Drug preference in normal volunteers: Effects of age and time of day

These experiments assessed the influence of two variables, age of subjects and time of drug administration, on the reinforcing properties of amphetamine and of diazepam in normal volunteers. Three groups of subjects were tested: i) a group of 40–55-year-old subjects (AGE group; N=11) who were tested in the morning, ii) a group of 21–35-year-old subjects (CTL group; N=12) who were also tested in the morning, and iii) a group of 21–35-year-olds who were tested in the late afternoon (AFT group; N=13). All subjects participated in three separate experiments comparing one drug (5 mg d,l-amphetamine, 5 mg diazepam or 10 mg diazepam) to placebo. Each experiment consisted of nine sessions: On the first four sessions subjects sampled two color-coded capsules on alternate sessions and on the following five sessions they chose and ingested the capsule they preferred. Subjective effects of the drugs were monitored using the Profile of Mood States (POMS) and a shortened version of the Addiction Research Center Inventory (ARCI). Subjects in all three groups chose 5 mg diazepam as often as placebo but preferred placebo to 10 mg diazepam. In contrast, they chose amphetamine either as often as or more often than placebo. The subjective effects of diazepam (i.e. sedation) were similar across all three groups, but after amphetamine the AGE group showed greater stimulant effects. In addition, the AFT group showed fewer positive mood effects after amphetamine than the CTL group. The AFT and AGE groups also differed from the CTL group on several measures of mood, independently of drug administration. Despite the modest group differences in subjective effects, it was concluded that neither time of day nor age of subjects substantially affected the reinforcing properties of either drug.