According to a new study published in the Journal of Concurrent Disorders, those who consistently use medical cannabis show little to no difference in their driving performance following cannabis inhalation. The study, titled Effects of therapeutic cannabis on simulated driving: A pilot study, was first reported on by NORML.

For the study, researchers from the University of Toronto, Health Canada, and the Centre for Addiction and Mental Health examined the influence of marijuana inhalation on simulated driving performance among a group of daily medical marijuana consumers. “Participants were asked to refrain from engaging in any cannabis use during the 48 hours immediately prior to the study sessions. Fourteen subjects completed the study.” Subjects performed on a driving simulator prior to and 30 minutes after inhaling cannabis. Three separate driving scenarios were programmed into the simulator.

The study found that subjects decreased their overall mean speed following cannabis consumption, and marijuana did not appear to influence subjects’ ability to maintain lateral control or their brake reaction time. “Despite refraining from the use in cannabis in the days leading up to the study, subjects nonetheless possessed residual levels of THC (4ng/ml on average) in their blood prior to smoking marijuana during the study session.”

The study concludes by stating: “The purpose of the present pilot study was to investigate the effects of therapeutic cannabis use on simulated driving. It was found that therapeutic cannabis reduced overall mean speed with no effects on straightaway mean speed, straightaway lateral control, or brake latency. … [F]urther investigation of the effects of therapeutic cannabis on driving are warranted.”

The full abstract of the study can be found below:

Background: Although medical cannabis has been available to Canadians since 2001, there is little research on the effects of cannabis on driving in individuals who use cannabis medically. This pilot study sought to determine the effects of therapeutic cannabis use on simulated driving. Methods: Eligible participants reported daily use of cannabis for therapeutic purposes, with a medical authorization. Prior to the test session, participants were asked not to smoke their regular dose. Participants (n=14) completed self-report questionnaires, including subjective effects questionnaires (visual analog scales), the Addiction Research Centre Inventory (ARCI), and Profile of Mood States (POMS), and provided blood (for determination of THC and metabolites). They also drove a simulator both before and after smoking their usual daily dose of cannabis. Outcome measures on simulated driving consisted of overall mean speed, straightaway mean speed, straightaway lateral control, and brake latency. Speed and lateral control were also measured under cognitive load. Results: After smoking cannabis, overall mean speed was reduced. No effects of therapeutic cannabis were found on straightaway mean speed or straightaway lateral control for either condition (standard or cognitive load) or on brake latency. After smoking therapeutic cannabis in the lab, changes in speed and lateral control were negatively correlated with the amount of cannabis smoked per day. Prior to smoking therapeutic cannabis in the lab, under baseline conditions, speed and lateral control under cognitive load were also correlated with the amount of cannabis used per day. Therapeutic cannabis use increased subjective reports and blood levels of THC and metabolites. Conclusions: The present study suggests that, even with repeated daily use, cannabis consumption among therapeutic users may alter driving behavior. This has implications for road safety and use of cannabis for therapeutic purposes.