DUIC - DRIVING UNDER THE INFLUENCE OF CANNABIS
A commentary on Marcotte, T. D. et al. Driving Performance and Cannabis Users’ Perception of Safety. JAMA Psychiatry 79, 1–9 (2022). Phil Molloy, MD, is a contributor to this blog post. Be sure to check out our journal club discussion on this topic under live events. This article originally appeared on www.prc-trials-plus.com.
The intersection of cannabis use and driving safety is a critical area of research, particularly in light of the increasing legalization and decriminalization of cannabis around the world. What is the best way to measure or study the impact of cannabis impairment on driving? The answer is obviously, to design the study to have real world applications or to mimic real world settings as much as possible. Researcher at the Center for Medical Cannabis Research in San Diego, decided to do just that, publishing a noteworthy study in this field entitled "Driving Performance and Cannabis Users’ Perception of Safety" by Marcotte, T. D. et al., published in JAMA Psychiatry in 2022.
The study was conducted between February 2017 and June 2019, involving 191 principally male participants, mean age of 29.9 years, with various histories of cannabis use. They were randomly assigned to 3 blinded arms: smoking marijuana with 5.9% THC, 13.4% THC, or placebo marijuana. They let participants use cannabis to their hearts content – i.e. “to their desired level of highness”, simulating real-world use consumption patterns. At multiple time points after smoking, they were subject to Composite Driving Scores (CDS) while driving in a simulator (explained in detail below). THC blood levels were checked, and subjects were asked to estimate their degrees of impairment at multiple time points, specifically judging when they felt they were no longer significantly impaired and safe to drive again. Most of the participants underestimated their levels of impairment as documented using the driving simulator.
The driving simulators were designed to provide scenarios where people drove an assortment of city and country roads and different driving circumstances. Over the course of an hour, they would have to drive through residential zones, business districts, traffic lights, freeway merging, maintaining lane position and speed, etc. They also would have to respond to what we call the amber light dilemma – having to decide, should they stop or go. Built within these driving simulations were two standardized sort of structured scenarios.
KEY FINDINGS
There was no association between blood levels of THC and impairment by CDS or by subjective reporting, in contrast to the situation with alcohol for example. This has been shown before by many other groups. Measuring THC levels in any body fluid does not correlate with impairment.
Compared to placebo, the active THC groups were significantly impaired, at 30 min. and at 90 min.; less so at 3 hrs. 30 min.; and were similar to placebo at 4 hrs. 30 min.
There was no significant difference between the 5.9% and the 13.4% arms, presumably due to the study design of encouraging smoking ad libitum, titrating amount smoked to their customary desired effect.
There was a poor correlation between self-reported impairment and judging when they were safe to resume driving again, especially noted at the 90 min measurement at which time subjects tended to describe themselves as no longer impaired and safe to resume driving, despite persistent impairments on the CDS simulated driving metrics.
One of the study's key findings was the association between chronic cannabis use and higher blood THC levels, yet this did not linearly translate to self-reported feelings of impairment or "highness." This discrepancy underscores the complex relationship between physiological THC absorption and subjective intoxication experiences. The study also revisits the concept of behavioral tolerance, where long-term regular cannabis users exhibit less pronounced impairment effects, possibly due to their body's adaptation to THC, highlighting the importance of smoking topography and individual consumption patterns in assessing impairment and safety. This aspect is particularly intriguing as it challenges the conventional understanding of drug tolerance and its implications for driving performance.
Previous research has highlighted that while THC impairs driving abilities, users often compensate by adopting safer driving practices, such as driving slower. Interestingly, certain stimuli elicit quicker responses from THC-affected drivers, possibly due to altered time perception under the influence, making brief waits seem longer.
The study also addresses the critical public safety issue of determining an appropriate waiting period before driving post-cannabis consumption. The lack of correlation between self-evaluated impairment and actual driving performance, especially 90 minutes after consumption, raises concerns about users' ability to judge their fitness to drive safely. There are implications for how healthcare providers should best educate and advise our cannabis using patients, specifically on how long to wait after consumption to resume safe unimpaired driving.
Furthermore, the study questions the validity of per se laws based on blood THC concentration, given the lack of a direct relationship with driving impairment. This finding is pivotal in shaping future legal frameworks and public safety guidelines regarding cannabis use and driving.
However, the study acknowledges several limitations, including the absence of a non-using control group, variability in 'highness' levels due to self-administered consumption, and the limited scope regarding infrequent users, older populations, and other consumption methods. These findings cannot be extrapolated to “gummies” and other oral forms of cannabis for example. Further research is needed to fully understand the implications of cannabis use on driving safety.
Clinical and Public Health Takeaways:
The goal is to keep impaired drivers off the road.
One can't assume impairment based upon the THC content of the product because users self-titrate to desired effects.
One can't assume impairment based upon the THC blood levels.
An important public message is that even among experienced regular users, there is a disconnect between subjective impairment and “I’m OK to drive now”, and actual impairment, which last several hours after consumption.
The schedule I status of THC is a major impediment to evaluating products that are actually in the marketplace being used by consumers.
In summary, the study by Marcotte et al. provides valuable insights into the complex dynamics of cannabis consumption, driving performance, and user perceptions of safety. It highlights the need for nuanced public safety guidelines, further research, and informed legal frameworks to address the challenges posed by the increasing prevalence of cannabis use in society.
If you want to learn more about this study, the authors are featured in a 35-minute podcast reviewing and explaining this study.
Disclaimer: This article is intended for informational purposes only and does not constitute medical advice. Always consult a healthcare professional for medical diagnosis or treatment recommendations.
