Cannabis Use Doubles Risk for Serious Road Accidents

Caroline Cassels

February 09, 2012

February 9, 2012 — Drivers who consume cannabis within a 3-hour period before driving are nearly twice as likely to cause a serious car accident, resulting in serious injury or death, as those who are not under the influence of drugs or alcohol, a new meta-analysis shows.

The systematic review of observational studies examining the risk for vehicular collision after consuming cannabis is the first of its kind.

"[T]his meta-analysis of studies examining acute cannabis consumption and motor vehicle collisions, with adequate control groups, found a near doubling of risk of a driver being involved in a motor vehicle collision resulting in serious injury or death," the investigators, led by Mark Asbridge, PhD, from Dalhousie University, Halifax, Nova Scotia, Canada, write.

Previous studies, the authors note, have failed to distinguish the effects of alcohol and other substances from the effects of the use of cannabis, resulting in a lack of consensus.

"An up-to-date systematic review is necessary to integrate the existing evidence on the role of cannabis use on collision risk, not only from a public policy and programme perspective, but also in view of the current gaps in scope and quality of literature and methodology," the investigators write.

The study is published online February 9 in the BMJ.

Most Widely Used Illicit Substance

Cannabis is the most widely used illicit substance in the world. The investigators note that rates of driving while under the influence of cannabis have increased. In a recent Scottish survey, 15% of respondents aged 17 to 39 years admitted to using cannabis within 12 hours of driving a vehicle.

Furthermore, the European Monitoring Centre for Drugs and Drug Addiction found that between 0.3% and 7.4% of drivers tested positive for cannabis from roadside surveys in the United Kingdom, Denmark, the Netherlands, Norway, the United States, and Australia.

To determine whether acute consumption of cannabis by drivers increases the risk for motor vehicle collisions, the investigators conducted a meta-analysis of observational epidemiological studies of motor vehicle collisions that included an appropriate control group, and studies that measured recent cannabis use in drivers via toxicological analysis of whole blood or self-report.

Experimental and simulator studies were excluded. The review included a total of 9 studies and 49,411 participants.

All motor vehicle accidents included in the study took place on a public road and involved 1 or more moving vehicles, including cars, vans, sport utility vehicles, trucks, buses, and motorcycles.

Collision outcomes included single or multiple vehicles, which might have led to injuries or death and that could be measured via administrative data or self-report.

The researchers found that driving under the influence of cannabis was associated with a significantly increased risk for motor vehicle collisions compared with unimpaired driving (odds ratio [OR], 1.92; 95% confidence interval [CI], 1.35 - 2.73; P = .0003).

The investigators also report that estimates of collision risk were higher in case-control studies (OR, 2.79; 95% CI, 1.23 - 6.33; P = .01) and studies of fatal collisions (OR, 2.10; 95% CI, 1.31 - 3.36; P = .002) than in culpability studies (OR, 1.65; 95% CI, 1.11 - 2.46; P = .07) and studies of nonfatal collisions (OR, 1.74; 95% CI, 0.88 - 3.46; P = .11).

"To our knowledge, this meta-analysis is the first to examine the association between acute cannabis use and the risk of motor vehicle collisions in real life settings. This review is important to enhance our understanding of the effects of cannabis consumption on collision risk, and provides a more definitive statement on the direction that efforts in public policy and intervention should take in addressing road safety," the researchers write.

Benefits of Roadside Drug Testing Questioned

In an accompanying editorial, Wayne Hall, PhD, from the University of Queensland in Australia, said that the study reinforces the premise that cannabis users should be deterred from driving while under the drug's influence.

However, he notes, "public health education about the dangers of driving while under the influence of cannabis is unlikely to be enough to deter cannabis users from driving — they will also need to be persuaded that they are at risk of their cannabis use being detected."

Although some countries, including jurisdictions in western Europe, Australia, and the United States, have introduced roadside saliva testing for cannabis and other drugs, such as methamphetamine and MDMA (3,4-methylenedioxymethamphetamine), the rationale for such testing remains unclear.

Dr. Hall notes that this is largely because the association between blood levels of tetrahydrocannabinol, the active ingredient in cannabis, and driving impairment is unclear and a "cut off value of drug detected in the saliva has not been defined."

"Better evidence is essential if lawmakers in countries that have already implemented roadside drug testing are to make informed decisions about whether it has produced public health benefits at an acceptable social and economic cost, and to inform those in countries that are considering introducing such testing," he writes.

The investigators and the editorialist have disclosed no relevant financial relationships.

BMJ. Published online February 9, 2012.

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