Since you have a history of blood clots, I assume that you are taking Coumadin (warfarin) or another blood-thinning (anticoagulant) drug. If so, you do have to be aware of your intake of vitamin K, an essential nutrient necessary for blood clotting that occurs in many vegetables. Vitamin K antagonizes the action of Coumadin. People taking anticoagulants should maintain a consistent intake of vitamin K so that their drug dosage does not have to be adjusted and readjusted. For instance, if you eat lots of vegetables, make sure that the total amount you eat each day is relatively constant and be sure to discuss any major change in your diet with your physician.
It is true that dried green tea leaves have a very high concentration of vitamin K, but a cup of brewed green tea provides only a small amount, 0.03 mcg per 3.5 fluid ounces (roughly half a cup). In fact, research published in the Journal of the American Dietetic Association in 1995 showed that tea is not a significant dietary source of vitamin K and, as you know, green tea provides lots of other health benefits. The only beverages you should avoid while on blood-thinners are alcoholic drinks of all types since alcohol can affect your response to these drugs.
It is important, however, be aware of foods that are particularly high in vitamin K if you are taking prescription anticoagulants. Vitamin K is abundant in leafy greens - a three and a half ounce serving of Swiss chard or kale provides between 800 and 830 mcg; parsley, 500-540 mcg; Brussels sprouts and spinach 400-440 mcg. Other fruits and vegetables including tomatoes, lima beans, apricots, potatoes, grapes, cauliflower, peaches, plums and squash contain only minimal amounts of vitamin K. Bottom line: it's more important to stay consistent with your intake of fruits, veggies and beverages containing vitamin K than to worry about avoiding any particular ones.
Andrew Weil, M.D.