"As with all drugs, there is such a thing as too much caffeine."

"According to a 2001 Institute of Medicine report, 600 mg of caffeine (or six cups of coffee) will bring on negative cognitive effects, otherwise known as the jitters, in most people — including Kramer from Seinfeld. And some people are so sensitive to caffeine that one cup will bring on nervousness and irritability, rather than the alertness that most of us feel. 'We also know that caffeine is bad for people with anxiety — for them, it's likely to hurt productivity... But for people on the more depressive end, caffeine would improve productivity. The effect of the drug really depends on the brain into which it's being infused.'"