Publication Abstract Display
Type: Poster
Title: Cannabis, driving performance and users'' perception of safety: A double-blind, placebo-controlled randomized clinical trial of smoked cannabis.
Authors: Marcotte TD, Umlauf A, Grelotti DJ, Sones EG, Sobolesky PM, Smith BE, Hoffman MA, Hubbard JA, Severson J, Huestis MA, Grant I, Fitzgerald RL
Date: 02-03-2022
Abstract:Background: With increasing legalization of cannabis for medicinal and recreational use, there are growing concerns regarding cannabis-impaired driving. There remains uncertainty regarding the magnitude and time course of cannabis effects on those most likely to be on the road – regular users smoking to a desired level of intoxication. The appropriate waiting period before driving after smoking is a significant public safety concern, with some suggesting 3-5 hours, and others recommending longer. Since this decision may be self-determined based upon "feeling impaired", it is important to understand the accuracy of these self-evaluations. Lastly, while greater frequency of use is associated with increased behavioral tolerance, the relationship to driving remains poorly understood, since individuals may counteract tolerance by ingesting greater amounts to achieve desired psychoactive effects. The aims of this study were to determine the magnitude and time course of driving impairment produced by cannabis with different THC percentages, concordance between perceived impairment and observed performance, and the effects of use history on driving performance. Methods: In this double-blind, placebo-controlled parallel randomized clinical trial, 191 healthy adult cannabis users were randomized to consume a cigarette with either 5.9% THC, 13.4% THC, or placebo (.02% THC) ad libitum. Participants completed a driving simulator assessment prior to smoking and at 4 timepoints post-smoking. Driving performance was summarized using the Composite Drive Score (CDS), comprised of key driving simulator variables. Additional measures included self-reported 1) perceptions of driving impairment and 2) cannabis use intensity (quantity x frequency for the past 6 months). Results: Participants were 61.8% male with a mean of 16.7 (9.8) days of use in the past month. Compared to Placebo, the THC group significantly declined on the CDS at 30min (Cohen’s d=.59) and 1h 30min (d=.55), but not at 3h 30min or 4h 30min. Participants reported feeling less impaired and being more willing to drive at 1h 30min despite performance being similar to that observed at 30min. Many THC participants performed similar to the Placebo group at each timepoint, and post-smoking simulator performance was similar for the high and low THC content groups. Simulator performance in the THC group did not differ by cannabis intensity over the past 6 months (p = .964). Discussion: Smoking cannabis ad libitum resulted in significant reductions in simulated driving performance, which resolved over 3h 30min – 4h 30min. However, not all drivers’ performance declined. THC content of the cigarettes did not significantly affect outcomes: users self-titrated, resulting in similar performance reductions. The THC group generally showed good agreement between subjective driving impairment and actual performance at 30min. However, at 1h 30min participants increasingly rated themselves as safe to drive, whereas simulator data indicated on-going reduced driving performance. These first few hours may constitute a period of greatest risk, since users may be less likely to refrain from driving or to attempt to compensate for reduced functioning. This is an important topic for public safety messaging, since a goal is to keep impaired drivers off of the road prior to becoming a danger.

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