Abstract
Given the increasing availability of high-potency Δ9-tetrahydrocannabinol (Δ9-THC) products, understanding potency-related effects of Δ9-THC on driving performance is an important public safety issue. This randomized, double-blind, placebo-controlled, within-subjects trial examined the effects of smoked cannabis with varying Δ9-THC concentrations on simulated driving. Adults aged 19–45 who regularly used cannabis and held valid driver’s licenses completed simulated driving tasks after smoking placebo, or cannabis containing low (6.25%/47 mg Δ9-THC), medium (12.5%/94 mg Δ9-THC), or high (22%/165 mg Δ9-THC) Δ9-THC levels. The primary outcome was mean speed (km/h); secondary measures included maximum speed, standard deviation of lateral position (SDLP), reaction time (RT), and subjective ratings of driving ability and intoxication. Mean speed did not differ across conditions. Maximum speed increased under medium (p = 0.006) and high (p = 0.02) potencies versus placebo. SDLP was higher across all Δ9-THC potencies (p < 0.001), and RT was longer under medium and high potencies (p < 0.001). Both SDLP (p < 0.001) and RT (p = 0.023) positively correlated with blood Δ9-THC concentrations. Participants reported poorer driving performance and reduced willingness to drive at higher potencies. Findings demonstrate potency-dependent impairments in simulated driving linked to Δ9-THC concentration, underscoring implications for road safety.
Trial registration: clinicaltrials.gov, ID NCT03656029; First posted date: 04/09/2018; URL: https://clinicaltrials.gov/study/NCT03656029.
Data availability
The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.
References
Brubacher, J. R. et al. Cannabis legalization and detection of tetrahydrocannabinol in injured drivers. N Engl. J. Med. 386 (2), 148–156 (2022).
Moore, C. et al. Alcohol and drug prevalence among seriously or fatally injured road users (US DOT National Highway Traffic Safety Administration, 2022).
Health Canada. 2024 Canadian Cannabis Survey 2024. Available from: https://www.canada.ca/en/health-canada/services/drugs-medication/cannabis/research-data/canadian-cannabis-survey-2024-summary.html
Simmons, S. M., Caird, J. K., Sterzer, F. & Asbridge, M. The effects of cannabis and alcohol on driving performance and driver behaviour: a systematic review and meta-analysis. Addict. (Abingdon England). 117 (7), 1843–1856 (2022).
Li, M-C. et al. Marijuana use and motor vehicle crashes. Epidemiol. Rev. 34 (1), 65–72 (2012).
Rogeberg, O. & Elvik, R. The effects of cannabis intoxication on motor vehicle collision revisited and revised. Addiction 111 (8), 1348–1359 (2016).
Veldstra, J. L., Bosker, W. M., de Waard, D., Ramaekers, J. G. & Brookhuis, K. A. Comparing treatment effects of oral THC on simulated and on-the-road driving performance: testing the validity of driving simulator drug research. Psychopharmacol. (Berl). 232 (16), 2911–2919 (2015).
Arkell, T. R. et al. Cannabidiol (CBD) content in vaporized cannabis does not prevent tetrahydrocannabinol (THC)-induced impairment of driving and cognition. Psychopharmacol. (Berl). 236 (9), 2713–2724 (2019).
Bramness, J. G., Khiabani, H. Z. & Morland, J. Impairment due to cannabis and ethanol: clinical signs and additive effects. Addict. (Abingdon England). 105 (6), 1080–1087 (2010).
Hartman, R. L. et al. Cannabis effects on driving lateral control with and without alcohol. Drug Alcohol Depend. 154, 25–37 (2015).
Lenne, M. G. et al. The effects of cannabis and alcohol on simulated arterial driving: influences of driving experience and task demand. Accid. Anal. Prev. 42 (3), 859–866 (2010).
Ramaekers, J. G., Robbe, H. W. & O’Hanlon, J. F. Marijuana, alcohol and actual driving performance. Human Psychopharmacol. 15 (7), 551–558 (2000).
Ronen, A. et al. Effects of THC on driving performance, physiological state and subjective feelings relative to alcohol. Accid. Anal. Prev. 40 (3), 926–934 (2008).
Anderson, B. M., Rizzo, M., Block, R. I., Pearlson, G. D. & O’Leary, D. S. Sex differences in the effects of marijuana on simulated driving performance. J. Psychoactive Drugs. 42 (1), 19–30 (2010).
Ronen, A. et al. The effect of alcohol, THC and their combination on perceived effects, willingness to drive and performance of driving and non-driving tasks. Accid. Anal. Prev. 42 (6), 1855–1865 (2010).
Brands, B. et al. Acute and residual effects of smoked cannabis: impact on driving speed and lateral control, heart rate, and self-reported drug effects. Drug Alcohol Depend. 205, 107641 (2019).
Brands, B., Di Ciano, P. & Mann, R. E. Cannabis, impaired driving, and road safety: an overview of key questions and issues. Front. Psychiatry. 12, 641549 (2021).
Burt, T. S., Brown, T. L., Milavetz, G. & McGehee, D. V. Mechanisms of cannabis impairment: implications for modeling driving performance. Forensic Sci. Int. 328, 110902 (2021).
ElSohly, M. A., Chandra, S., Radwan, M., Majumdar, C. G. & Church, J. C. A comprehensive review of cannabis potency in the United States in the last decade. Biol. Psychiatry: Cogn. Neurosci. Neuroimaging. 6 (6), 603–606 (2021).
Ontario Cannabis Store. Ontario Cannabis Store (OCS). Dried Cannabis Flower. (2024). Available from: https://ocs.ca/collections/dried-flower
Robbe, H. Marijuana’s impairing effects on driving are moderate when taken alone but severe when combined with alcohol. Hum. Psychopharmacology: Clin. Experimental. 13 (S2), S70–S8 (1998).
Marcotte, T. D. et al. Driving performance and cannabis users’ perception of safety: a randomized clinical trial. JAMA Psychiatry. 79 (3), 201–209 (2022).
Di Ciano, P. et al. Cannabis and driving in older adults. JAMA Netw. open. 7 (1), e2352233–e (2024).
Cooper, Z. D. & Haney, M. Opioid antagonism enhances marijuana’s effects in heavy marijuana smokers. Psychopharmacol. (Berl). 211 (2), 141–148 (2010).
Fares, A. et al. Combined effect of alcohol and cannabis on simulated driving. Psychopharmacol. (Berl). 239 (5), 1263–1277 (2022).
Guy, W., Wilson, W. H., Brooking, B., Manov, G. & Fjetland, O. Reliability and validity of SAFTEE: preliminary analyses. Psychopharmacol. Bull. 22 (2), 397–401 (1986).
Rabkin, J. G. & Markowitz, J. S. Side effect assessment with SAFTEE: pilot study of the instrument. Psychopharmacol. Bull. 22 (2), 389–396 (1986).
Lansdown, T. C. & Saunders, S. J. Driver performance, rewards and motivation: a simulator study. Transp. Res. Part. F: Traffic Psychol. Behav. 15 (1), 65–74 (2012).
North, A. C. & Hargreaves, D. J. Music and driving game performance. Scand. J. Psychol. 40 (4), 285–292 (1999).
Peterson, L. R. & Peterson, M. J. Short-term retention of individual verbal items. J. Exp. Psychol. 58, 193–198 (1959).
R Core Team. R: A Language and Environment for Statistical Computing Vienna, Austria 2023. Available from: https://www.R-project.org/
Bates, D., Mächler, M., Bolker, B. & Walker, S. Fitting linear mixed-effects models using lme4. J. Stat. Softw. 67 (1), 1–48 (2015).
Pinheiro, J. B. D. & R Core Team. nlme: Linear and Nonlinear Mixed Effects Models 2023. Available from: https://CRAN.R-project.org/package=nlme
Lenth, R. Estimated Marginal Means, aka Least-Squares Means 2023 1.8.6. Available from: https://cran.r-project.org/package=emmeans
Verster, J. C. & Roth, T. Standard operation procedures for conducting the on-the-road driving test, and measurement of the standard deviation of lateral position (SDLP). Int. J. Gen. Med. 4, 359–371 (2011).
Arkell, T. R., Spindle, T. R., Kevin, R. C., Vandrey, R. & McGregor, I. S. The failings of per se limits to detect cannabis-induced driving impairment: results from a simulated driving study. Traffic Inj. Prev. 22(2), 102-107 (2021).
Di Ciano, P. et al. The utility of THC cut-off levels in blood and oral fluid for detection of impaired driving. Cannabis and cannabinoid research. ;in progress. (2022).
McCartney, D., Arkell, T. R., Irwin, C., Kevin, R. C. & McGregor, I. S. Are blood and oral fluid ∆(9)-tetrahydrocannabinol (THC) and metabolite concentrations related to impairment? A meta-regression analysis. Neurosci. Biobehav Rev. 134, 104433 (2022).
Mura, P., Kintz, P., Dumestre, V., Raul, S. & Hauet, T. THC can be detected in brain while absent in blood. J. Anal. Toxicol. 29 (8), 842–843 (2005).
Huestis, M. A. Human cannabinoid pharmacokinetics. Chem. Biodivers. 4 (8), 1770–1804 (2007).
Hartman, R. L. & Huestis, M. A. Cannabis effects on driving skills. Clin. Chem. 59 (3), 478–492 (2013).
Preuss, U. W. et al. Cannabis use and car crashes: a review. Front. Psychiatry. 12, 643315 (2021).
Costales, B., Babalonis, S. L., Brown, J. D. & Goodin, A. J. Cannabis effects on driving performance: clinical considerations. Med. Cannabis Cannabinoids. 6 (1), 8–14 (2023).
Ramaekers, J. G., Kauert, G., Theunissen, E. L., Toennes, S. W. & Moeller, M. R. Neurocognitive performance during acute THC intoxication in heavy and occasional cannabis users. J. Psychopharmacol. 23 (3), 266–277 (2009).
Acknowledgements
We would like to extend our deepest gratitude to the dedicated nurses who were instrumental in the success of this study: Paige Blanchard, Emily Simpkin, Jiah Jethava, and Sharon Atendido. Additionally, we wish to acknowledge the invaluable support of the staff at the CAMH (Centre for Addiction and Mental Health) research pharmacy and clinical laboratory. Furthermore, we are thankful for the guidance and oversight provided by the members of the Drug Safety Monitoring Board (DSMB) and Research Quality Assurance team.
Funding
This study was supported by Public Safety Canada. The funding agency was not involved in the design, data collection, analysis, or interpretation of the results of this study. Additionally, the funder had no role in the writing of the manuscript nor the decision to submit it for publication.
Author information
Authors and Affiliations
Contributions
Bruna Brands: conceptualization, methodology, funding acquisition, supervision, writing—original draft; Adam Zaweel: data curation, investigation, visualization, writing—review and editing; Madison Wright: data curation, investigation, visualization, writing—review and editing; Patricia Di Ciano: project administration, writing—review and editing; Christine Wickens: writing—review and editing; Justin Matheson: data interpretation, writing—review and editing; Andrew Fares: data curation, investigation, visualization, writing—review and editing; Omer Hasan: data interpretation, software, writing—review and editing; Marcos Sanches: formal analysis, writing—review and editing; Beth Sproule: writing—review and editing; Marilyn Huestis: consulting, data interpretation, writing—review and editing; Timothy Brown: consulting, data interpretation, writing—review and editing; Bernard Le Foll: funding acquisition, conceptualization, methodology, study oversight, medical oversight, data interpretation, writing—review and editing.
Corresponding author
Ethics declarations
Competing interests
Dr. Bernard Le Foll obtained funding from Indivior for a clinical trial sponsored by Indivior. Dr. Le Foll has in-kind donations of placebo edibles from Indiva. Dr. Le Foll obtained industry funding from Canopy Growth Corporation (through research grants handled by the Centre for Addiction and Mental Health and the University of Toronto). He participated in a session of a National Advisory Board Meeting (Emerging Trends BUP-XR) for Indivior Canada and is part of Steering Board for a clinical trial for Indivior. Dr. LeFoll consulted with Shinogi, ThirdBridge, and Changemark. He is part of a scientific advisory board for NFL Biosciences. He received travel support to attend an event by Bioprojet and is supported by CAMH, Waypoint Centre for Mental Health Care, a clinician-scientist award from the Department of Family and Community Medicine of the University of Toronto and a Chair in Addiction Psychiatry from the Department of Psychiatry of University of Toronto. Dr. Christine Wickens serves on the Executive Committee of the International Council on Alcohol, Drugs and Traffic Safety (ICADTS) and on the Canadian Society of Forensic Science’s Drugs and Driving Committee, which acts as an advisory body to the Department of Justice with respect to issues of drug impaired driving. In the past three years, she served on the Board of Directors of the Canadian Association of Road Safety Professionals (CARSP) and consulted for the AAA Foundation for Traffic Safety. She was awarded grant funding from Transport Canada, Ministry of Transportation Ontario, Canadian Institutes for Health Research, CAMH Foundation, and CAMH Remedial Measures Programs. She received travel reimbursement for academic presentations delivered to the Canadian Mental Health Association (CMHA) and the Canadian Council of Motor Transport Administrators (CCMTA). Professor Huestis is currently a consultant to many pharmaceutical and diagnostic companies, none of which had any role in this research and none of whom’s products were used or mentioned in the manuscript. She also is an unpaid member of the scientific advisory board of the Smart Approaches to Marijuana organization. All other authors declare that they have no conflicts of interest.
Additional information
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Supplementary Information
Below is the link to the electronic supplementary material.
Rights and permissions
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
About this article
Cite this article
Brands, B., Zaweel, A., Wright, M. et al. Potency-related effects of smoked cannabis on simulated driving performance: a randomized, controlled crossover trial. Sci Rep (2026). https://doi.org/10.1038/s41598-026-43045-2
Received:
Accepted:
Published:
DOI: https://doi.org/10.1038/s41598-026-43045-2
