The review selected studies that assessed associations between blood THC and driving. Its twelve included articles were not a count of all research on whether cannabis affects driving.
Eligibility defined the evidence set
The authors required a correlational analysis or a related measure such as linear regression or a general linear model. They included human studies of acute THC-dominant cannabis effects with on-road or simulated driving, and excluded reviews, case reports, conference abstracts and other specified designs.
This question differs from asking whether an active-cannabis group performed differently from placebo. A study could provide information about cannabis effects yet fall outside a review focused on blood-concentration associations.
Do not expand the denominator
Ten of the twelve included articles reported no significant linear relationships for the measures examined. That is a statement about this selected evidence set. It cannot be restated as ten of every twelve cannabis-driving experiments finding no impairment. A null association also does not establish that all concentrations are harmless.
Use the review for its actual purpose
The review helps assess whether a blood number tracks particular performance measures under studied conditions. Task complexity, route, timing and cannabis-use history remain relevant. A forensic summary should distinguish the eligibility question from the broader collision-risk or treatment-effect questions that require their own evidence.
Related reading: cannabis screening and impairment.
Research cited
Behzad and colleagues (2025). Association of driving with blood delta-9-tetrahydrocannabinol: a systematic review. DOI: 10.1093/ijnp/pyaf021.
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