"In general, more than 1.5 grams of caffeine a day can cause the typical symptoms of caffeinism: anxiety, insomnia, IRRITABILITY and palpitations." - Richard A. Friedman, M.D. in the NY Times That's about 6 cups of coffee or 2 venti Starbucks. In my family we say that someone who is complaining, or being irritable, or just being a pain in the butt is "yelling their head off." So if I'm ever yelling my head off, you'll probably know why.