Onset of peak impairment after diazepam and after alcohol.

Single doses of diazepam induced peak impairment of performance on cognitive and psychomotor tasks early (20 min after ingestion), when blood levels had reached less than two thirds of their eventual plateau. Alcohol did not have this effect. Early peak impairment and the acute tolerance that follows it contribute greatly to the lack of correlation between diazepam plasma level and performance impairment.