In 1999, in France, before considering modifications in drug legislation, the government requested a study of the effect of illicit drugs on the risk of road crashes. It implemented a systematic screening of illicit drugs for all drivers involved in fatal crashes between October 2001 and September 2003. Within the European DRUID project, the study was restricted to car drivers.

The project reported here is a responsibility analysis and, as such, it belongs to the framework of case-control studies;​ the outcome of interest is “being responsible for a fatal crash”. It was assessed with a method adapted from Robertson and Drummer. Cases are the 4,946 car drivers who are responsible for the crash;​ controls are the 1,986 car drivers selected from the non-responsible car drivers, in a way that makes the control group similar to the general driving population.

The effect of cannabis on fatal crash responsibility is significant after adjustment for age, sex and alcohol:​ adjusted odds ratio is 1.89 [1.43-2.51]. The dose-response effect is significant (p=0.0001). For alcohol (≥0.1 g/l), the adjusted odds ratio for responsibility is 8.39 [6.95-10.11]. No interaction was found between alcohol and cannabis. For amphetamine, cocaine and opiates, adjusted odds ratios were not significantly different from 1. However the statistical power is low.

The study finds similar odds ratios for alcohol as previously published. For cannabis, the significant odds ratio together with the significant dose-response effect indicates a causal relationship between cannabis and road crashes. A multiplicative effect between cannabis and alcohol was noted.