Does Green Tea Affect Coumadin?

According to the National Cancer Institute, green tea is the most popular tea in Japan and China 14. While black tea is the most common tea beverage in the United States, green tea is also available here 4. For most people, green tea is simply a beverage, but for people who take the blood thinner Coumadin, green tea has the potential to cause problems 4.

About Green Tea

Green tea is made from unwilted leaves of the Camellia sinensis plant 4. Unlike black and oolong tea, it does not undergo fermentation. The tea is brewed from the dried leaves or buds, which are available in tea bags or loose form. It may also be available in dried instant form, or as a green tea extract supplement 4. Although it has not been evaluated for effectiveness by the FDA, notes that green tea may be used for stomach disorders, vomiting, diarrhea, to prevent dental cavities, to lower cholesterol levels, as an antioxidant, to reduce cancer, and as a stimulant 4. Green tea contains a variety of substances, one of which is vitamin K. In one study from Portugal, the concentration of vitamin K1 in tea leaves was between 120 and 625 μg/100 g. It is the vitamin K that can cause problems with warfarin, also known by the brand name Coumadin 4.

About Coumadin

According to the National Institutes of Health Drug Nutrient Task Force, warfarin is a medication used to help prevent blood clots. The drug works by decreasing the activity of vitamin K, which is essential for the clotting process, and increases the length of time needed for a clot to form. In order to know if the Coumadin is effective, the patient’s blood is drawn at regular intervals for a laboratory test called the INR. The dose of Coumadin is then adjusted as necessary to keep the INR in the desired range. Because vitamin K is a key factor in the process of blood clotting, eating or drinking foods with a high vitamin K level can decrease the INR and have an impact on the dose of Coumadin needed.

Green Tea and Coumadin and MedlinePlus both warn that people who take Coumadin should not drink green tea or take green tea supplements 4. In one case report from the April 1999 “Annals of Pharmacology,” a 44-year-old man who had begun drinking 1/2 to 1 gallon of green tea each day was found to have an INR of 1.37 one week later 4. Prior to that time, his INR had been above 3. Once the green tea was discontinued, the patient’s INR came back up to 2.55 4.


If you are taking Coumadin, you should not drink green tea or take green tea supplements without discussing your condition with a health care professional 4.