Daily cannabis users may show less driving impairment than occasional users, but the evidence is not conclusive | Research

The takeaway

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.

Medical cannabis patients who drive, policymakers setting impaired driving standards, driving safety researchers

Daily users had blood THC of 36.4 ng/mL vs 6.4 ng/mL for occasional users with similar subjective effects

What the researchers 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.

Why it 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 numbers in context

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 the study worked

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.

What this study cannot 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.

How to read the evidence

Controlled simulator study with reasonable sample size but using self-supplied cannabis and ad libitum dosing

When this study was published

Published in 2021. Cannabis products used (15-30% THC) reflect current market potency levels.

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.

Questions still open

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?

Common 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.

Read the original research

Simulated driving performance among daily and occasional cannabis users.

Accident; analysis and prevention, 160, 106326

View on PubMed ↗Publisher’s paper ↗

Citation

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