After smoking the same cannabis cigarette, occasional users showed significantly more impairment on tracking and divided attention tasks than frequent users, suggesting tolerance to psychomotor effects.
Read this if you want to understand how cannabis tolerance affects driving-related skills.
Occasional smokers significantly more impaired on tracking and attention
What the researchers found
Fourteen frequent cannabis smokers (4+ times per week) and 11 occasional smokers (less than twice per week) smoked a single 6.8% THC cigarette in a controlled setting. Performance was tested on tracking, divided attention, working memory, and risk-taking tasks at multiple time points.
Occasional smokers showed significantly more difficulty on the critical tracking task 1.5 hours after smoking. Divided attention was also more impaired in occasional users, with group differences in tracking error, hit rate, false alarms, and reaction time. However, working memory and risk-taking showed no group differences.
The findings suggest that frequent cannabis users develop some tolerance to the psychomotor-impairing effects of THC, which has direct implications for interpreting impairment in driving-under-the-influence cases.
Why it matters
Driving impairment laws often treat all cannabis users the same. This study shows that the same dose produces very different levels of impairment depending on use frequency, which complicates the development of fair impairment thresholds.
The numbers in context
14 frequent smokers (4+/week) and 11 occasional smokers (<2/week). One 6.8% THC cigarette. Occasional smokers significantly more impaired at 1.5 hours. No group differences on working memory or risk-taking tasks.
How the study worked
Controlled laboratory study where 14 frequent and 11 occasional cannabis smokers entered a secure research unit approximately 19 hours before smoking one 6.8% THC cigarette. Performance was assessed at -1.75, 1.5, 3.5, 5.5, and 22.5 hours post-smoking using critical tracking, divided attention, n-back, and Balloon Analog Risk tasks.
What this study cannot tell us
Relatively small sample sizes. Single dose of moderate-potency cannabis. Laboratory tasks may not fully capture real-world driving complexity. 19 hours of abstinence before the session may not reflect typical use patterns for frequent users.
How to read the evidence
Controlled laboratory study with standardized dosing and multiple time-point assessments, though sample sizes were modest.
When this study was published
Published in 2015. Cannabis potency has increased since this study used 6.8% THC.
The bigger picture
Tolerance to cannabis effects is well-documented for subjective "high" but less studied for actual psychomotor performance. This study confirms tolerance extends to skills relevant for driving, which challenges one-size-fits-all impairment testing approaches.
Questions still open
- Should impairment testing account for use frequency? At what level of tolerance does psychomotor impairment become negligible? Do frequent users still show impairment at higher doses?
Common questions
Does frequent cannabis use reduce impairment?
Which skills were most affected?
Read the original research
Smoked cannabis' psychomotor and neurocognitive effects in occasional and frequent smokers.
Journal of analytical toxicology, 39(4), 251-61
Citation
Desrosiers, Nathalie A; Ramaekers, Johannes G; Chauchard, Emeline; Gorelick, David A; Huestis, Marilyn A. (2015). Smoked cannabis' psychomotor and neurocognitive effects in occasional and frequent smokers.. Journal of analytical toxicology, 39(4), 251-61. https://doi.org/10.1093/jat/bkv012
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