If you have stomach problems and drink a lot of green tea on your stomach, the fruit acids found in green tea can irritate the mucous membranes. If you have no stomach problems and you are not drinking green tea on an empty stomach, then it has many health benefits.
I recommend an article about the beneficial properties of different types of tea and precautions in drinking tea " Tea Longevity "
Marina wrote everything right. Green tea is good for your health and is a rich source of antioxidants. For him to harm the body, you need to drink a lot of tea, this will create a load on the mail. It is unlikely that you will drink more than 3 liters of tea per day.
Please note, we tend to drink tea when we are thirsty. It is not right. Tea, as it were, is a dessert in our diet. When we are thirsty, the body needs water. It is water that should be drunk during the day, and tea should be consumed several times a day during rest.
For green tea, it is important to follow the instructions for preparation. Useful substances are destroyed if they are poured with boiling water. It is customary to brew tea with water close to 90 degrees, but in practice the temperature is lower.
If you want to know more about tea, I recommend the video blog of Grigory Potemkin (realchinatea).
On my own I can recommend trying milk oolong. It is not green tea (semi-fermented), but it has many health benefits and a pleasant aromatic taste.
In the quantities in which people drink it - definitely not. It is a good emulsifier of fats and a source of tannin, which provides some protection against stomatitis and improves digestion. But everything is good in moderation. Potential negative effects include the aggravation of diseases such as GERD, hypertension, peptic ulcer disease (due to caffeine, which, by the way, is less than in black tea and coffee). We can say that sugar in this tea is more harmful than the tea itself.