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What is warfarin?

Warfarin is a medication that helps “thin” your blood to decrease your body’s chance of forming harmful clots. Unwanted blood clots may cause strokes, heart attacks, or other potentially harmful events such as clots in the legs (deep vein thrombosis) or lungs (pulmonary embolism).

How does warfarin work?

There are proteins in your blood to help form clots. These proteins are made by your liver with the help of vitamin K. Warfarin works by blocking the effects of vitamin K, making it harder for your body to form clots. In order to make sure the amount of warfarin you’re taking is right for you, your healthcare professional will test your blood periodically. The blood test checks your protime (PT) or international normalized ratio (INR) to measure how long it takes for your blood to clot. If your PT/INR is outside your target range, your warfarin dose will need to be adjusted.

How does your diet affect warfarin?

Since vitamin K and warfarin work against each other, the amount of vitamin K in your diet can change warfarin’s effects. It is important to keep your dietary intake of vitamin K consistent. Foods such as green leafy vegetables and certain oils have higher contents of vitamin K. If you decrease your intake of vitamin K (eat fewer foods containing vitamin K) your dose of warfarin may need to be lowered to prevent bleeding. If you increase your intake of vitamin K, your dose of warfarin may need to be increased to prevent blood clots. A major change in your vitamin K intake can affect your PT/INR, but normal daily variation in the foods you eat is okay. You do not have to avoid foods that are high in vitamin K, just keep your diet consistent. You should let your healthcare professional know if there is going to be a major change in your diet, so your PT/INR can be closely monitored. Many people are on special diets, such as the Atkin’s or South Beach diets, to lose weight.

These diets are high protein diets and can also affect the way warfarin works in your body. Once you take a dose of warfarin, some of it binds to the protein in your bloodstream. While warfarin is attached to this protein, it has no effect on your body. It’s thought that high protein diets can increase the number of proteins in your body and cause more warfarin to be bound to protein. This causes a decrease in warfarin available to prevent clots, so your warfarin dose may need to be increased. Always check with your healthcare professional before starting any special diets, so your PT/INR can be closely monitored. Certain foods can also affect how your liver clears warfarin from your body, causing warfarin levels to increase or decrease.

Examples include alcohol, and possibly cranberry products (e.g., juice, supplements) or grapefruit or grapefruit juice. Too much alcohol, cranberry products, or grapefruit products can potentially increase warfarin’s effect and increase your risk of bleeding. Avoid or limit your intake of alcohol, cranberry products, and grapefruit or grapefruit juice. Make sure your healthcare professional knows if your diet contains any of these products, so your PT/INR can be closely monitored.

Should you avoid multivitamins that contain vitamin K?

Many multivitamin supplements contain small amounts of vitamin K. It’s not necessary to avoid them, just make sure you take them on a regular basis. Always let your healthcare provider know when you start or stop taking a multivitamin which contains vitamin K.

What about vitamin K supplements?

There are other supplements that only contain vitamin K. They may be labeled as vitamin K1(phylloquinone or phytonadione) or vitamin K2 (menaquinone or menatetrenone). These supplements generally contain more vitamin K than regular multivitamins. Some patients with difficult to control INRs may benefit from taking a vitamin K-only supplement. However, you should not take these supplements on your own. Your healthcare provider will tell you if you should be taking vitamin K supplements, and how much. If you use a vitamin K supplement, select a product with vitamin K1. It is the most commonly used form of vitamin K. Be sure to stick with the same brand of vitamin K to avoid differences between products.

Other things to remember when taking warfarin

In addition to foods, many prescription and over-the-counter drugs, including vitamins and herbal supplements, can affect your warfarin level. You should not start, stop, or change doses of any drugs or supplements without first talking with your healthcare professional. Try to keep a healthy, well-balanced diet and keep your vitamin K intake consistent. Take your warfarin dose around the same time each day. If you miss a dose, take it as soon as you remember. If it is almost time for the next dose, skip the missed dose and continue your regular dosing schedule. Don’t double up doses without checking with your healthcare professional. Tell your healthcare professional immediately if you have unusual bleeding or bruising, black or bloody stools, blood in the urine, or stomach pain.

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