Caffeine is known to overcome G2/M (and possibly other types of) cell cycle arrest through its effects on DNA repair machinery and is used as a positive control in studies of cell cycle arrest. In this paper, for example, researchers use concentrations of caffeine in the 1-4 mM range to overcome nocodazole-mediated arrest. Is this a physiologically relevant concentration?
I found a lot of interesting caffeine data including the LD50 (estimated to be around 150-200 mg/kg) but have not been able to turn up any good numbers on reasonable concentrations that might be found in a human.