Rationale: Caffeine produces mild psychostimulant effects that are thought to underlie its widespread use. However, the direct effects of caffeine are difficult to evaluate in regular users of caffeine because of tolerance and withdrawal. Indeed, some researchers hypothesize that the psychostimulant effects of caffeine are due largely to the reversal of withdrawal and question whether there are direct effects of caffeine consumption upon mood, alertness, or mental performance in nondependent individuals.
Objective: This study investigated the physiological, subjective, and behavioral effects of 0, 50, 150, and 450 mg caffeine in 102 light, nondependent caffeine users.
Methods: Using a within-subjects design, subjects participated in four experimental sessions, in which they received each of the four drug conditions in random order under double blind conditions. Participants completed subjective effects questionnaires and vital signs were measured before and at repeated time points after drug administration. Forty minutes after the capsules were ingested, subjects completed behavioral tasks that included tests of sustained attention, short-term memory, psychomotor performance, and behavioral inhibition.
Results: Caffeine significantly increased blood pressure, and produced feelings of arousal, positive mood, and high. Caffeine increased the number of hits and decreased reaction times in a vigilance task, but impaired performance on a memory task.
Conclusion: We confirm that acute doses of caffeine, at levels typically found in a cup of coffee, produce stimulant-like subjective effects and enhance performance in light, nondependent caffeine users. These findings support the idea that the drug has psychoactive effects even in the absence of withdrawal.