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FAQs About Warfarin (brand name Coumadin)

Henry I. Bussey, Pharm.D., FCCP, FAHA
March, 2000
Updated December, 2006

This section is designed to answer general questions you may have about warfarin. If you do not find answers to your questions about warfarin, please send your question via e-mail to and you will receive a response from a member of our editorial board. Please feel free to print this page.

What is warfarin?
Warfarin is the most commonly used oral anticoagulant in the US. An anticoagulant is a drug used to prevent unwanted and dangerous blood clots. Although anticoagulants are often called "blood thinners," warfarin does not actually make your blood thinner. Warfarin interferes with the body's ability to make a blood clot (see How does warfarin work?). Warfarin is also sold as Coumadin and Jantoven, which are brand names for the generic drug warfarin.

Warfarin is a product that was initially developed at the University of Wisconsin around 1950 as a product of spoiled sweet clover. Excessive intake by cattle caused them to bleed because warfarin interferes with the blood clotting process. In the U.S. a refined form of this compound has been used widely for about 50 years and currently is taken by millions of patients each year to prevent blood clots. Unwanted blood clots may cause strokes, heart attacks, and other very serious (and possibly deadly) events - such as blood clots in the legs or lungs. If it is used correctly, warfarin can be one of the most valuable drugs we have. If warfarin therapy is not managed very carefully, however, it also can be one of the most dangerous drugs we have because of the potential to induce bleeding.

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How does warfarin work?
In order for your blood to form a clot, you have to have certain proteins in your blood. Normally, your liver makes these proteins and it requires vitamin K to do so. You get vitamin K from many of the foods you eat (especially green vegetables and certain oils). Warfarin reduces your liver's ability to use vitamin K to make these blood clotting proteins, which makes it harder for your blood to clot. Vitamin K and warfarin tend to work against each other. For this reason warfarin is often referred to as a vitamin K antagonist (VKA). If your intake of vitamin K increases, you will need more warfarin to keep your blood from clotting. If your intake of vitamin K is reduced, your dose of warfarin will also have to be reduced in order to keep you from bleeding. Fortunately, both the amount of vitamin K and warfarin in your body tends to rise and fall somewhat slowly. Therefore, one way to think about this balance between warfarin and vitamin K is that it is the WEEKLY (rather than the daily) intake that is being balanced against each other. Additionally, because the level of warfarin rises or falls rather slowly, any change in dose may take several days or even a few weeks to reach a new stable level. Similarly, if you miss a dose of warfarin (or take an extra dose) the level of warfarin (and therefore its effect) may be altered for several days.

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How is warfarin taken?
The clinician who is responsible for managing your warfarin therapy will tell you exactly how to take it. If the instructions are not perfectly clear, ask your clinician to explain the instructions again or to write down the instructions for you. Once you clearly understand the instructions, take your warfarin exactly as your clinician instructs you. Often times the dosage schedule may require taking different doses on different days of the week. This is required to "fine tune" the weekly dose of warfarin to meet you needs.

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When during the day should I take my dose of warfarin?
The time of day when you take your dose of warfarin does not affect how it works. However, it is a good idea to take it at the same time every day just to reduce the chances that you might forget to take it. It may also help if you take it at the same time that you do something else that is routine to you - such as take it when you eat your dinner or when you brush your teeth at night. Many clinicians may prefer that you take your warfarin in the evening. This is done so that on those days when your blood test is checked, the dose can be adjusted that day, if needed, rather than having to wait until the next day's dose. If it is hard for you to remember to take your warfarin at the designated time, discuss this with your clinician. It may help for you to take it at a different time of day or there may be other methods (such as a calendar or a weekly "pill" box) that may help you remember.

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What should I do if I miss doses of warfarin or take extra doses?
Some times your clinician may instruct you to either skip doses of warfarin or take extra doses. This is a perfectly acceptable way of quickly adjusting your warfarin level. You, however, should NOT attempt to adjust your therapy on your own. It is possible, however, that you may either forget to take your warfarin or may become confused and take extra doses. What to do if that occurs depends on several factors (as discussed below).

Even one extra dose of warfarin may create a dangerous situation that could last for several days. Therefore, unless your clinician instructs you to do otherwise, you should report ANY extra doses you may have taken. Also, when you call, be prepared to report (as best you can recall) the size of any extra doses taken and the date(s) when the extra dose(s) were taken.

If you miss only ONE dose of warfarin, what you should do depends on when you realize the error. If it is approximately 12 hours or less since you were supposed to have taken the dose, then go ahead and take it. If it is much beyond that 12 hour period, either wait until your next dose is due or follow the instructions of your clinician. As an example; if you usually take your dose at bedtime but find in the morning that you missed last night's dose, then just take it that morning. Resume the usual schedule at bedtime that same day. If, however, you remember one night that you missed the last night's dose, do NOT take two doses that night (unless directed to do so by your clinician). Also, it is very important to make a note of any missed doses so that you can tell your clinician the date that you missed any doses and the size of the doses you missed. This information may be critical for your clinician to appropriately interpret the results of your blood tests. In most cases, missing only ONE dose does not mean that you need to call your clinician, but you may want to discuss this with your own clinician. If, however, you miss two or more doses, you should notify your clinician immediately.

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How often will my blood be tested and what does the test mean?
How often your blood is tested will depend on how stable the blood test results have been, whether your dose of warfarin has been changed recently, and whether other factors that may alter your dose of warfarin have happened recently. If your blood tests and all other factors have been stable for some time, your blood test should be performed at least every four weeks. If the blood tests, warfarin dose, or other important factors (such as diet, other medications, etc.) have changed in the past several weeks or months, then your blood test may have to be done significantly more often (one or more times per week). The test that is used to adjust your warfarin dose is called the prothrombin time (PT) and it measures how many seconds it takes for your blood to form a clot in the laboratory under carefully controlled conditions. Although the test is measured in seconds, the only accurately way to report the test is in INR (International Normalized Ratio) units. For more information on the PT test and the INR, see Understanding the PT-INR Test).

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What is the INR and what does it mean?
The INR is the standard unit used to report the result of the PT test. For a more detailed explanation of the PT test and the INR, please see Understanding the PT-INR Test.

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What if I am allergic to warfarin?
If you are allergic to warfarin, your anticoagulation clinician may consider switching you to low molecular weight heparin or, perhaps, to one of the new oral anticoagulation medications that are currently in development. Gaining access to one of the new agents before it is approved for marketing by the FDA usually requires special circumstances and a special application by your physician.

In many cases, however, the allergy is to the dye used in the tablets rather than to the warfarin itself. If your clinician can determine that you are allergic to the dye rather than the warfarin, he/she may consider using a different strength of tablet that has a different color dye, or he/she may consider using the dye-free 10mg tablets of warfarin for your treatment.

If you are used to taking fewer than 5 mg of warfarin per day, you may wonder how you could use 10 mg tablets, since it is difficult to break the tablet into more than 2 pieces. Since warfarin usually is metabolized very the slowly, the exact daily dose is not as important as the average dose over a several day period. So, your clinician may consider having you take about the same amount of warfarin each week using different daily doses that are feasible with the larger, dye-free tablet.

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Can things other than the dose of warfarin change my INR?
Definitely. This is one reason that your INR blood test needs to be measured rather frequently. Many things can alter the INR in a patient taking warfarin. Image of pills Such factors include changes in your diet, vitamins, food supplements, herbal preparations, other medications (either prescription or those bought "over the counter"), change in your level of exercise, getting sick, smoking, and consuming alcohol. As mentioned above, a change in the INR may cause a big change in your risk of bleeding or blood clot formation. Therefore, you should keep you clinician informed of any of these types of changes and discuss in advance any changes you are planning in any of the above factors (such as going on a trip, starting an exercise program, trying to stop smoking, going on a new diet, taking an herbal preparation, etc.).

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What side effects may occur with warfarin?
Side effects of warfarin are unusual if you take your medication regularly as instructed, have your clinician check your INR often, and notify your clinician of any changes in your diet, vitamins, food supplements, herbal preparations, other medications (either prescription or those bought "over the counter"), change in your level of exercise, getting sick, smoking, and consuming alcohol. By far, bleeding is the most common and most serious side effect of warfarin. Bleeding can occur at any site or into any organ, but a number of measures can greatly reduce this risk (see What can I do to minimize my risk of bleeding?). Other rare side effects (that occur in less than 1% of patients) include skin necrosis, purple toe syndrome, and allergic reaction. Warfarin-induced skin necrosis is thought to be due to the formation of many small blood clots in small blood vessels. This can lead to tissue damage that may require amputation or skin grafting. For unknown reasons, this condition usually occurs over fatty areas of the body such as the breasts or buttocks. Patients with a condition called protein C deficiency may be especially prone to this rare side effect. The purple toe syndrome is not well understood, but is thought to be due to cholesterol deposits breaking loose from the walls of blood vessels, flowing to the toe, and blocking usual blood flow to the affected toe. Allergic reactions to warfarin are rare, and it is difficult to confirm that warfarin is the cause. Even so, if the patient develops a rash that cannot be attributed to another cause, warfarin - or perhaps the dye used to color the tablet - should be considered a potential allergen. If the dye is the culprit, the clinician may change to another strength tablet (which is a different color and therefore contains a different dye). Another alternative would be to change to another oral anticoagulant such as Miradon.

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What can I do to minimize my risk of bleeding?
On average, your risk of serious bleeding can be kept below 2 % per year, if several requirements are met. Your INR blood test should be monitored frequently by a reliable laboratory and dosage adjustments should be made by a clinician who is an experienced expert in this form of therapy. You should take your medication exactly as instructed, and notify your clinician if any of several potential changes occur. Specifically, you should notify your clinician of any changes that may alter your response to warfarin. These changes include changes in your diet, vitamins, food supplements, herbal preparations, other medications (either prescription or those bought "over the counter"), change in your level of exercise, getting sick, smoking, consuming alcohol, or any travel plans (since travel may inadvertently result in a change in diet or physical activity).

In addition to keeping your clinician informed of changes that may alter your response to warfarin, you can also reduce your risk of serious bleeding by monitoring yourself for signs and symptoms of bleeding. The most common "early warning" signs are unusual bruising (bruises that develop without a cause or bruises that tend to grow or expand), nose bleeds, and bleeding gums. Signs (things you may see) that may indicate more serious bleeding include pink or brown urine, red or black stools (bowel movements), coughing up blood, or vomiting blood or "coffee ground" colored material. New symptoms (changes in how you feel) that may suggest serious bleeding is occurring include a severe headache, dizziness, fatigue or weakness. If the "early warning" signs occur, you should contact your clinician immediately. If the indicators of more serious bleeding occur, you should secure medical attention immediately (including going to an emergency department if necessary).

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What do I need to know about drug interactions with warfarin?
Numerous "drugs" (including prescription medications, medications you can buy without a prescription, vitamins, dietary supplements, and herbal preparations) can produce a dangerous change in the effect of warfarin. Such changes may increase your risk of blood clot formation or bleeding; either of which may result in serious harm or even death. Therefore, it is very important that you keep your anticoagulation clinician up-to-date on any such product(s) you are taking. Additionally, you should not start, stop, or change the dosage of any such product without the approval of your anticoagulation clinician. Furthermore, if another clinician instructs you to make such a change, you should NOT presume that the change is safe or that the other clinician will notify your anticoagulation clinician of the change. YOU should immediately notify your anticoagulation clinician of the change.

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What do I need to know about diet interactions with warfarin?
Because warfarin exerts its effect by reducing the ability of your liver to use vitamin K to make normally functioning blood clotting proteins, any dietary change that alters your vitamin K intake can alter your response to warfarin. Such dietary changes may increase your risk of blood clot formation or bleeding; either of which may result in serious harm or even death. Therefore, it is very important that you keep your anticoagulation clinician up-to-date on any planned changes in your diet (including vitamins, nutrition supplements, herbal preparations, etc.). Image of a leafy green salad This does NOT mean, however, that you should avoid vitamin K containing foods. In fact, this is a popular misconception among both patients and some clinicians. Reducing the amount of vitamin K in your diet may both cause your INR to increase to a dangerous level and make the INR more difficult to control. The most important dietary consideration is consistency. This does not mean that you have to eat the same thing every day. Both warfarin and vitamin K tend to be eliminated rather slowly from your body. For that reason, the idea of "balancing" your weekly intake of vitamin K against your weekly dose of warfarin is a valid concept . Therefore, if you eat the same types of foods each week, it will help to stabilize your INR and avoid dietary-related INR fluctuations. Even so, the occasion may arise when you may want to eat a food that is not a common part of your diet. For that reason, you may wish to have information about the vitamin K content of certain foods. As a "rule of thumb", green vegetables (especially leafy green vegetables) and certain oils have a high content of vitamin K. Most fruits, meats, dairy products, and grains are low in vitamin K. As a resource to use in making consistent dietary decisions, ClotCare provides a list of certain foods and their relative content of vitamin K (i.e. low, moderate, or high). Click here to download this list. If the "new" food is high in vitamin K, that does not mean that you should not eat it. You should, however, eat only a small amount and/or leave out of your diet that week some other high vitamin K containing food. If the food is low in vitamin K, then no such modification in your usual diet is needed. Just to re-emphasize, you should NOT try to avoid vitamin K containing foods; rather you should maintain a consistent (week-to-week) intake in your diet -- consistency is the key!

For a more detailed explanation of the relationship between vitamin K and warfarin, please review the Vitamin K and Warfarin: What You Should Know section of ClotCare.

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What if I want to become pregnant or suspect that I am pregnant?
Warfarin has been proven to cause birth defects and increases the risk of hemorrhage in both the mother and the newborn. It is, therefore, very important that you NOT become pregnant while taking warfarin. The period of greatest risk of birth defects is from week 6 to week 12 of the pregnancy. Therefore, if you think you may be pregnant, contact your clinician immediately. If you wish to become pregnant, discuss this with your clinician in advance. It is possible that your antithrombotic therapy might be switched to another agent that does not cause birth defects (such as one of the injectable heparin preparations). There may be other options as well, but this will depend on the reason you are taking warfarin and perhaps a number of other factors that relate to you individually.

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Can I breast feed my baby while taking warfarin?
Yes. Although warfarin is dangerous to take while pregnant or if you are trying to become pregnant (see "What if I want to become pregnant or suspect that I am pregnant?"), it is OK for you to breast feed your infant because not enough warfarin gets into the breast milk to do any harm.

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Should I wear an "Emergency ID" bracelet or carry an "Emergency Card" in my wallet or purse?
Yes. Either or both of these devices are good ideas for several reasons. If you happen to be in an accident or require emergency surgery for some other reason, the warfarin will cause you to bleed more than usual. However, there are a number of ways that the effect of warfarin can be reversed and thereby prevent excessive blood loss. Obviously, those providing medical care to you in such a situation must know that you are taking warfarin in order to know that they need to reverse it. It is also wise for such emergency identification to contain the name and phone number of the clinician who is responsible for managing your anticoagulation so that they can be contacted readily. Additionally, this information may be of use to you as well in less urgent situations that may require you to contact your clinician.

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Are there other lifestyle factors that I should know about?
Yes. Other "risk factors", smoking, alcohol ingestion, contact sports and other physical activities, and sharp objects are all factors that require consideration.

You should follow your clinician's instructions on managing other "risk factors" that are known to increase your risk of blood clotting or bleeding. Specifically, high blood pressure can increase both your risk of stroke and bleeding and, therefore, should be carefully controlled. High cholesterol levels can increase your risk of clotting. Similarly, the use of estrogen replacement therapy in post-menopausal women or the use of oral contraceptives may increase the risk of blood clot formation. Whether adequate anticoagulation eliminates the estrogen-related increased risk of clotting is controversial.

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Smoking, in general, increases the risk of blood clot formation, causes or worsens other health conditions, and should be discouraged. However, your clinician should be kept informed of any change in the amount you smoke because smoking tends to increase how fast your body metabolizes warfarin.

Alcohol has several negative effects in patients taking warfarin. It can increase or decrease how fast your body metabolizes warfarin. Whether it increases or decreases your warfarin metabolism may depend on how much you drink, how often you drink, and whether the alcohol has damaged your liver. Additionally, alcohol irritates the lining of your stomach and this may cause internal bleeding. It is, therefore, best not to drink alcohol; but if you do, you should limit your consumption to only 1 or 2 drinks a day. As with smoking, it is best not to use alcohol; but it also is very important to keep your clinician informed of any change in the amount of alcohol you ingest or the frequency with which you do so.

Contact sports and other physical activities that may cause injury should be avoided because of the risk to induce bleeding. Although cuts and scrapes may be obvious sources of bleeding, there is also the potential for injury or mild trauma to cause bleeding into a muscle, joint, or organ (including the brain). Football and rugby would be obvious sports to avoid; but falls while skiing or the violent jerking of a roller coaster ride may also cause injury that may result in serious bleeding. Although you should avoid these types of activities, should any such injury - or severe headache - occur, you should report these to your clinician immediately.

Sharp objects - from kitchen knives to chain saws - should be avoided or handled with extreme caution since a cut may result in uncontrolled bleeding. Although many clinicians advise their patients to use an electric razor rather than a "safety razor", ClotCare feels that this may not be necessary since most shaving "nicks" are small and bleeding cessation is probably more dependent of the role of blood cells called platelets rather than the protein-based blood clotting pathway that warfarin inhibits.

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What does the FDA say about warfarin (brand name Coumadin)?

The FDA may require distribution of Medication Guides, FDA-approved patient information, for selected prescription drugs that pose a serious and significant public health concern. Medication Guides will be required if the FDA determines that one or more of the following circumstances exist:

  • patient labeling could help prevent serious adverse effects
  • the drug product has serious risk(s) (relative to benefits) of which patients should be made aware because information concerning the risk(s) could affect patients' decision to use, or to continue to use, the product
  • the drug product is important to health and patient adherence to directions for use is crucial to the drug's effectiveness

The FDA's medication guide for warfarin (brand name Coumadin) is available online at

Although this medication guide provides a lot of useful information, it should be realized that individual clinicians may have a different perspective on several issues raised in this document. A few such issues include (but are not limited to):

  1. Dietary vitamin K intake – Although the medication guide discusses the importance of maintaining a steady or consistent dietary intake of vitamin K, some of the comments tend to suggest that a patient on warfarin should avoid or limit the intake of certain foods. Recent information has indicated that a healthy (higher) intake of vitamin K may actually help to improve stability of the warfarin therapy as measured by the INR. Consequently, many clinicians endorse the importance of consistency of dietary vitamin K intake; but they may be reluctant to discourage the consumption of any particular foods.

  2. Intake of cranberry juice and cranberry products – Various reports have suggested that cranberry juice or cranberry products may either increase or decrease the effect of warfarin. Most such reports have involved patients who were consuming rather large amounts of cranberry juice or cranberry products. These reports seem to be conflicting.  Additionally, it is not clear from these reports whether consuming cranberry juice (or cranberry products) in usual amounts is likely to change the effect of warfarin.

  3. Consumption of alcohol – Although alcohol consumption can change the effect of warfarin, many clinicians allow patients to drink alcohol in moderation, such as 1 or 2 glasses of wine in an evening.

  4. Breast feeding while taking warfarin – Available data suggest that warfarin does not pass into breast milk to a measurable degree. Therefore, breastfeeding is generally considered safe for mothers who are taking warfarin. However, the data are less clear for other warfarin–like anticoagulants, which are often used outside of the U.S.  Thus the safety of breastfeeding while taking one of these other oral anticoagulants is less certain.

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Thursday, July 18, 2024