Ya well you can OD on green tea... go dizzy, get high n get nervous tremors.
But that only happens when you act like an idiot, like I did, and brew 30gms in 500ml and drink the concentrate undiluted.
If you brew it yourself, take care to brew it for 3/4 mins max, drink it warm with a few drops lemon juice mixed in each cup. And always remember, if its bitter, its either too concentrated or you've brewed it for too long, n the tea has got too much tannins. DON'T drink this very bitter tea. The tannins are carcinogenic in the long run.
Preferably brew a concentrate, and store it at room temperature. Heat up some water and add some concentrate to it when you wanna drink it. Drink no more than 1litre a day, add no sugar to it, and you won't have any problems.
Best of luck!