Green Tea Interactions

Green tea leaves contain vitamin K, the cofactor the liver needs to make the clotting factors warfarin works against. Large or fluctuating green tea intake can lower the INR and weaken warfarin's anticoagulant effect, as documented in a published case report. Moderate, steady intake is generally not a problem.

Green tea polyphenols, especially the catechin EGCG, bind non-heme iron in the gut and form insoluble complexes that the intestine cannot absorb. The effect is most pronounced when green tea is consumed together with an iron supplement or an iron-rich plant meal, and it can be blunted by spacing the two apart and by pairing iron with a vitamin C source.

Green tea catechins, especially EGCG, partly inhibit the proton-coupled folate transporter (PCFT) in the small intestine, the main carrier for absorbing dietary folate and folic acid. In a controlled human study, taking folic acid together with green tea modestly lowered its peak blood level and total absorption compared with water. The direction is well established but the effect is small, and it is easily managed by separating the two in time.