Dose-related caffeine discrimination in normal volunteers: individual differences in subjective effects and self-reported cues.

A within subject design was used to study a caffeine versus placebo drug discrimination in five volunteers who were not explicitly instructed that the drug conditions involved caffeine and placebo. The caffeine (200 or 300mg) versus placebo discrimination was acquired by all subjects and remained stable (78-90% accuracy) throughout the study which spanned 5 to 8.7 months across the subjects. A full caffeine dose-response function (50 to 400 or 600mg) was determined repeatedly under test conditions in each subject; caffeine produced orderly dose-related increases in caffeine identification in all subjects. The present study evaluated individual subject data to examine the correspondence between the subjective effects of caffeine versus placebo and the cues subjects reported as being important to making the discrimination. Although the subjective effects and self-reported cues differed across subjects, there was a correspondence within subjects. Prominent self-reported cues for caffeine included jittery/nervous/anxious (four subjects) and alert/active (one subject); self-reported cues for placebo included tired and/or headache (three subjects) and absence of drug effect (two subjects). The reporting of tired and headache as cues for the placebo condition suggests that caffeine withdrawal may produce stimulus effects relevant to the caffeine versus placebo discrimination.