According to the results of a study published recently in the Journal of Safety Research, taking prescribed cannabis before the start of a journey does not affect the ability of patients to drive safely.
In the study, funded by the Motor Accident Insurance Commission, Australian researchers measured the ability of 38 participants to judge distance, control speed, follow cars at a safe distance, and spot hazards while using a driving simulator under the influence of a prescribed dose of medical cannabis.
Participants were then asked to perform the same driving-simulator tests the following week, abstaining from medical cannabis use for 11.5 hours before.
Researchers found no difference in the results of the driving simulator tests. Patients showed no change in driving ability when under the influence of a dose of medicinal cannabis when compared to the results from the test taken after abstaining.
Patients were also tasked with completing a self-perceived hazard perception test and a self-perceived on-road driving skills and safety test. Both were completed after they had taken their prescription.
The results of the self-perceived tests were not associated with the results of their driving-simulator tests. This may identify a higher degree of care in drivers who are taking medicinal cannabis because the driver’s assessment of their ability is lower than their actual level of ability.
“The findings from this study suggest that a dose of vaporized cannabis (consumed in accordance with prescription) may not affect hazard perception ability or driving-related risk-taking behavior among medicinal cannabis patients,” the study authors wrote.
“However, the findings indicate that cannabis consumption may lead to a reduction in participants’ perceptions of their hazard perception performance and ability to predict traffic conflicts during real driving, although it did not change their perceptions of their overall driving skill, safety or crash risk. In addition, the lack of relationship between objective and subjective ratings of hazard perception performance highlights that individuals lack insight into their own abilities, irrespective of whether or not they have consumed cannabis.”
Researchers acknowledged that the sample size in the study was small and that patients’ experience with cannabis could have influenced the outcomes. On average, participants reported an estimated 23 years of cannabis use, both illegal and medical. Due to a long history of cannabis use and frequent dosing, typically five times a day, it is likely that participants had a high tolerance to cannabis. This may “lead to a reduction of the common neurocognitive effects, mitigating negative effects associated with THC,” given that the sample consumed doses that would normally be “directly related to impairment in infrequent user groups”.
Researchers recommend conducting further studies involving participants who do not share the same extensive cannabis history as those in the current study.
“The results from this study suggest that some driving-related skills and behaviors (e.g., hazard perception skill, speeding propensity, and gap acceptance) may not be acutely affected following medicinal cannabis consumption in a sample of tolerant users. It should also be considered that the rapid symptom relief provided by cannabis flower may assist in maintaining performance rather than reducing performance (as seen in studies of recreational or infrequent users), providing an explanation for these results. Furthermore, while it appears that cannabis flower may influence perceived skill in terms of hazard perception/prediction, overall self-assessment of driving ability appears to be unreliable even in the absence of cannabis consumption. Future research is needed to clarify whether these effects are observed in patients who are less tolerant to the substance (e.g., do not have a history of illicit or medicinal cannabis use prior to obtaining their prescription),” the authors concluded.
