Twenty-two days later his INR was 1.37. One month later the patient's INR was 1.14. It was subsequently discovered that the patient began drinking one-half to one gallon of green tea per day about one week prior to the INR of 1.37. On discontinuation of the green tea, the patient's INR increased to 2.55. The exogenous administration of vitamin K inhibits the effect of warfarin and reduces a patient's degree of anticoagulation.
Green tea can be a significant source of vitamin K and thus antagonize the effect of warfarin.
Warfarin is a highly effective oral anticoagulant, but it requires close monitoring to prevent complications. Patients receiving warfarin need to be routinely questioned about their intake of vitamin K-containing foods and beverages.