Green Tea can cause allergic reactions.

Green tea contains caffeine which has been shown to increase anxiety and impair sleep. Caffeine can interfere with many prescription medications causing adverse reactions or dangerous interactions.

Results suggest that tea consumption inhibits the utilization of thiamin.
– Plant Foods Hum Nutr 1991 Oct;41(4):337-53 — Niacin, thiamin, iron and protein status of humans as affected by the consumption of tea (Camellia sinensis) infusions. — Wang RS, Kies C.

Findings confirm that alcohol consumption is associated a greater number of errors and provide some evidence for task-specific antagonism of alcohol’s cognitive effects by caffeine.
– Hum Psychopharmacol 2002 Apr;17(3):151-6 — Interactions between alcohol and caffeine in relation to psychomotor speed and accuracy. — Mackay M, Tiplady B, Scholey AB.

Green Tea has been associated with documented reports of potential interactions with warfarin.
– Am J Health Syst Pharm 2000 Jul 1;57(13):1221-7; quiz 1228-30 —
Potential interactions between alternative therapies and warfarin. — Heck AM, DeWitt BA, Lukes AL.

Overconsumption may cause irritability, insomnia, nervousness, and tachycardia. Because studies on its possible teratogenic effect are inconclusive, caffeine consumption is contraindicated during pregnancy. Lactating women should also limit caffeine intake to avoid sleep disorders in infants.

– DerMarderosian A. The Review of Natural Products. St. Louis, MO: Facts and Comparisons, Wolters Kluwer Co. 1999.