Daily cannabis users may show less driving impairment than occasional users, but the evidence is not conclusive
In a driving simulator study of 85 adults, occasional cannabis users showed statistically significant driving impairment after smoking, while daily users showed a trend toward impairment that did not reach statistical significance.
Quick Facts
What This Study Found
Occasional users showed significant increases in lane weaving (SDLP) compared to non-users after smoking (p=0.02, effect size 0.64). Daily users also increased SDLP but the change versus non-users was not significant (p=0.08). Daily users drove significantly slower after cannabis use. Blood THC was much higher in daily users (36.4 ng/mL) than occasional users (6.4 ng/mL) despite similar subjective high ratings.
Key Numbers
85 participants; 24 occasional, 31 daily, 30 non-users; blood THC: occasional 6.4 ng/mL vs daily 36.4 ng/mL; subjective high similar (~50/100); occasional SDLP increase significant (p=0.02); daily SDLP increase not significant (p=0.08)
How They Did This
Within-subjects design with 85 adults (24 occasional users, 31 daily users, 30 non-users) completing driving simulator tests before and after ad libitum smoking of self-supplied cannabis (15-30% THC). Blood samples collected pre and post smoking.
Why This Research Matters
The tolerance question is central to medical cannabis driving policy. If daily users are less impaired despite much higher blood THC levels, blood-level-based driving laws may be even more unreliable for this population.
The Bigger Picture
These findings reinforce that THC blood levels are poor predictors of impairment, especially when tolerance is considered. Daily users had 5-6 times higher THC levels but similar or less impairment than occasional users.
What This Study Doesn't Tell Us
Simulated rather than real-world driving, self-supplied cannabis introduced variability, relatively small groups, ad libitum dosing means groups may have consumed different amounts, non-users did not smoke anything.
Questions This Raises
- ?At what frequency of use does tolerance develop enough to protect driving performance?
- ?Should driving laws account for tolerance?
- ?Does the compensatory behavior of driving slower actually reduce crash risk?
Trust & Context
- Key Stat:
- Daily users had blood THC of 36.4 ng/mL vs 6.4 ng/mL for occasional users with similar subjective effects
- Evidence Grade:
- Controlled simulator study with reasonable sample size but using self-supplied cannabis and ad libitum dosing
- Study Age:
- Published in 2021. Cannabis products used (15-30% THC) reflect current market potency levels.
- Original Title:
- Simulated driving performance among daily and occasional cannabis users.
- Published In:
- Accident; analysis and prevention, 160, 106326 (2021)
- Authors:
- Brooks-Russell, Ashley(20), Brown, Tim(4), Friedman, Kyle(2), Wrobel, Julia, Schwarz, John, Dooley, Gregory, Ryall, Karen A, Steinhart, Benjamin, Amioka, Elise, Milavetz, Gary, Sam Wang, George, Kosnett, Michael J
- Database ID:
- RTHC-03026
Evidence Hierarchy
Participants are randomly assigned to treatment or placebo groups to test cause and effect.
What do these levels mean? →Frequently Asked Questions
Are daily cannabis users less impaired when driving?
In this simulator study, daily users showed less measurable driving impairment than occasional users after smoking, despite having 5-6 times higher blood THC levels. However, the study could not conclusively establish this difference.
Do daily cannabis users compensate when driving high?
Yes. Daily users in this study drove significantly slower after cannabis use compared to both occasional users and non-users, suggesting a compensatory behavior that may partially offset impairment.
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Cite This Study
https://rethinkthc.com/research/RTHC-03026APA
Brooks-Russell, Ashley; Brown, Tim; Friedman, Kyle; Wrobel, Julia; Schwarz, John; Dooley, Gregory; Ryall, Karen A; Steinhart, Benjamin; Amioka, Elise; Milavetz, Gary; Sam Wang, George; Kosnett, Michael J. (2021). Simulated driving performance among daily and occasional cannabis users.. Accident; analysis and prevention, 160, 106326. https://doi.org/10.1016/j.aap.2021.106326
MLA
Brooks-Russell, Ashley, et al. "Simulated driving performance among daily and occasional cannabis users.." Accident; analysis and prevention, 2021. https://doi.org/10.1016/j.aap.2021.106326
RethinkTHC
RethinkTHC Research Database. "Simulated driving performance among daily and occasional can..." RTHC-03026. Retrieved from https://rethinkthc.com/research/brooks-russell-2021-simulated-driving-performance-among
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.