Determining the magnitude and duration of acute Δ9-tetrahydrocannabinol (Δ9-THC)-induced driving and cognitive impairment: A systematic and meta-analytic review

McCartney, Danielle; Arkell, Thomas R.; Irwin, Christopher; McGregor, Iain S. · 2021 · OpenAlex-citations

DOI: 10.1016/j.neubiorev.2021.01.003

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Summary

This systematic review and meta-analysis addresses the critical public health issue of road safety in the context of increasing cannabis legalization. The primary research question focuses on characterizing the magnitude and duration of acute impairment caused by Δ9-tetrahydrocannabinol (Δ9-THC) on driving performance and related cognitive skills. The study was motivated by the lack of comprehensive meta-analytic evidence regarding how long impairment persists, a gap that complicates the establishment of consistent legal frameworks and public health guidelines for safe driving intervals post-cannabis use. The authors conducted a systematic search of Web of Science and Scopus databases from 2000 to April 2020, identifying 80 eligible publications comprising 1,534 outcomes. The review included placebo-controlled experimental trials measuring simulated or on-road driving performance, as well as discrete cognitive skills such as divided attention, tracking, and information processing. Studies were categorized by route of administration (oral vs. inhaled) and participant usage patterns (regular vs. occasional/non-regular users). Data were synthesized using four-level meta-analyses to account for dependency among effect estimates, with Hedges’ *g* used to quantify impairment magnitude. Methodological quality was assessed using the Rosendal scale, and only studies scoring ≥50% were included in quantitative synthesis. The results demonstrated that Δ9-THC significantly impaired several measures of driving performance and cognitive skills at peak effect levels (*p* < 0.05). Specifically, lateral control (e.g., lane weaving), tracking, and divided attention were notably affected. Meta-regression analyses revealed that regular cannabis users experienced significantly less impairment than occasional users (*p* = 0.003), likely due to tolerance. The magnitude of impairment depended on dose, time post-administration, and the specific skill assessed. Crucially, the analysis predicted that for occasional users inhaling 20 mg of Δ9-THC, most driving-related cognitive skills would recover to baseline (Hedges’ *g* = –0.25) within approximately 5 hours, with nearly all skills recovering within 7 hours. Oral administration was associated with a longer duration of impairment. The study concludes that individuals should wait at least 5 hours after inhaling cannabis before engaging in safety-sensitive tasks like driving. These findings provide evidence-based guidance to inform public health recommendations and legal policies, addressing the current inconsistency in regulations regarding cannabis-impaired driving. The results highlight that while impairment is significant, it is not indefinite, and tolerance plays a substantial role in mitigating acute effects.

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Topics

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• cannabis
• polydrug
• illicit drugs