Caffeine is arguably the most robust and popular self-administered drug known to man, with almost 90 percent of North Americans consuming some form of it regularly. Most people aren’t harmed by moderate doses of caffeine-say 200 to 300 milligrams, or about two to three cups of brewed coffee a day. But it should be consumed in the morning hours to give the drug time to clear. Remember, with a half-life of six hours, caffeine lingers in your body all day–and into the night if you’ve had an afternoon pick-me-up.