A study published in Traffic Injury Prevention indicated that daily cannabis consumers exhibit tolerance to the acute effects of cannabis on psychomotor function. When the researchers observed changes in driving performance, the effect size was minimal.
University of Colorado Anschutz researchers compared the simulated driving performance of daily cannabis consumers, occasional users, and controls (non-users). Chronic consumers used either high-potency cannabis flower or concentrates averaging roughly 78% tetrahydrocannabinol (THC). Occasional users only inhaled cannabis flower. The participants drove for 20 minutes on a computer-simulated course following consumption and once again 80 minutes later.
The researchers found that daily consumers exhibited few changes in psychomotor performance compared to the controls. Daily consumers exhibited improvements in standard deviation in lateral positioning (SDLP) after cannabis ingestion. Both daily and occasional cannabis users reduced their speed following consumption, but participants in the control group typically increased their speed.
Occasional cannabis consumers demonstrated minor detriments in SDLP performance following cannabis inhalation, while chronic consumers did not. These changes were not statistically significant compared to the control group. The researchers noted that the relative absence of significant differences in performance across the participant groups was unexpected, given the use of high-potency THC products and the relatively high level of self-reported drug effect. According to the researchers, “It was notable that the daily use group who inhaled concentrates showed the least number of significant differences as compared to the control group, having little to no change in the average SDLP and speed across the three drives.” The researchers found the absence of lane departures or SDLP among the daily-concentrate group to be consistent with tolerance to acute cannabis’s acute impairing effects.
The study also did not identify any correlations between THC/blood concentrations and impaired driving performance, and this finding is also consistent with other studies. The researchers noted that the findings suggest that the presence of THC in the blood is an inconsistent and inappropriate indicator of psychomotor impairment.
The study highlights the importance of developing standardized impairment thresholds in the presence of large inter-individual variability in driving performance and cannabis tolerance associated with chronic use.
Sources: NORML, Traffic Injury Prevention