Green Tea and Warfarin
With a bunch of varieties available, tea is becoming the most frequently consumed beverage after water. The average American drinks 155 cups of tea a year and green tea extracts are rapidly growing as a dietary supplement.1 According to the National Institutes of Health, green tea, black tea, and oolong tea are all made from lightly steaming and drying leaves from the Camellia sinensis plant. Drinking tea originated from a Chinese legend where leaves from a nearby plant landed in a Chinese emperor’s pot of boiling water.2
Based on scientific evidence reviewed by the Natural Medicines Comprehensive Database green tea provides effective health benefits. Effective uses of green tea include preventing certain cancers and reducing blood pressure and increasing mental alertness. 3,4,5
Although results from epidemiologic and clinical studies are mixed, green tea polyphenols may play a beneficial role in weight management and cardiovascular disease.6, 7 The FDA also recently approved an ointment containing green tea used for treating highly contagious sexually transmitted diseases.
Interactions with Warfarin?
There is little to no evidence of an interaction between green tea and warfarin. Green tea does contain caffeine as well as moderate quantities of vitamin K; which may lead to a decrease in INR (International Normalized Ratio). But, brewed green tea contains low amounts of vitamin K, approximately 0.03 micrograms (ug) of vitamin K per 100 gm of brewed tea.8 The actual concentration of vitamin K in brewed tea will depend on the amount of leaves brewed and the dilution of the tea leaves.9
If you plan on drinking green tea for any of the above benefits, consuming a consistent amount of vitamin K per day will help your INR to remain in range. As always, talk to your doctor about any changes to your diet, medication or supplements.
- Schardt D. Does tea ward off disease? Nutr Action Healthletter. 2007; 34(2):9-11.
- Patel SH. Camellia sinensis: historical perspectives and future prospects. J Agromedicine. 2005;10(2):57-64.
- Sun CL, Yuan JM, Koh WP, Yu MC. Green tea, black tea and breast cancer risk: a meta-analysis of epidemiological studies. Carcinogenesis. 2006;27(7):1310-1315.
- Sun CL, Yuan JM, Koh WP, Yu MC. Green tea, black tea and colorectal cancer risk: a meta-analysis of epidemiologic studies. Carcinogenesis. 2006;27(7):1301-1309.
- Kuriyama S, Shimazu T, Ohmori K, et al. Green tea consumption and mortality due to cardiovascular disease, cancer, and all causes in Japan: the Ohsaki study. JAMA. 2006;296(10):1255-1265.
- Cabrera C, Artacho R, Giménez R. Beneficial effects of green tea— a review. J Am Coll Nutr. 2006;25(2):79-99.
- T.O. Cheng, All teas are not created equal. The Chinese green tea and cardiovascular health, Int J Cardiol 108 (2006), pp. 301-308.
- T.O. Cheng, Danshen: a popular Chinese cardiac herbal drug, J Am Coll Cardiol 47 (2006), p. 1498.
- S.L. Booth, J.A. Sadowski and J.A.T. Pennington, Phylloquinone (vitamin K1) content of foods in the US Food and Drug Administration's Total Diet Study, J Agric Food Chem 43 (1995), pp. 1574-1579.