Driving by frequent cannabis users ‘the morning after’ last use of smoked cannabis: an observational driving simulator study

Abstract Background It is well-established that cannabis can affect driving in the hours after cannabis use, but the exact duration of these effects, and relationship with delta-9-tetrahydrocannabinol (THC) concentrations in blood and oral fluid, remains to be determined. Methods Frequent (≥ 4 times a week) users of smoked cannabis drove a simulator the morning after (12-15 hours) last use of smoked cannabis; a control group of non-cannabis users matched for age and sex was also included. Concentrations of THC, cannabidiol (CBD) and metabolites were measured in oral fluid and blood at the time of the drive. Results In total, 65 participants (mean age 30 years; 33 males) in each group completed all study procedures. Participants were generally well-matched (age, sex, driving experience, amount of driving per year/week, hours of sleep) but differed in racial breakdown and years of education. Under both standard and dual task (distacted) conditions, standard deviation of lateral position (SDLP) was higher in the control group (standard: 0.305 meters; dual task: 0.272 meters; n=65) compared to the cannabis group (standard: 0.28 meters; dual task: 0.256 meters; n=65); these differences were small (Cohen’s d -0.389 (standard) and -0.359 (dual task)) and were not significant after correction for multiple comparisons. Measures of speed and following distance were not impacted. Neither blood nor oral fluid THC, CBD or metabolites was significantly correlated with any measure of driving after correction for multiple comparisons; mean concentrations of blood THC was above 2 ng/mL. After correction for multiple comparisons, trends between driving and concentrations of the psychoactive metabolite 11-hydroxy-THC (11-OH-THC) were found. Participants who smoked cannabis the night before reported higher levels of subjective intoxication, and more willingness to drive before the drive, that was not significant after correction for multiple comparisons. Conclusions The regular cannabis use group showed no significant impairment in driving performance 12-15 hours after last cannabis use the night before, compared to the control group. Blood and oral fluid THC concentrations may not be an accurate correlate of driving behavior. Large-scale studies are needed to determine whether less frequent users are impaired the morning after last use, and whether the present findings also extend to different routes of administration.