Even for the most caffeine-dependent person, there are still side effects that set in after too much -- usually more than 300mg (about three cups of coffee). Signs you've gone overboard are the usual suspects, like nervousness, jitteriness, and heart palpitations.
But to reach the fatal levels of caffeine intoxication, someone would have to take in 10g, or 10,000mg. That's when the really scary stuff starts to set in.
"Your blood pressure, your heart rate, your temperature all go up really, really high," says Dr. Adam Splaver, clinical cardiologist and co-founder of NanoHealthAssociates. "You can go into cardiogenic arrest, and shock, and die. Some people can even have a heart attack." All from a totally legal drug.