Green and black teas are made of the same leaves, but through different processes. The result is two distinct products. Although generally safe, green tea presents a problem if you have a condition in which blood coagulates in your veins and prevents flow. The leaves or their extract may interact with medications that counteract the problem, creating an unhealthy effect. Check with your doctor regarding interactions before mixing blood thinners and green tea.
About Blood Thinners
Most blood thinners are either anticoagulants or antiplatelet drugs. Your doctor prescribes them when you are at risk for a heart attack or stroke. Anticoagulants interfere with the chemical reaction that forms blood clots. As a result, it takes longer than normal for blood to coagulate. Warfarin and heparin are two anticoagulants. Antiplatelet drugs such as aspirin keep platelets from aggregating and forming clots. Platelets are cell fragments without a nucleus that are present in blood. They function as clotting agents.
Green Tea and Anticoagulants
Green tea contains vitamin K, a nutrient vital for blood coagulation--you may bleed to death from a cut without it. This healing property makes green tea a bad companion for anticoagulants. As the drug works to prevent coagulation, the vitamin K in the tea encourages clotting, neutralizing the medication's effect.
Green Tea and Antiplatelets
Interestingly, green tea also has a component that prevents clotting, says the University of Maryland Medical Center. When you drink the beverage concurrently with taking aspirin or another antiplatelet medication, you intensify anti-clotting activity. The heightened effect can cause hemorrhaging.
Despite green tea's potential adverse interaction with blood thinners, you may not have to give it up completely. Sheldon G. Sheps, M.D., a hypertension specialist, writes on MayoClinic.com that you do not need to avoid vitamin K while on anticoagulants. You just have to limit your daily intake of the nutrient to the micrograms generally recommended. Dr. Sheps does not address whether the same is true if you are taking antiplatelet drugs. Your best alternative is to raise these issues with your doctor. Get a recommendation for how much green tea she believes is safe for you. The recommended daily intake for vitamin K for an adult man is 120 mcg. A grown woman needs 90 mcg of the nutrient, says Dr. Sheps.