The subjective effects of caffeine, the most widely used psychoactive drug, vary widely from person to person. The factors responsible for these individual differences remain largely unknown. Data from 36 healthy adults who were light to moderate users of caffeine were examined with the goals of characterizing the subjective effects of caffeine and identifying factors which might correlate with or predict these effects. Subjective effects of 100 and 300mg caffeine were measured with standardized questionnaires. The low dose of caffeine was found to produce negligible subjective effects in the group as a whole, while the high dose produced mild stimulant and anxiogenic effects. As expected, there was considerable intersubject variability in the subjective effects of caffeine. There was no correlation between stimulant and anxiogenic effects after the high dose. Light caffeine users were more accurate than heavy caffeine users in identifying the high dose of caffeine as an active drug, but there was no evidence that light users were generally more sensitive to the subjective effects of caffeine. Current alcohol use, prior recreational use of stimulants, and baseline level of self-reported arousal appeared to influence subjective response to caffeine. Other variables (personality, gender, baseline anxiety, and prior use of other drugs) showed no relationship with mood response to caffeine.
L. D. Chait (Mon,) studied this question.