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Why Is Taking Coumadin & Cranberry Juice or Tablets Harmful Together?

author image Deborah Lundin
Deborah Lundin is a professional writer with more than 20 years of experience in the medical field and as a small business owner. She studied medical science and sociology at Northern Illinois University. Her passions and interests include fitness, health, healthy eating, children and pets.
Why Is Taking Coumadin & Cranberry Juice or Tablets Harmful Together?
Cranberries act as a natural blood thinner. Photo Credit 5PH/iStock/Getty Images

As with many prescription drugs, if you take Coumadin, the brand name for warfarin, there are certain foods that may cause undesirable interactions with the medication. Coumadin thins your blood to reduce the chance of blood clots. When you take Coumadin, you should avoid foods like spinach and kale with high levels of vitamin K. This vitamin thickens the blood and can make the Coumadin less effective. The opposite is true in the case of cranberries. Cranberry works to thin the blood, and drinking cranberry juice or taking cranberry supplements when you take Coumadin can increase your bleeding risk.


Coumadin is an anticoagulant, according to Bristol-Myers Squibb. Its purpose is to thin your blood and reduce the risk of blood clots. Thinning of the blood puts you at higher risk for bleeding, and you must be careful to avoid injury. Even the smallest bump can cause a bruise in those taking Coumadin. When your physician prescribes Coumadin, take it exactly as prescribed. You will need your International Normalized Ratio, or INR, checked on a regular basis through a blood test called a prothrombin time, or PT. This measures the clotting tendency of your blood and helps your physician determine the correct dosage of Coumadin.

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Cranberry Juice

According to the Institutes of Health, cranberries have long been used for medicinal purposes, beginning with Native Americans. Cranberry juice is still taken to prevent and treat urinary tract infections. Cranberries contain a significant amount of natural salicylic acid, which is a main ingredient in aspirin. Salicylic acid has a natural ability to thin the blood and reduce the risk of blood clots.


In patients taking Coumadin for blood thinning, the optimal range of the INR is between 2.0 and 3.0. Some patients at high risk for clotting may go up to a 2.5 to 3.5 INR range. This range must be monitored to make sure you are not at too high a risk for bleeding. Too much cranberry juice can increase your bleeding risk. Two recent studies have shown a connection between the ingestion of cranberry juice and Coumadin and an increased INR. A study published in “Clinical Therapeutics” showed that a 78-year-old man taking warfarin who drank cranberry juice had an INR of 6.45. A study published in “The Annals of Pharmacotherapy” looked at a 46-year-old female on warfarin who had a normal INR of 2.0. After drinking approximately 1.5 quarts of cranberry juice for two days, her INR increased to 4.6. After repeating the experiment a few months later and drinking the cranberry juice for three to four days, her INR had increased to 6.5.


If you are at risk for blood clots or heart attack, your physician may prescribe Coumadin. Talk to her about possible food and vitamin interactions with the drug. Follow the prescription exactly as it is written, and have your PT test done as scheduled. Limit your intake of cranberry juice and cranberries to make sure your INR level stays within range and you are not at increased risk for bleeding.

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