Actions and interactions of diazepam and alcohol on psychomotor skills in young and middle-aged subjects.

Healthy pretrained men participated in three separate controlled double-blind and cross-over trials. There were 10 students in Trial I, 11 army officials or policemen aged 38-59 years in Trial II, and 11 students in Trial III. After single oral doses of diazepam 10 mg (D), placebo, alcohol 0.5 g/kg (A) or D+A, given at one-week intervals in balanced order, the psychomotor skills (choice reaction, tracking, attention, flicker fusion) were repetitively measured up to 2.5 hrs. Serum D concentrations were measured by gas chromatography. D alone impaired flicker fusion in trials I and II, and also reactive and coordinative skills in trial III. A alone slightly impaired skills and failed to potentiate D effects. Increasing age rendered the performance worse but this was due to increased base-line errors rather than to enhanced responses to D. Serum D concentrations tended to rise in the presence of A suggesting an inhibition of D demethylation by alcohol. Both total and free concentrations of D were similar in both age groups. The results suggest that in spite of unaltered sensitivity to D, middle-aged subjects have a lowered margin of safety when driving or using machinery under D treatment.