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Warfarin Institute of America

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GREEN TEA INTERACTIONS WITH WARFARIN

Green tea is used by many people because it is thought it may help protect against cancer. (1) However, green tea contains vitamin K. When brewed, the concentration of vitamin K is low, but in large quantities this may still be enough to cause a decrease in the INR of a patient taking warafrin. There is one case report available in the literature

Case report:(^1,2)

44 year-old male with hypertension, Marfan’s syndrome, and St. Jude mechanical heart valve in the aortic position, taking 7.5 mg warfarin daily for 14 months with fluctuating INRs. His INR dropped from 3.79 to 1.37 in 22 days, and further to 1.14 in another month. The patient stopped drinking green tea and the INR recovered to 2.55 in one week. There is a strong temporal relationship of an interaction here between green tea and warfarin.

In some cultures, the green tea leaves are eaten after the liquid has been consumed. This would cause a much higher intake of vitamin K.

References:

1: Taylor JR, Wilt VM. Probable antagonism of warfarin by green tea. Annals of Pharmacotherapy 1999; 33:426-8.

2: http://www.micromedex.com Complementary and Alternative Medicine AltMedDex, as seen on March 1, 2001.

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Last updated November 13, 2004