As usual, too much of a good thing can be very, very bad. A
  moderate portion of caffeine simply increases the
  circulation and is harmless as long as you have no medical
  conditions, such as an allergy, that can react adversely.

  However, large doses can be lethal. The average human can
  be killed by ten grams of caffeine. But before you swear
  off coffee and colas, keep in mind that you would have to
  drink 100 cups of coffee in four hours to get your ten
  grams. And if you're drinking that much coffee, it seems
  likely that a busted bladder could kill you before the
  caffeine gets a chance.