The expected drug and its expected effect interact to determine placebo responses to alcohol and caffeine

This study tested placebo responses in psychomotor performance when caffeine or alcohol was expected. Fifty male university students were assigned to one of four placebo groups or to a no-treatment control group. Two groups received placebo caffeine and two received placebo alcohol. Subjects performed 12 trials on a pursuit rotor task and performance was measured by the percent time on target. Then they received information about the expected drug effect on the task. One caffeine placebo group (C+) and one alcohol placebo group (A+) were led to expect enhanced performance on the task. The other caffeine placebo group (C-) and alcohol placebo group (A-) were led to expect impaired performance. Subjects subsequently performed 12 trials on the task. An interaction was obtained between the expected type of effect and the expected type of drug. The C+ group displayed superior performance compared to the C- group, and the reverse relationship was observed between the A+ and A- group. In addition, subjects led to expect alcohol-induced impairment (A-) performed better than subjects led to expect caffeine-induced impairment (C-). Subjects also reported greater motivation to resist impairment when they expected alcohol rather than caffeine. The research indicates that understanding and predicting placebo responses may require consideration of the drug that is expected as well as its expected effect.