Warfarin and Vitamin K

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Topic Overview

Warfarin is a pill that you take regularly to help prevent blood clots or to keep a clot from getting bigger. Coumadin is the common brand name for warfarin.

To ensure that warfarin is effectively thinning your blood, it's important to eat about the same amount of vitamin K every day.

  • Vitamin K normally helps your blood clot so wounds don't bleed too much.
  • Warfarin works against vitamin K, making your blood clot more slowly.

So warfarin and vitamin K work against each other in your body. That is why, when you take warfarin, it's important that you not suddenly eat a lot more or a lot less vitamin K-rich food than you usually do.

How to get a steady amount of vitamin K

It's up to you how much vitamin K you choose to eat. For example, if you already eat a lot of leafy green vegetables, that's fine. Just keep it about the same amount each day.

And if you take a multivitamin that contains vitamin K, be sure you take it every day.

Check with your doctor before you make big changes in what you eat, such as starting a diet to lose weight.

Adding vitamin K

If you want to start eating more of a food that's rich in vitamin K, talk to your doctor about how to add it safely. Your warfarin dose may need to be adjusted.

Use this list to get an idea of what foods are sources of vitamin K.

Vitamin K content of select foods footnote 1
 Food (no salt added) Serving Size Vitamin K (mcg)

Kale, boiled, drained

1 cup


Spinach, frozen, boiled, drained

1 cup


Spinach, boiled, drained

1 cup


Collards, boiled, drained

1 cup


Broccoli, boiled, drained

1 cup


Brussels sprouts, boiled, drained

1 cup


Parsley, raw

10 sprigs


Cabbage, boiled, drained

1 cup


Spinach egg noodles, cooked, enriched

1 cup


Spinach, raw

1 cup


Broccoli, raw

1 cup


Lettuce, green leaf, raw

1 cup


Coleslaw, fast food

¾ cup


Okra, boiled, drained

1 cup


Green peas, canned, drained

1 cup


Lettuce (such as romaine), raw

1 cup


Vegetables, mixed, frozen, boiled, drained

1 cup


Lettuce, butterhead (such as Boston or Bibb), raw

¼ head


Blueberries, frozen, sweetened

1 cup


Peas, edible pods, boiled

1 cup


Green peas, frozen, boiled

1 cup


Tuna fish, light, in oil, drained

3 oz


Celery, raw

1 cup


Lettuce, iceberg, raw

¼ head


Soy beans (edamame), boiled

1 cup


Kiwi, raw

1 medium


Scallion or spring onion, raw

1 medium


Asparagus, boiled, drained

4 spears


Blackberries, raw

1 cup


Blueberries, raw

1 cup


Marinara sauce for pasta, ready-to-serve

½ cup


Cucumber, with peel, raw

¼ large


Canola oil

1 Tbsp


Olive oil

1 Tbsp


Pistachios, dry roasted, salt added

1 oz (47 nuts)


Tea, brewed, prepared with tap water

6 fl oz


Green and black tea leaves do contain vitamin K before they are steeped in water, but a small serving of the hot tea itself does not.

Check with your doctor before you take any supplements or herbal products. Some of these may contain vitamin K. If you already take a product that contains vitamin K, do not stop taking it without talking with your doctor first.

How vitamin K and warfarin affect your risks and your test results

To find out how well warfarin is working, you will get blood tests to measure how long it takes for your blood to clot. Your lab results are called your Prothrombin Time (PT) and International Normalized Ratio (INR) values. You may just hear about your INR.

Your INR needs to be in a safe range—not too high and not too low. Vitamin K can change how warfarin works, which changes your INR.

  • Vitamin K lowers your INR values. The lower your INR, the less time it takes for your blood to clot. A low INR means that warfarin isn't working well enough to prevent a dangerous blood clot.
  • Warfarin raises your INR values. The higher your INR, the more time it takes for your blood to clot. A high INR means that warfarin is working too well, so you bleed more quickly and easily. This can be dangerous.

Keeping your warfarin and vitamin K intake steady every day helps keep you in a safe INR range.

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  1. Agricultural Research Service (2010). USDA National Nutrient Database for Standard Reference, Release 23. Available online: http://www.ars.usda.gov/SP2UserFiles/Place/12354500/Data/SR23/nutrlist/sr23w430.pdf.

Other Works Consulted

  • Agency for Healthcare Research and Quality (2010). Blood Thinner Pills: Your Guide to Using Them Safely (AHRQ Publication No. 09-0086-C). Rockville, MD: Agency for Healthcare Research and Quality. Available online: http://www.ahrq.gov/consumer/btpills.htm.


ByHealthwise Staff
Primary Medical Reviewer E. Gregory Thompson, MD - Internal Medicine
Specialist Medical Reviewer Rakesh K. Pai, MD, FACC - Cardiology, Electrophysiology

Current as ofFebruary 20, 2015