden High Blood Pressure. Do not take warfarin if you have very high blood pressure or normal blood pressure that suddenly becomes very high (over 200/120). Very high blood pressure can increase your risk for bleeding with warfarin.

What should I know about warfarin before using it?

Do not take warfarin unless it has been prescribed to you by a health care provider. Take it as prescribed. It is important for all health care providers to know that you take warfarin. It is a good idea to keep a card in your wallet or something else to let others know you take warfarin. 

Do not share warfarin with other people, even if they have the same condition as you. It may harm them.

Keep warfarin out of the reach of children.

People who are 65 and older can be at greater risk of some side effects from warfarin. Talk to your health care provider about your risks if you are in this age group.

Talk to your health care provider about how often to get your INR checked. Your health care provider will set your INR goal (the range of numbers your INR should fall between). Your INR will tell you how fast your blood is clotting. Your health care provider may adjust your dose of warfarin, as needed, to try and keep your INR within the target range.

Avoid doing any activity or sport that may cause a serious injury. Call your health care provider right away if you fall, injure yourself, or hit your head. 

Call your health care provider if you have severe diarrhea, infection, or a fever. These things can increase your risk for bleeding with warfarin.

What should I tell my health care provider before using warfarin?

Tell your health care provider about all of your health conditions and any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using. This will help them know if warfarin is right for you.

In particular, make sure that you discuss any of the following.

Current and Past Health Conditions. Tell your health care provider if you have any of the following.

Upcoming Surgery. Tell your health care provider if you have any upcoming surgeries, including dental procedures or cataract surgery. 

Heparin-Induced Thrombocytopenia. Tell your health care provider if you have used heparin and experienced a condition called heparin-induced thrombocytopenia (HIT) or heparin-induced thrombocytopenia with thrombosis syndrome (HITTS). These conditions can occur when heparin triggers your immune system and causes low platelet levels and blood clots. Warfarin can make these conditions worse, possibly leading to amputation or even death.

Other Medicines and Supplements. Warfarin may interact with other medicines and supplements. Before taking warfarin, tell your health care provider about any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using. See the Interactions section for more details.

Pregnancy. Warfarin may cause harm to a fetus. Tell your health care provider if you are pregnant or plan to become pregnant. If you become pregnant, stop taking warfarin right away and contact your health care provider. If you have a mechanical heart valve and are at high risk for blood clots, your health care provider may consider continuing warfarin during your pregnancy. Use effective birth control while taking warfarin and for at least one month after your last dose. If you are pregnant, you should not handle crushed or broken warfarin tablets.

Breastfeeding. Warfarin does not pass into breast milk. Tell your health care provider if you are breastfeeding or plan to breastfeed. Your health care provider will let you know if you should take warfarin while breastfeeding. If you take warfarin while breastfeeding, check your baby for any bruising or bleeding.

Interactions

Does warfarin interact with foods or drinks?

Try to eat a balanced and consistent diet while taking warfarin. Talk to your health care provider if you make changes to your diet. Dark, leafy green vegetables and certain vegetable oils have vitamin K in them. Vitamin K helps your blood clot. Warfarin works to keep your blood thin by preventing blood clots. Tell your health care provider if you change how much of these foods you eat.

Warfarin may interact with grapefruit juice. If you drink grapefruit juice, talk to your health care provider.

It is unknown if drinking alcohol will affect warfarin, but both warfarin and alcohol can increase your risk of bleeding. It is best to limit the amount of alcohol you drink.

Does warfarin interact with other medicines (drug interactions)?

Always tell your health care provider about any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using.

In particular, make sure that you discuss if you are using any of the following before taking warfarin.

  • A medicine that is used to treat or prevent blood clots, such as an anticoagulant or antiplatelet 
  • A nonsteroidal anti-inflammatory drug (NSAID), such as ibuprofen, naproxen, and others, which is in many prescription and OTC products used for pain, swelling, and fever
  • A selective serotonin reuptake inhibitor (SSRI), such as fluoxetine, paroxetine, or sertraline, or a serotonin and norepinephrine reuptake inhibitor (SNRI), such as duloxetine or venlafaxine, which are commonly used for anxiety and depression
  • A medicine used to treat bacterial or fungal infections
  • An herbal medicine including coenzyme Q10 (CoQ10)echinacea, garlic, ginkgo bilobaginsenggoldenseal, or St. John’s wort

Many other medicines may alter the blood levels of warfarin. Tell your health care provider about all medicines that you take or have recently taken.

This may not be a complete list of medicines that can interact with warfarin. Always check with your health care provider.

Does interact with other drugs you are taking?

Enter your medication into the WebMD interaction checker

Overdose/Missed Dose

What should I do if I accidentally use too much warfarin?

If you or someone else has used too much warfarin, get medical help right away, call 911, or contact a Poison Control center at 800-222-1222.

What should I do if I miss a dose of warfarin?

If you miss a dose, take it as soon as you remember, as long as it is the same day as the missed dose. If you remember a missed dose a day later, skip the missed dose. Do not take double or extra doses to make up for the missed dose, unless your health care provider tells you to.

It is important that you take warfarin as prescribed. Keep track of any missed doses and tell your health care provider about them, including when any doses were missed. This can help with decisions about possible dosing changes based on your lab work. Your health care provider may also change how often you get your INR checked.