Is there any side effect for green tea?

Possibly: Green tea has many health benefits. It does have some caffeine so you may have some jitters and fast heart beat if you drink a lot of it. It is best not to drink it at bedtime. Also you need to check with your doctor if you are on medications as it can have interactions with some medications like coumadin (warfarin).

Green tea: Firstly this is healthy. The big issue is staining of the teeth. Also if you just drink tea then there is a risk of dehydration as it is a diuretic. So you must drink other fluids like water etc....