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Warfarin and Vitamin K

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

Here's why:

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

So warfarin and vitamin K work against each other in your body, and it's important to keep them in balance. 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. If you do, your warfarin dose may need to be adjusted.

It's up to you to decide 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 to 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. And 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.

Also check with your doctor before you add or change any supplements or herbal products. Some of these may contain vitamin K. If you already take a product that contains vitamin K, don't stop taking it without talking with your doctor first.

Foods that are sources of vitamin K

The following are lists of some foods that are high, moderate, and low in vitamin K.footnote 1

Examples of foods that are high in vitamin K (more than 100 mcg per serving)

Food

Serving Size

Vitamin K (mcg)

Kale, cooked

½ cup

531

Collard greens, cooked

½ cup

530

Spinach, cooked

½ cup

444

Swiss chard, cooked

½ cup

287

Kale, raw

1 cup

274

Spinach, raw

1 cup

145

Endive, raw

1 cup

116

Broccoli, cooked

½ cup

110

Brussels sprouts, cooked

½ cup

109

Examples of foods that are moderate in vitamin K (25–100 mcg per serving)

Food

Serving Size

Vitamin K (mcg)

Cabbage, cooked

1 cup

85

Spinach noodles, cooked

½ cup

81

Green leaf lettuce

1 cup

63

Romaine lettuce

1 cup

57

Broccoli, raw

½ cup

45

Okra, cooked

½ cup

44

Asparagus spears, cooked

5 spears

38

Black-eyed peas, cooked

½ cup

32

Kiwi fruit

1 medium

31

Prunes, dried

5

25

Examples of foods that are low in vitamin K (less than 25 mcg per serving)

Food

Serving Size

Vitamin K (mcg)

Cabbage, raw

½ cup

21

Green peas

½ cup

19

Parsley, raw

1 tablespoon

18

Celery, raw

½ cup

18

Artichoke, cooked

1 medium

18

Blackberries

½ cup

14

Blueberries

½ cup

14

Iceberg lettuce

1 cup

13

Carrots, cooked

½ cup

11

Cucumber, with peel

½ cup

9

Cauliflower, raw

½ cup

8

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.

How vitamin K and warfarin affect 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, and this 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.

References

Citations

  1. Academy of Nutrition and Dietetics (2018). Vitamin K content of foods. Nutrition Care Manual. https://www.nutritioncaremanual.org. Accessed July 17, 2019.

Credits

Current as of: September 8, 2021

Author: Healthwise Staff
Medical Review:
E. Gregory Thompson MD - Internal Medicine
Martin J. Gabica MD - Family Medicine
Adam Husney MD - Family Medicine

 

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