Warfarin and Vitamin K: Understanding How Food Affects Warfarin
Key takeaways:
Vitamin K is a nutrient your body uses to form natural blood clots. Warfarin (Coumadin, Jantoven) prevents unwanted blood clots by blocking vitamin K from working in your body.
Sudden changes in the amount of vitamin K in your diet can change the way warfarin works. Adding vitamin K will make warfarin less effective. Lowering vitamin K can make warfarin too effective.
Common foods high in Vitamin K include kale, spinach, and broccoli. It’s also found in salad dressing and mayonnaise. It’s OK to eat foods high in vitamin K while taking warfarin. But you should try to be consistent with how much and how often you eat them.
Warfarin (Coumadin, Jantoven) is a blood thinner used to treat and prevent unwanted blood clots. But your diet has an effect on your warfarin dose, which can make it tricky to manage. More specifically, it’s foods that contain vitamin K — like your leafy green vegetables — that can affect how well warfarin works in the body.
If you love big spinach salads, broccoli side dishes, or you just can’t do without kale in your morning smoothie, there’s no need to worry. You can keep greens as a part of your diet. But it’s important to know how these foods interact with warfarin so you can safely enjoy the meals you love.
Why is vitamin K harmful for people taking warfarin?
Vitamin K isn’t always harmful if you’re taking warfarin. But large changes in the amount of vitamin K in your diet can be.
This is because warfarin and vitamin K work in opposite ways. Vitamin K is a nutrient your body uses to help form natural blood clots. And warfarin prevents blood clots by blocking vitamin K. So the amount of vitamin K in your diet can affect how well warfarin works.
Large amounts of vitamin K can make it harder for warfarin to work, and make blood clots more likely. But removing vitamin K from your diet while taking warfarin could make your blood too thin and lead to bleeding risks.
How does your diet affect your warfarin dose?
Your diet can affect how much warfarin you need to take. That’s because your warfarin dose is based on a blood test called the international normalized ratio (INR). The INR measures how fast your blood clots.
The lower your INR is, the more likely it is that clots could form. The higher your INR, the “thinner” your blood becomes, which makes clots less likely. But if it’s too high, you may be at risk for unwanted bleeding. When you’re taking warfarin, you’ll have your INR levels checked often to see how well it’s working. And your warfarin dose may change based on your INR results.
Changes in your diet can also change your INR — even if your warfarin dose stays the same. If you eat more vitamin K, it can lower your INR. If you eat less vitamin K, it can raise your INR.
This is where consistency comes into play. If you eat about the same amount of vitamin K each week, your healthcare provider can adjust your warfarin dose to balance its effects and keep your INR at a safe level. The same is true for other food and drinks that affect warfarin.
What foods contain vitamin K?
Certain foods have more vitamin K than others. The most well-known foods high in vitamin K are leafy green vegetables, such as spinach and kale. But it’s also in canola, soy, and olive oils. So keep in mind that products made with these oils, such as salad dressing and mayonnaise, will also have large amounts of vitamin K.
You can find common foods that are considered high in vitamin K in the table below.
Very High Vitamin K (more than 100 mcg/serving) | High Vitamin K (25-100 mcg/serving) | Low Vitamin K (less than 25 mcg/serving) |
|---|---|---|
Kale, cooked or raw (1 cup) | Cabbage, raw (1 cup) | Carrots (1 cup) |
Spinach, cooked or raw (1 cup) | Basil, dried (1 tbsp) | Bananas (1 medium) |
Broccoli, cooked (½ cup) | Sage, dried (1 tbsp) | Cashews (1 oz) |
Collard greens, cooked (½ cup) | Thyme,dried (1 tbsp) | Sweet potatoes (1 cup) |
Turnip greens, cooked (½ cup) | Broccoli, raw (1 cup) | Tomato (1 cup) |
Chard, cooked or raw (1 cup) | Kiwifruit (1 large) | Mushrooms, white (1 cup) |
Mustard greens, cooked (1 cup) | Avocado (1 cup) | Corn, cooked (1 cup) |
Brussels sprouts, cooked (1 cup) | Green beans, cooked (1 cup) | Cucumber (1 cup) |
Parsley, fresh (1 cup) | Mayonnaise (2 tbsp) | Bell pepper, raw (1 cup) |
Margarine (1 cup) | Tuna fish (1 can) | Celery (1 cup) |
Coleslaw (1 cup) | Asparagus (½ cup) | Zucchini (1 cup) |
What other food or drinks can affect warfarin?
Besides vitamin K-rich foods, other food and drinks may also affect warfarin. But some have greater effects than others.
Alcohol
Alcohol can change the way warfarin works in your body. It’s recommended that you consume alcohol in moderation or avoid it altogether if you’re taking warfarin.
Occasionally drinking a small amount of alcohol isn’t likely to cause a problem. But binge drinking can raise warfarin levels and make your INR go up. This puts you at greater risk for a bleed. Chronic alcohol use can have the opposite effect. It can lower your INR and make blood clots more likely.
Don’t be afraid to let your healthcare provider know if there’s been a change in your alcohol consumption each time your INR is measured. Being open about your lifestyle can help them make the best decision about your warfarin dose moving forward.
If you or someone you know struggles with substance use, help is available. Call SAMHSA’s National Helpline at 1-800-662-HELP (4357) to learn about resources in your area.
Cranberry juice
Although cranberry juice contains little vitamin K, it may still interact with warfarin. Several case reports found that drinking large amounts of cranberry juice with warfarin may increase your risk of bleeding.
However, most experts agree that drinking cranberry juice in moderation (about 8 fluid ounces per day or less) while taking warfarin shouldn’t affect your blood’s ability to clot.
Green tea
Although some sources mention that green tea may affect warfarin levels, this doesn’t seem likely. The information is based on a single report of a person who drank a half-gallon or more per day of green tea while taking warfarin and experienced low INR levels.
It’s mentioned in the report that this interaction was due to the amount of vitamin K in such a large volume of green tea. But moderate amounts of green tea seem to contain very little vitamin K and are not likely to affect warfarin.
Mango
Mango contains an ingredient that may raise warfarin levels in the body and increase your INR. If you consume mango regularly, it’s likely that your warfarin dose has already been adjusted to account for its effects.
But if your INR level changes and you’ve recently consumed more or less mango than usual, be sure to let your healthcare provider know. This will help them make the best decision about your warfarin dose moving forward.
How much vitamin K can I have if I’m taking warfarin?
As mentioned, it’s all about consistency. There’s not a specific amount of vitamin K you’re “allowed” to have while taking warfarin. But you should try to eat about the same amount each day. So if you usually eat a spinach salad for lunch, try to keep that consistent.
It’s best to avoid sudden changes to your vitamin K intake while taking warfarin. If you do make any changes, be sure to let your healthcare provider know.
Keep in mind that you also shouldn’t avoid vitamin K altogether. It’s an important vitamin for keeping your bones and heart healthy, so it’s best to consume at least the minimum recommended amount each day.
Recommended minimum daily amount of vitamin K
Female | Male |
90 mcg/day | 120 mcg/day |
The recommended daily amount of vitamin K is 90 mcg for females and 120 mcg for males. If you have questions about your vitamin K intake, contact your healthcare provider.
Can too much vitamin K cause blood clots?
On its own, large amounts of vitamin K aren’t associated with any adverse effects.
But if you’re taking warfarin to prevent blood clots, large amounts of vitamin K can make it less effective. And this can increase your risk for blood clots. To help prevent this, limit your alcohol intake and try not to make any sudden changes to your diet. You also shouldn’t stop or start any new medications, vitamins, or dietary supplements without your provider’s OK.
It’s also good to be aware of the signs and symptoms of a blood clot, which include:
Pain, swelling, or redness in an arm or leg
Difficulty breathing
Chest pain
Severe headache
Seek emergency care if you experience any of these symptoms. And be sure to be aware of symptoms of bleeding, such as blood in your urine, while brushing your teeth, or large bruises that don’t heal.
The bottom line
Vitamin K can prevent warfarin from working as well in your body. But you can still eat food that’s high in vitamin K if you’re taking warfarin. Just be consistent with how much and how often you do. Try to avoid sudden changes in your diet and limit your alcohol intake. This can help keep your warfarin levels stable.
Why trust our experts?
References
Centers for Disease Control and Prevention. (2022). Binge drinking.
Centers for Disease Control and Prevention. (2022). Know the risks, signs & symptoms of blood clots.
Crader, M. F., et al. (2022). Warfarin drug interactions. StatPearls.
Di Minno, A., et al. (2017). Old and new oral anticoagulants: Food, herbal medicines and drug interactions. Blood Reviews.
Kuruvilla, M., et al. (2001). A review of warfarin dosing and monitoring. Baylor University Medical Center Proceedings.
Tan, C. S. S., et al. (2021). Warfarin and food, herbal or dietary supplement interactions: A systematic review. British Journal of Clinical Pharmacology.
Taylor, J. R., et al. (1999). Probable antagonism of warfarin by green tea. Annals of Pharmacotherapy.
U.S. Department of Agriculture FoodData Central. (2023). Vitamin K (phylloquinone).
U.S. Office of Dietary Supplements. (2021). Vitamin K.
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