Effects of single oral doses of codeine, diazepam (Valium), and alcohol on simulated driving were investigated by using a modification of the English Sim‐L‐car. The driving time was 40 minutes, subjects were told to adapt speed to surroundings and traffic. Placebo increased the inaccuracy of speed estimations. Alcohol increased the numbers of steering wheel reversals and neglected instructions. Diazepam 10 mg increased the number of collisions and neglected instructions, but the greatest increase in collisions was after codeine 50 mg. Diazepam generally enchanced the effect of alcohol.

1 Psychomotor skills and visual functions related to driving were measured double-blind cross-over in ten healthy volunteers before, and 1, 3, 5 and 7 h after a single oral administration of diazepam (10 mg), medazepam (15 mg) or lorazepam (2.5 mg). The late effects of lorazepam were tested in seven other subjects 12 and 24 h after the administration. 2 Lorazepam impaired almost all the measured skills more (P < 0.05 to 0.001) than diazepam, medazepam or the placebo. The lorazepam impairment of reactive skills and flicker fusion discrimination remained statistically significant (P < 0.05) for as long as 12 h. Medazepam impaired only reactive skills and flicker fusion, the latter remaining impaired (P < 0.05) for as long as 5 h after the administration. The magnitude and duration of the effects of diazepam were intermediate between those of lorazepam and medazepam. Diazepam impaired perceptual speed and reactive and co-ordinative skills as well as flicker fusion discrimination and visual parameters related to driving. Slight impairments in performance were measurable for up to 5 h after administration but at 7 h the results resembled those measured after the placebo. 3 The lack of alterations in adaptation to darkness, sensitivity to brightness or visual discrimination ability in bright counterlight at a time when flicker fusion discrimination was severely depressed suggests that an impaired ability to discriminate flickering light is of no or little clinical significance to driving ability. 4 It is concluded that patients receiving a 2.5 mg dose of lorazepam should not drive or operate machinery for 24 h after the administration. After diazepam (10 mg) or medazepam (15 mg) patients should refrain from driving or participating in skilled performances for only 5 to 7 hours.

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