A meta-regression of 28 studies found blood and oral fluid THC concentrations are weak to negligible predictors of driving impairment, especially in regular cannabis users.
Policymakers, law enforcement, cannabis users who drive, traffic safety researchers
No significant THC-impairment link in regular users
What the researchers found
Higher blood THC, 11-OH-THC, oral fluid THC, and subjective intoxication were associated with greater impairment in occasional users, but correlations were negligible to weak (r = -0.08 to -0.43). No significant biomarker-performance relationships were found in regular (weekly+) cannabis users.
Why it matters
Many jurisdictions use blood or oral fluid THC levels to identify impaired drivers, similar to blood alcohol. This meta-analysis shows THC biomarkers are fundamentally different from blood alcohol as impairment indicators.
The numbers in context
28 publications; 822 driving outcomes; correlations ranged from -0.08 to -0.43; no significant relationships in regular users
How the study worked
Meta-regression analysis of 28 publications and 822 driving-related outcomes examining relationships between blood/oral fluid THC concentrations, subjective intoxication, and driving performance.
What this study cannot tell us
Analyses in regular users were less robust due to fewer studies. Most studies used simulated rather than real-world driving.
How to read the evidence
Meta-regression of 28 publications with 822 outcomes provides strong evidence, though most underlying studies used simulated driving.
When this study was published
Published in 2022
The bigger picture
Unlike alcohol, where blood concentration reliably predicts impairment, THC levels do not. This has profound implications for cannabis DUI laws that set per se THC limits as a proxy for impairment.
Questions still open
- If THC levels do not predict impairment, what should replace per se THC limits in driving laws? Would performance-based impairment testing be more accurate?
Common questions
Can a blood test tell if someone is too impaired to drive from cannabis?
Why is THC different from alcohol for impairment testing?
Read the original research
Are blood and oral fluid Δ9-tetrahydrocannabinol (THC) and metabolite concentrations related to impairment? A meta-regression analysis.
Neuroscience and biobehavioral reviews, 134, 104433
Citation
McCartney, Danielle; Arkell, Thomas R; Irwin, Christopher; Kevin, Richard C; McGregor, Iain S. (2022). Are blood and oral fluid Δ9-tetrahydrocannabinol (THC) and metabolite concentrations related to impairment? A meta-regression analysis.. Neuroscience and biobehavioral reviews, 134, 104433. https://doi.org/10.1016/j.neubiorev.2021.11.004
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