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}

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.