Caffeine is the world's most widely used behaviorally active drug. More than 80 percent of adults in North America consume caffeine regularly. Average per capita caffeine intakes in the United States, Canada, Sweden, and the United Kingdom have been estimated at 211 milligrams, 238 milligrams, 425 milligrams, and 444 milligrams per day, respectively; the world's per capita caffeine consumption is about 70 milligrams per day. These dose levels are well within the range of caffeine doses that can alter human behavior: As little as 32 milligrams of caffeine, less than the amount of caffeine in most 12-ounce cola soft drinks, can improve vigilance performance and reaction time; and doses as low as 10 milligrams, less than the amount of caffeine in some chocolate bars, can alter self-reports of mood. These data suggest that a large number of people are daily consuming behaviorally active doses of caffeine.

Caffeine-containing foods and beverages are ubiquitously available in and widely...