A Patient's Guide to Taking Warfarin

pharmacist holding prescription behind the counter

Warfarin (brand names Coumadin and Jantoven) is a prescription medication used to prevent harmful blood clots from forming or growing larger. Beneficial blood clots prevent or stop bleeding, but harmful blood clots can cause a heart attack, stroke, deep vein thrombosis or pulmonary embolism. Because warfarin interferes with the formation of blood clots, it's called an anticoagulant (PDF). Many people refer to anticoagulants as *blood thinners; however, warfarin does not thin the blood but instead causes the blood to take longer to form a clot.

How Does Warfarin Work?

The formation of a blood clot in the body is a complex process that involves multiple substances called clotting factors. Warfarin decreases the body’s ability to form blood clots by blocking the formation of vitamin K–dependent clotting factors. Vitamin K is needed to make clotting factors and prevent bleeding. By giving a medication that blocks the clotting factors, your body can stop harmful clots from forming and prevent clots from getting larger.

Monitoring and Dosing Tips

The goal of warfarin therapy is to decrease the clotting tendency of blood, not to prevent clotting completely. The effect of warfarin must be monitored carefully with blood testing. Based on the results of the blood test, your daily dose of warfarin will be adjusted to keep your clotting time within a target range. The blood test used to measure the time it takes for blood to clot is referred to as a prothrombin time test, or protime (PT). The PT is reported as the International Normalized Ratio (INR).

The INR is a standardized way of expressing the PT value. The INR ensures that PT results obtained by different laboratories can be compared. It's important to monitor the INR at least once a month and sometimes as often as twice weekly to make sure the level of warfarin remains in the effective range. If the INR is too low, blood clots will not be prevented, but if the INR is too high, there is an increased risk of bleeding. This is why those who take warfarin must have their blood tested so frequently.

Unlike most medications that are administered as a fixed dose, warfarin dosing is adjusted according to the INR blood test results; the dose usually changes over time.

Difference Between Brand-Name and Generic Medications

Generic drugs are supposed to have the same dosage, therapeutic effects, route of administration, side effects and strength as the original drug. The U.S. Food and Drug Administration requires that all generic drugs be as safe and effective as brand-name drugs.

Generic drugs are often less expensive than their brand-name counterparts, because the generic manufacturers have not incurred the expenses of developing and marketing a new drug. In the United States, trademark laws do not allow generic drugs to look exactly like the brand-name drug; however, the generic drug must have the same active ingredients. In the case of Coumadin (the brand-name product) and warfarin (the generic product), the manufacturers attempted to keep the colors consistent with the strength of the pills. The goal is to allow the patient to identify the color-coded dose and prevent mix-ups or errors. Therefore, if the color or dose of the dispensed tablet appears different from the pill taken previously, the patient should immediately notify the pharmacist or health care professional. 

If you plan to travel abroad while on these medications, ensure you have an adequate supply for your stay. If you need to purchase these medications outside the U.S., first ask your health care professional to confirm the accuracy and quality of the medication.

Warfarin must be taken exactly as prescribed.

Never increase or decrease your dose unless told to do so by your health care professional. If you miss a dose, call your health care office for advice.

Side Effects

The major complications associated with warfarin are clotting due to underdosing or bleeding due to excessive anticoagulation. The most serious bleeding is gastrointestinal or intracerebral (within the brain). Excessive bleeding can occur in any area of the body, and patients taking warfarin should report any falls or accidents, as well as signs or symptoms of bleeding or unusual bruising, to their health care professional.

Signs of unusual bleeding include:

  • Bleeding from the gums
  • Bleeding into a joint
  • Blood in the urine
  • Bloody or black stool
  • A nosebleed
  • Vomiting blood

An unusual headache or a headache that is more severe than usual may signal a brain bleed (hemorrhage).

When to Call Your Health Care Professional

If you have the following signs of bleeding, you should immediately call 911 or your health care professional.

  • Severe headache, confusion, weakness or numbness
  • Coughing up large amounts of bright red blood
  • Vomiting blood
  • Bleeding that will not stop
  • Bright red blood in stool
  • Fall or injury to the head
  • Headache that is severe or unusual
  • Heavier than normal menstrual bleeding

Some simple changes to decrease the risk of bleeding while taking warfarin include:

  • Use a soft-bristle toothbrush.
  • Floss with waxed floss rather than unwaxed floss.
  • Shave with an electric razor rather than a blade.
  • Take care when using sharp objects, such as knives and scissors.
  • Avoid sports and activities that have a risk of falling or injury.

Warfarin and Lifestyle

Changes in daily living can affect the INR. It's important to know common do’s and don’ts for warfarin therapy.


Warfarin is not recommended during pregnancy. A woman who becomes pregnant or plans to become pregnant while taking warfarin should immediately notify her health care professional.

Medical or Dental Procedures

It's important to tell all your health care professionals that you are taking warfarin. If you are having surgery, dental work or other medical procedures, you may need to stop taking warfarin.


Check with your health care professional if you plan to travel. While traveling, it's important to always carry your medication with you. Don't put it into a checked bag.

Common Do's and Don'ts

What to Do What Not to Do
Watch for signs and symptoms of bleeding. Never double a dose because you missed a dose.
Tell your health care professional when you get sick or hurt. Don’t start new medications, herbs or supplements without talking to your health care professional.
Take warfarin exactly as prescribed. Don’t make changes to your warfarin dose unless your health care professional tells you to.
Tell anyone giving you medical or dental care that you are taking warfarin.
Keep your blood test appointments.

Warfarin Interacts With Other Medications

People who take warfarin should consult with their health care professional before taking any new medication, including over-the-counter medicines, herbs and vitamins. Many medications can alter the effectiveness of warfarin, resulting in an INR that is either too high or too low. Some of the most common over-the-counter pain relievers, such as ibuprofen (brand name Advil) and naproxen (brand name Aleve), enhance the anticoagulant effects of warfarin and increase the likelihood of harmful bleeding.

Warfarin Interacts With Alcohol and With Certain Foods

  • Alcohol - Alcohol can affect how the body metabolizes warfarin. People on warfarin therapy should avoid drinking alcohol or it should be consumed in moderation. Speak with your health care professional if you are taking warfarin and consume alcohol. The antiplatelet effect of alcohol increases the risk of major bleeding, even if the INR remains within the target range. It can also decrease the effect of warfarin.
  • Foods - Some foods can interfere with the effectiveness of warfarin. The most important point to remember is to eat what you normally eat and not to make any major changes in your diet without contacting your health care professional.
  • Vitamin K - Eating an increased amount of foods rich in vitamin K can lower the PT and INR, making warfarin less effective and potentially increasing the risk of blood clots. People who take warfarin should eat a relatively similar amount of vitamin K each week. The highest amount of vitamin K is found in green and leafy vegetables such as broccoli, lettuce and spinach. It's not necessary to avoid these foods, but it is important to try to keep the amount of vitamin K you eat consistent. Download our quick reference guide to foods that are high and low in Vitamin K. (PDF)

Wear Medical Identification

Those who take warfarin long-term should always wear a medical alert bracelet, necklace or similar alert tag. If an accident occurs and the person is too ill to communicate, a medical alert tag will help responders provide appropriate care. The alert should include a list of major medical conditions and the reason warfarin is needed, as well as the name and phone number of an emergency contact.

Where to Get More Information

Your health care professional is the best source of information for questions and concerns related to your medical problem. Because no two people are exactly alike, and recommendations can vary from one person to another, it's important to seek guidance from a health care professional who is familiar with your condition.

* Some medications are commonly called blood thinners because they can help reduce a blood clot from forming. There are two main types of blood thinners that patients commonly take: anticoagulants such as warfarin, dabigatran (Eliquis) and rivaroxaban (Xarelto), and antiplatelet drugs such as aspirin or clopidogrel. Each type of medication has a specific function to prevent a blood clot from forming or causing a blocked blood vessel, heart attack or stroke. 
The American Heart Association receives support from pharmaceutical and biotech companies, device manufacturers and health insurance providers whose products may be mentioned in this article. The American Heart Association maintains strict policies preventing supporters from influencing science-based health information. View a list of supporters.