Drivers who consume cannabis within three hours of driving are nearly twice as likely to cause a vehicle collision as those who are not under the influence of drugs or alcohol, claims a paper published recently on the British Medical Journal website.

The paper's authors, from Dalhousie University, reviewed nine studies with a total sample of 49,411 people to determine whether the consumption of cannabis increases the risk of a motor vehicle collision.

This is the first review to look at various observational studies concerned with the risk of vehicle collision after the consumption of cannabis. Previous studies have failed to separate the effects of alcohol and other substances from the use of cannabis, resulting in a lack of agreement.

Cannabis is the most widely used illicit substance globally and recent statistics have shown a significant increase in use across the world. Rates of driving under influence have also increased. A roadside survey carried out in Scotland in 2007 showed that out of 537 drivers tested, 15% aged 17-39 admitted to having consumed cannabis within 12 hours of driving.

All motor vehicle collisions involved in the study took place on a public road and involved one or more moving vehicles such as cars, vans, sports utility vehicles, trucks, buses and motorcycles. Results were taken through blood samples or direct self-report.

Results show that if cannabis is consumed before driving a motor vehicle, the risk of collision is nearly doubled. Previous results have also found that there is also a substantially higher chance of collision if the driver is aged 35 or younger.

In conclusion, the authors suggest that the consumption of cannabis impairs motor tasks important to safe driving, increasing the chance of collisions and that future reviews should assess less severe collisions from a general driving population.

The author of an accompanying editorial, from the University of Queensland Centre for Clinical Research, questions the benefits of roadside drug testing on public health. He argues that further evidence of this is required so that countries already carrying out drug testing can help to inform those countries that have yet to introduce it.