Placebo-effects contribute to differences in the acquisition of drug discrimination by humans: a retrospective analysis.

Adult human volunteers (n = 50) were trained to discriminate triazolam (TRZ, 0.32mg/70kg, p.o.) from placebo. Based on a criterion that required greater than 80% capsule-appropriate responding during each of four test sessions, 19 subjects were designated non-discriminators (NDs) and 31 were designated discriminators (Ds). NDs and Ds did not differ significantly in age, weight, gender or previous drug use and generally reported similar effects following TRZ. NDs reported greater effects following placebo than Ds on several measures, including 'good', 'bad', 'high' and sedative drug effects, suggesting that NDs in this study were 'placebo reactors'. These results show that NDs and Ds of TRZ differed in self-reported responses and suggest a close relationship between acquisition of a drug discrimination and self-reported effects of drugs. Moreover, greater placebo effects may hinder acquisition of TRZ discrimination.