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

The increasing legal availability of cannabis has important implications for road safety. This systematic review characterised the acute effects of Delta(9)-THC on driving performance and driving-related cognitive skills, with a particular focus on the duration of Delta(9)-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 Delta(9)-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) Delta 9-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 similar to 5-hs (and almost all within similar to 7-hs) of inhaling 20 mg of Delta(9)-THC; oral Delta(9)-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.

Automated summary

The study found that cannabis has acute effects on driving performance and related cognitive skills, with some measures showing impairment. Long-term cannabis users experienced less impairment compared to other users, and factors such as ingestion method, dosage, and time affected the duration of Delta(9)-THC's effects. The recommendations suggest waiting at least 5 hours after inhaling cannabis before engaging in safety-sensitive tasks.