Cannabis and the Risk of Road Crashes

THC-positive cases (no other drugs or alcohol detected) showed an elevated relative risk compared to drug free drivers suggesting increased crash risk with recent cannabis use. When alcohol or other psychoactive drugs were also present this crash risk increased substantially. Recent use of cannabis therefore appears to be associated with an increased risk of a fatal car crash similar to a BAC of 0.10 g/100 mL.

The most controversial aspect of the involvement of drugs in accident causation is that of cannabis. Previous reports using responsibility analysis of Australian drivers have relied on coroners records in which forensic laboratories only measured the inactive form of cannabis (carboxy-?9-THC). Following the use of cannabis, this species is present in blood for up to several days and therefore its presence cannot be used to imply recent use of cannabis, and therefore likely impairment. Those studies show an odds ratio of a control driver of close to 1.0, implying no increase in risk for a cannabis user. Since 1998, most Australian forensic laboratories have measured THC in fatal road crashes.

While alcohol over-involvement has been demonstrated by a number of epidemiological studies, there are only a relatively few that have clearly demonstrated the risk of crashes while using cannabis. This presentation reviews the evidence of the involvement of cannabis in road crashes and includes the results of a major Australian study conducted over several years that has examined the over-representation of drugs in fatal road crashes. The authors have used a form of responsibility analysis to provide a measure of involvement of cannabis and other drugs in about 3400 fatal crashes (1).