Important Drug and Food Information
From the Warren Grant Magnuson Clinical Center, National Institutes of HealthDrug-Nutrient Interaction Task Force
Important information to know when you are taking:
The food you eat can affect how your medicine works. It is important to learn about possibledrug-nutrient interactions for any medicines you take. This handout was developed to provide youwith information about the interaction between Coumadin® and vitamin K.
Why was Coumadin prescribed for you?
Warfarin (Coumadin ) is a medicine prescribed for people at increased risk of forming bloodclots. Sometimes medical conditions can make blood clot too easily and quickly. This couldcause serious health problems because clots can block the flow of blood to the heart or brain.
Warfarin (Coumadin ) can prevent harmful blood clots from forming.
How does Coumadin work?
Blood clots are formed through a series of chemical reactions in your body. Vitamin K is
essential for those reactions. Coumadin works by decreasing the activity of vitamin K;lengthening the time it takes for a clot to form.
International Normalized Ratio (INR) and Prothrombin Time (PT) are laboratory test valuesobtained from measurements of the time it takes for a clot to form. Individuals at risk for
developing blood clots take Coumadin to prolong the usual time it takes for a clot to form,resulting in a prolonged INR/PT. Doctors usually measure the INR / PT every month inpatients taking Coumadin® to make sure it stays in the desired range.
What can help keep INR / PT in the desired range?
To help Coumadin® work effectively, it is important to keep your vitamin K intake as consistentas possible. Sudden increases in vitamin K intake may decrease the effect of Coumadin®. Onthe other hand, greatly lowering your vitamin K intake could increase the effect of Coumadin®.
To keep INR / PT stable and within the recommended range, it is important to:
• take the correct dose of Coumadin® at the same time every day
• have your INR / PT checked regularly
• keep your vitamin K intake consistent from day to day
To help make it easier to keep your intake of vitamin K consistent:
• limit intake of foods considered “high” in vitamin K to no more than 1 serving each day
• limit intake of foods “moderately high” in vitamin K to no more than 3 servings each day
• report any significant changes in your diet or your weight to your doctor
In other words,
■ Watch how often you eat foods high in vitamin K.
■ Watch how much you eat of foods high in vitamin K.
Do you want to know more about vitamin K?
Green leafy vegetables are among the best food sources of vitamin K. The average intake ofvitamin K for most adults in the U.S. is 70 to 80 micrograms (mcg) per day. The Daily Valuefor vitamin K, an estimate of daily need, is 80 micrograms. The Percent Daily Values (%DV),listed on the tables below, help consumers determine if a food contains a little or a lot of aspecific nutrient.
As indicated below, it is important to limit intake of foods that provide more that 60% of theDaily Value for vitamin K to help keep INR / PT in the desired range.
Foods high in Vitamin K (more than or equal to 200% DV)
Eat no more than 1 serving per day
Serving size % Daily Value
Foods moderately high in Vitamin K (60 to 199% DV)
Eat no more than 3 servings per day
Serving size % Daily Value
Iceberg lettuce, red cabbage, asparagus, and soybean oil are often reported as beinghigh in vitamin K. They contain much smaller amounts than foods listed in the tablesabove. These, and other foods and beverages not listed in the tables above (includingcoffee and tea), may be consumed as desired. Food Values are from the U.S. Depart-
ment of Agriculture, Agricultural Research Service. 2003. USDA National NutrientDatabase for Standard Reference, Release 16. Nutrient Data Laboratory Home Page,http://www.nal.usda.gov/fnic/foodcomp
What else should you know about Coumadin®?
Alcohol intake greater than 3 drinks daily can increase the effect of Coumadin®. However,some medical doctors advise those taking Coumadin® to avoid all alcoholic beverages.
Check with your doctor about this issue.
• Dietary supplements and herbal medications
Many dietary supplements can alter the INR/ PT. Dietary supplements known to affect theINR/PT include: arnica, bilberry, butchers broom, cat’s claw, dong quai, feverfew, forskolin,garlic, ginger, ginkgo, horse chestnut, insositol hexaphosphate, licorice, melilot (sweet clover)pau d’arco, red clover, St. John’s wort, sweet woodruff, turmeric, willow bark, and wheat grass.
Much is unknown about dietary supplements. The safest policy is for individuals onCoumadin® to avoid all dietary supplements unless their physicians approve. This includes anyvitamin/mineral supplements that list vitamin K on the label. If they are taken regularly on adaily basis, they pose less of a problem than if taken off and on.
Evidence suggests that vitamin E has blood-thinning effects. Vitamin E intakes above 1,000International Units (IU) per day may increase the risk of excess bleeding. Research suggeststhat doses up to 800 IU may be safe for individuals on Coumadin®, but the evidence is notconclusive. It is best for those taking Coumadin® to ask their physicians about taking VitaminE supplements.
Some antibiotics can either lower vitamin K levels in the body or interfere with the activity ofCoumadin® . Check with your physician or pharmacist about whether you will need to adjustyour vitamin K intake or Coumadin® dose when you take antibiotics.
What are three important things to remember about Coumadin and vitamin K?
1. Coumadin® is a very important drug for you. Follow the prescription exactly, and keep your
follow-up appointments for blood tests such as the INR/PT.
2. Coumadin® interacts with vitamin K in your body, so you need to keep vitamin K intake
constant from day to day. It is also important to avoid herbal products and dietary supple-ments that may affect vitamin K and Coumadin® unless approved by a qualified health careprovider.
3. Post the phone numbers of your doctor, pharmacist, and registered dietitian for ready
reference when you have a question or concern about Coumadin®, vitamin K, and yourINR / PT.
Where applicable, brand names of commercial products
This information is prepared specifically for patients
are provided only as illustrative examples of acceptable
participating in clinical research at the Warren Grant
products, and do not imply endorsement by NIH; nor
Magnuson Clinical Center at the National Institutes
does the fact that a particular brand name product is not
of Health and is not necessarily applicable to
identified imply that such product is unsatisfactory.
individuals who are patients elsewhere. If you have questions about the information presented
here, talk to a member of your healthcare team.
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