Will drinking a cup of green tea be of benefit or harmful to a person who is on blood thinner?

Green tea contains Vitamin K which can cause the Coumadin to work less effectively. You need to discuss starting Green tea therapy with the Dr who prescribes your Coumadin so he/she can schedule an INR soon after you begin and he/she may want to have them done a little more often for a while, to see how the green tea affects you. You do not have to avoid Vitamin K containing foods while on Coumadin but you must consume a consistent amount so your Coumadin can be adjusted around it. If you plan on trying a small cup daily (I wouldnt advise more than that) you need to continue on having a cup a day. Starting and stopping the foods that affect Coumadin can have drastic effects on your INR. Depending on the food, it can either cause excessive bleeding or increase your risk for a blood clot.

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You should always be aware of signs and symptoms of bleeding: bloody or tarry looking stools, nosebleeds, bleeding gums, large unexplained bruises(when you are on Coumadin and you bump into something, you will likely bruise, but if you are finding large bruises and you know you have not bumped the area, this indicates a problem) Green tea does contain a pretty good amount of caffeine so if you are caffeine sensitive you might not like the effects. Hope this helps some.