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.