Drinking too much water can cause a fatal electrolyte imbalance in your blood. Breathing too pure of oxygen will seriously, perhaps fatally, damage your lungs. Many metals are simultaneously important in trace amounts and debilitating or fatal in larger (albeit still small) quantities.

That's just for things that you are supposed to do, only too much of. Graphene is not useful or an important part of your body's functions; doing things you aren't even supposed to be doing with it (breathing it in, for example) can only have a negative effect.

Speaking of pure carbon, carbon nanotubes are known to cause lung tumors.