Green tea is safe for most adults when used in moderate amounts. But :
1). There have been some case reports of liver problems in people taking concentrated green tea extracts. The problems do not seem to be connected with green tea infusions or beverages. Although these cases are very rare and the evidence is not definitive, experts suggest that concentrated green tea extracts be taken with food, and that people discontinue use and consult a health care practitioner if they have a liver disorder or develop symptoms of liver trouble, such as abdominal pain, dark urine, or jaundice.
2). Green tea and green tea extracts contain caffeine. Caffeine can cause insomnia, anxiety, irritability, upset stomach, nausea, diarrhea, or frequent urination in some people.
3). Green tea contains small amounts of vitamin K, which can make anticoagulant drugs, such as warfarin, less effective.
courtesy : http://nccam.nih.gov/health/greentea