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Driving Performance and Cannabis Users' Perception of Safety: A Randomized Clinical Trial.

Thomas D Marcotte, Anya Umlauf, David J Grelotti, Emily G Sones, Philip M Sobolesky, Breland E Smith, Melissa A Hoffman, Jacqueline A Hubbard, Joan Severson, Marilyn A. Huestis, Igor Grant, Robert L. Fitzgerald

JAMA Psychiatry March 1, 2022 DOI: 10.1001/jamapsychiatry.2021.4037 (opens in new tab)

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Characteristics Double-blind, placebo-controlled parallel randomized clinical trial Peer reviewed
Sample size 191
Population Regular cannabis users recruited at the Center for Medicinal Cannabis Research, University of California San Diego; mean age 29.9 years, 61.8% male, mean 16.7 days of use in the past month
Intervention Placebo
Dose 5.9% or 13.4% THC cannabis smoked ad libitum
Duration Assessed before smoking and at multiple time points up to 4 hours 30 minutes postsmoking
Measures Composite Drive Score (CDS), self-perceptions of driving impairment, cannabis use history
Topics Cannabis
Registration NCT02849587
Key findings Smoked cannabis significantly reduced simulated driving performance at 30 minutes and 1 hour 30 minutes after smoking, with borderline impairment at 3 hours 30 minutes and no difference from placebo by 4 hours 30 minutes. Impairment did not differ by THC content (5.9% vs 13.4%) or by past-6-month use intensity, and many participants reported readiness to drive at 1 hour 30 minutes despite performance not having improved.

Abstract

Expanding cannabis medicalization and legalization increases the urgency to understand the factors associated with acute driving impairment. To determine, in a large sample of regular cannabis users, the magnitude and time course of driving impairment produced by smoked cannabis of different Δ9-tetrahydrocannabinol (THC) content, the effects of use history, and concordance between perceived impairment and observed performance. This double-blind, placebo-controlled parallel randomized clinical trial took place from February 2017 to June 2019 at the Center for Medicinal Cannabis Research, University of California San Diego. Cannabis users were recruited for this study, and analysis took place between April 2020 and September 2021. Placebo or 5.9% or 13.4% THC cannabis smoked ad libitum. The primary end point was the Composite Drive Score (CDS), which comprised key driving simulator variables, assessed prior to smoking and at multiple time points postsmoking. Additional measures included self-perceptions of driving impairment and cannabis use history. Of 191 cannabis users, 118 (61.8%) were male, the mean (SD) age was 29.9 (8.3) years, and the mean (SD) days of use in the past month was 16.7 (9.8). Participants were randomized to the placebo group (63 [33.0%]), 5.9% THC (66 [34.6%]), and 13.4% THC (62 [32.5%]). Compared with placebo, the THC group significantly declined on the Composite Drive Score at 30 minutes (Cohen d = 0.59 [95% CI, 0.28-0.90]; P < .001) and 1 hour 30 minutes (Cohen d = 0.55 [95% CI, 0.24-0.86]; P < .001), with borderline differences at 3 hours 30 minutes (Cohen d = 0.29 [95% CI, -0.02 to 0.60]; P = .07) and no differences at 4 hours 30 minutes (Cohen d = -0.03 [95% CI, -0.33 to 0.28]; P = .87). The Composite Drive Score did not differ based on THC content (likelihood ratio χ24 = 3.83; P = .43) or use intensity (quantity × frequency) in the past 6 months (likelihood ratio χ24 = 1.41; P = .49), despite postsmoking blood THC concentrations being higher in those with the highest use intensity. Although there was hesitancy to drive immediately postsmoking, increasing numbers (81 [68.6%]) of participants reported readiness to drive at 1 hour 30 minutes despite performance not improving from initial postsmoking levels. Smoking cannabis ad libitum by regular users resulted in simulated driving decrements. However, when experienced users control their own intake, driving impairment cannot be inferred based on THC content of the cigarette, behavioral tolerance, or THC blood concentrations. Participants' increasing willingness to drive at 1 hour 30 minutes may indicate a false sense of driving safety. Worse driving performance is evident for several hours postsmoking in many users but appears to resolve by 4 hours 30 minutes in most individuals. Further research is needed on the impact of individual biologic differences, cannabis use history, and administration methods on driving performance. ClinicalTrials.gov Identifier: NCT02849587.

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Modulation of Mediotemporal and Ventrostriatal Function in Humans by Δ9-Tetrahydrocannabinol Healthy, native English-speaking, right-handed men of white race/ethnicity who had used... 2009 Randomized controlled trial n = 15
Cannabidiol (CBD) content in vaporized cannabis does not prevent tetrahydrocannabinol (THC)-induced impairment of driving and cognition Healthy volunteers with a history of light cannabis use 2019 Randomized, double-blind, within-subjects crossover n = 14

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