To ScottL or any other practicing doctors, heads up matey.
Green tea may inhibit warfarin.
Cheng TO.
Department of Medicine, The George Washington University Medical Center, 2150 Pennsylvania Avenue, N.W., Washington, DC 20037, USA.
PMID: 16757048 [PubMed - as supplied by publisher]
from the Letter to editior
Of the numerous positive responses to my recent review article on the cardiovascular effects of Chinese green tea [1], one seems to be most interesting and has important clinical implications. While praising me for the completeness of my article, the responder wrote "Unfortunately, although I like it, I can't drink it, I'm taking warfarin". The purpose of this Letter to the Editor is to address this specific question.
Green tea may contain significant quantities of vitamin K [2], [3] and [4]. This exogenous administration of vitamin K may antagonize the anticoagulant effect of warfarin, leading to a decrease in the International Normalized Ratio (INR). Although brewed green tea contains low amounts of vitamin K (0.03 ug of vitamin K per 100 gm of brewed tea [5]), the dry green leaves contain 1428 ug of vitamin K per 100 gm of leaves compared with only 262 ug of vitamin K per 100 gm of dry black tea leaves [4] and [5]. The actual concentration of vitamin K in the final product will, of course, depend on the dilution and amount of tea leaves used to brew the tea [2]. Also the quantity of tea consumed per day will obviously play a major role in determining the actual amount of exogenous vitamin K consumption.
Because warfarin usage is so common in everyday cardiologic practice [6] and because tea is one of the most widely consumed beverages in the world [7], next to only water [1], the possible inhibitory property of green tea on warfarin should be kept in mind. When unexpected decrease of the INR is encountered in any patient on warfarin, who has otherwise been under good anticoagulant control, the physician should inquire if the patient has been drinking green tea and how much a day.
References
[1] T.O. Cheng, All teas are not created equal. The Chinese green tea and cardiovascular health, Int J Cardiol 108 (2006), pp. 301-308.
[2] J.R. Taylor and V.M. Wilt, Probable antagonism of warfarin by green tea, Ann Pharmacother 33 (1999), pp. 426-428.
[3] 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.
[4] S.L. Booth, J.A. Sadowski, J.L. Weihrauch and G. Ferland, Vitamin K1 (phylloquinone) content of foods: a provisional table, J Food Comp Anal 6 (1993), pp. 109-120.
[5] S.L. Booth, H.T. Madabushi, K.W. Davidson and J.A. Sadowski, Tea and coffee brews are not significant dietary sources of vitamin K1 (phylloquinone), J Am Diet Assoc 95 (1995), pp. 82-83.
[6] T.O. Cheng, Danshen: a popular Chinese cardiac herbal drug, J Am Coll Cardiol 47 (2006), p. 1498.
[7] R. Cooper, D.J. Morre and D.M. Morre, Medicinal benefits of green tea: Part I. Review of noncancer health benefits, J Altern Complement Med 11 (2005), pp. 521-528.
