The effects of cannabis intoxication on motor vehicle collision revisited and revised

To determine whether and to what extent acute cannabis intoxication increases motor vehicle crash risk.

Study 1 replicates two published meta-analyses, correcting for methodological shortcomings. Study 2 is an updated meta-analysis using 28 estimates from 21 observational studies. These included studies from three earlier reviews, supplemented by results from a structured search in Web of Science and Google Scholar, and by the personal libraries of the research team. Risk estimates were combined using random-effects models and meta-regression techniques.

Study 1 replicates the analysis of Asbridge et al., based on nine studies from five countries, published 1982-2007; and Li et al., based on nine studies from six countries, published 2001-10. Study 2 involves studies from 13 countries published in the period 1982-2015.

In study 1, total counts extracted totalled 50 877 (27 967 cases, 22 910 controls) for Asbridge et al. and 93 229 (4236 cases and 88 993 controls) for Li et al. Study 2 used confounder-adjusted estimates where available (combined sample size of 222 511) and crude counts from the remainder (17 228 total counts), giving a combined sample count of 239 739.

Odds ratios (OR) were used from case-control studies and adjusted OR analogues from culpability studies. The impact of the substantial variation in confounder adjustment was explored in subsample analyses.

Study 1 substantially revises previous risk estimates downwards, with both the originally reported point estimates lying outside the revised confidence interval. Revised estimates were similar to those of study 2, which found cannabis-impaired driving associated with a statistically significant risk increase of low-to-moderate magnitude [random-effects model OR 1.36 (1.15-1.61), meta-regression OR 1.22 (1.1-1.36)]. Subsample analyses found higher OR estimates for case-control studies, low study quality, limited control of confounders, medium-quality use data and not controlling for alcohol intoxication.

Acute cannabis intoxication is associated with a statistically significant increase in motor vehicle crash risk. The increase is of low to medium magnitude. Remaining selection effects in the studies used may limit causal interpretation of the pooled estimates.