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The increasing legal availability of cannabis has important implications for road safety. This systematic review characterised the acute effects of Δ9-THC on driving performance and driving-related cognitive skills, with a particular focus on the duration of Δ9-THC-induced impairment. Eighty publications and 1534 outcomes were reviewed. Several measures of driving performance and driving-related cognitive skills (e.g. lateral control, tracking, divided attention) demonstrated impairment in meta-analyses of “peak” Δ9-THC effects (p’s<0.05). Multiple meta-regression analyses further found that regular cannabis users experianced less impairment than ‘other’ (mostly occasional) cannabis users (p = 0.003) and that the magnitude of oral (n = 243 effect estimates EE) and inhaled (n = 481 EEs) Δ9-THC-induced impairment depended on various factors (dose, post-treatment time interval, the performance domain (skill) assessed) in other cannabis users (p’s<0.05). The latter model predicted that most driving-related cognitive skills would ‘recover’ (Hedges’ g=–0.25) within ∼5-hs (and almost all within ∼7-hs) of inhaling 20 mg of Δ9-THC; oral Δ9-THC-induced impairment may take longer to subside. These results suggest individuals should wait at least 5 -hs following inhaled cannabis use before performing safety-sensitive tasks.
McCartney et al. (Sat,) studied this question.