Is there an interaction between green tea and warfarin?

Fråga: Is there an interaction between green tea and warfarin?

Sammanfattning: Green tea contains vitamin K and could therefore antagonize the effect of warfarin. Brewed green tea contains low amounts of vitamin K, but when consumed in large quantities, it could cause clinically significant changes in the INR values.

Svar: Warfarin is a vitamin K-antagonist that inhibits the role of vitamin K in the coagulation procedure. Green tea may contain varying amounts of vitamin K and can thus possibly antagonize the effect of warfarin (1).

There is a large variability in the amount of vitamin K between different tea brands. In one study vitamin K was measured in two different brands of dried green tea leaves. The brands contained 1654 and 482 ug per 100 g. When the brand with the lower amount of vitamin K was brewed it only contained 0.03 ug per 100 ml (2). Another study showed that the dried green tea leaves contained 1428 ug per 100 g. This could be compared with dried black leaves that only contained 262 ug per 100 g (3). The concentration of vitamin K in brewed green tea will depend on the amount of tea leaves used, the infusion time, brand and the quantity consumed per day (4).

There is one case report that describes the use of green tea causing a decrease in the International Normalized Ratio (INR) in a 44-year old male patient treated with 7.5 mg warfarin daily. The patient had been treated with warfarin for 14 months for his mechanical valve replacement with an aim INR of 2.5-3.5. The last measured INR-value was 3.8. Twenty-two days later his INR decreased to 1.4 and one month later to 1.1. The patient had begun drinking 1/2-1 gallon (1 gallon = 3.8 litre) green tea daily about a week before INR 1.4 was measured. He discontinued the green tea and one week later his INR was 2.6 (1).

In this case there is a possible interaction between warfarin and green tea since the patient drank enormous amounts of green tea. Patients treated with warfarin, who choose to drink green tea, should have a consistent consumption over time, use a consistent brand and method of brewing so that the INR value will not fluctuate.