Does the co-use of cannabis exacerbate the effect of alcohol on the risk of crashing? A systematic review and meta-analysis

This is the second of two closely related systematic literature reviews with meta-analyses by the same authors. The first concluded that there is no convincing epidemiological evidence that the prior use of cannabis (as indicated by the presence of THC in a body fluid) directly increases the risk of crashing. The purpose of this second review is to consider the possibility that the co-use of cannabis indirectly increases the risk of crashing by exacerbating the effect of alcohol on the risk of crashing. The starting point for both reviews is a systematic literature search that identifies seventeen epidemiological studies of the relationship between the prior use of cannabis and the risk of crashing. Twelve of those studies also investigated the joint effects of cannabis and alcohol on the risk of crashing. Although the authors of some studies claimed to have found an exacerbation (or equivalent) effect, the results of a meta-analysis of exacerbation odds ratios failed to support the claim. It is concluded that there is no credible evidence from the twelve epidemiological studies that the co-use of cannabis exacerbates the effect of alcohol on the risk of crashing.