Researchers from Dalhousie University found that, individuals are almost two times more likely to cause a vehicle collision if they consume cannabis within three hours of driving than individuals not under the influence of alcohol or drugs. The study is published today on bmj.com.

In order to find out whether cannabis consumption increases the risk of a motor vehicle collision, the researchers examined nine investigations involving a total of 49,411 individuals.

This is the first study to examine different observational investigations concerned with the risk of vehicle collision after consuming cannabis. Earlier ones were unable to separate the use of cannabis from the effects of alcohol and other substances, which made it difficult to come to any conclusions from the findings.

Worldwide, cannabis is the most commonly used illegal substance. According to recent statistics, cannabis use has considerably increased across the world, as well as rates of driving under the influence.

In 2007, a roadside survey conducted in Scotland revealed that 15% of 537 drivers aged between 17 to 39 years reported using cannabis within 12 hours of driving.

In the study, all vehicle collisions occurred on a public road and involved one or more moving vehicles, such as cars, trucks, buses, motorcycles, vans, and sports utility vehicles. Results were obtained via blood samples or direct self-report.

According to results of the study, consumption of cannabis prior to driving a motor vehicle almost doubles the risk of vehicle collision. In addition, earlier results showed that there is a considerably increased risk of collision if the driver is 35 years of age or younger.

The researchers conclude that consumption of cannabis impairs motor tasks important to safe driving, and that it increases the chance of collisions. In addition, they believe future studies should evaluate less severe collisions from a general driving population.

The author of an associated report, from the University of Queensland Center for Clinical Research, questions the benefits of roadside drug testing on public health. He states that more evidence is needed so that countries already conducting drug testing can assist to inform countries that have not yet introduced it.

Written by Grace Rattue