The Belgium study estimated the odds ratio for motor vehicle risk with cannabis at about 5.8.
Asbridge and colleagues recently published a review of acute cannabis consumption and MVA risk. This review published in the British Medical Journal preceded the Belgium study. It included analysis of nine independent studies using a variety of methodologies.
Examining the effect of cannabis use on MVA risk by specific study methodology found the following odds ratio estimates:
- Case-control studies: 2.79
- High quality studies: 2.21
- Fatal collision studies: 2.10
- Medium quality studies: 1.78
- Non-fatal collision studies: 1.74
- Culpability studies: 1.65
In the perspective of previous studies, the Belgium study produced a higher estimate of the role of cannabis in accident risk.
There are individual studies that have failed to find an association of MVA risk with cannabis use. However, the Asbridge et al meta-analysis suggests the preponderance of evidence finds an elevated risk of MVA with cannabis use. Using a best estimate of the odds ratio from combining studies produces a summary odds ratio of 1.92.
The authors note experimental studies of cannabis use on cognitive function have been done "suggesting that cannabis impairs performance of the cognitive and motor tasks necessary for safe driving, increasing the risk of collision".
A key methodological issue in estimating the role of cannabis use in MVA risk is the potential role of alcohol and other drug effects. Many drivers using cannabis have additional drugs and alcohol in blood toxicology tests. This makes estimating the specific effects of cannabis difficult.
Additionally, there is very little research looking at the effect of acute cannabis use versus chronic cannabis use on cognitive performance and MVA accident risk. This is an topic that deserves additional research attention.
The evolving use of driving simulator technology provides an opportunity to better understand the role of alcohol, cannabis, other illicit drugs and prescription drugs on motor vehicle driving performance.
In upcoming posts, I will examine how driving simulators are providing important insight in the effect of a variety of cognitive factors in driving performance.
Halloween photo from Busch Gardens in Tampa, Florida from the authors files.
Asbridge M, Hayden JA, & Cartwright JL (2012). Acute cannabis consumption and motor vehicle collision risk: systematic review of observational studies and meta-analysis. BMJ (Clinical research ed.), 344 PMID: 22323502