Caffeine is the most widely used behaviorally active drug in the world. At dietary doses, caffeine poses little if any medical risk to healthy people. Significantly, caffeine produces many of the effects that define drugs of abuse: It can serve as a discriminative stimulus. Under some circumstances, it can be a positive reinforcer. Repeated exposure to caffeine produces tolerance to its cardiovascular and behavioral effects. Finally, chronic caffeine administration can produce physical dependence, as evidenced by a withdrawal syndrome that emerges when daily caffeine consumption is abruptly terminated.
Although caffeine can have all of these effects, its effects are often subtle and difficult to demonstrate experimentally. As a result, more sensitive procedures have been developed to capture caffeine's effects. Nowhere is the sensitivity of this emerging methodology better illustrated than in the search for the lowest caffeine doses that affect human behavior.
Although caffeine is consumed regularly in coffee, tea, soft drinks, and chocolate, there has been little evidence that the amount of caffeine found in individual portions of these sources can affect human behavior.
The difficulty of detecting behavioral effects of caffeine has led observers to conclude that "a coherent account of the salient selective effects of caffeine on behavior cannot be given, that "the behavioral effects of caffeine eat levels causing selective changes are modest and even subtle", and that caffeine "bestows little if any benefit on psychomotor and cognitive performance". (Evans, S. M., & Griffiths, R. R. 1992).
Only rarely have doses below 100 mg altered self-reports of mood or performance. An impressive pair of studies found that caffeine enhanced auditory vigilance and reaction time at doses as low as 32 mg and affected self-reports of mood at doses as low as 64 mg. In the first of this pair of studies, four doses of caffeine and a placebo were administered in capsules, in counterbalanced order and under double-blind conditions, to each of 20 participants....
