Any kind of chemical or material that is ingested in large amounts has potential risks. Green tea in moderate amounts has no known toxicity. It is also a weak antibacterial. Tea comes from camellia sinensis, an evergreen with white blossoms that thrives in tropical and semi-tropical climates. No area of the United States supports its growth.
Green tea is the least processed, youngest, and freshest of teas.
Other teas are fermented. To make green tea, the leaves are steamed and then rolled and dried. Other teas are processed in a more heavy handed way that promotes oxidation of enzymes in the leaf, a process called fermentation. The leaves blacken as they oxidate. According to Fujiki, it takes about 10 cups of green tea a day to get its potentially protective benefits.