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

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

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



Commentary* by Dr. Laurence Jerome: Clear evidence of a dose response impairment to THC in non-clinical samples in a well controlled study. The increased impact in ADHD drivers will likely be magnified if they are not being treated. Important educational value for our ADHD patients and their families.



Importance: Expanding cannabis medicalization and legalization increases the urgency to understand the factors associated with acute driving impairment.

Objective: 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.

Design, Setting, and Participants: 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.

Interventions: Placebo or 5.9% or 13.4% THC cannabis smoked ad libitum.

Main Outcomes and Measures: 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.

Results: 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 post smoking blood THC concentrations being higher in those with the highest use intensity. Although there was hesitancy to drive immediately post smoking, increasing numbers (81 [68.6%]) of participants reported readiness to drive at 1 hour 30 minutes despite performance not improving from initial post smoking levels.

Conclusions and Relevance: 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 post smoking 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.


* Dr. Laurence Jerome, former CADDRA Board and Advisory Council member, is a medical practitioner specializing in child, adolescent, and adult psychiatry, with a focus on ADHD throughout the lifespan.


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