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Marijuana Influenced Drivers more Likely to have a Fatal Vehicle Crash

Kathleen Blanchard's picture
Marijuana consumers twice as likely to have a vehicle crash

Marijuana users are found in a new study to be more than twice as likely to be involved in a vehicle crash; especially a fatal collision.

The finding comes from a review of multiple studies showing cannabis consumption within three hours of driving - without alcohol or other drugs on board - can raise the chances of collision.

Researchers from Dalhousie University reviewed nine studies with a total sample of 49,411 people. Results were taken from blood sample or self-reported marijuana use.

Most of the studies used 1 ng/ml of cannabis or any amount greater than zero as the cutoff for a positive test result. One study used self-reporting and another used 2g/ml.

The authors looked at just the effects of marijuana on driving reactions, excluding any effects from alcohol or other drugs for the analysis.

Criteria for a vehicle crash was “a collision or incident that may or may not lead to injury, occurring on a public road and involving at least one moving vehicle”; defined by the World Health Organization.

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All of the accidents included one or more moving vehicles of all types that took place on a public road.

Seven out of nine studies found marijuana use within a few hours of driving raises the risk of having a collision. Two studies concluded the risk of a motor vehicle accident from cannabis use was lower than that of unimpaired drivers.

The reviewers for the study write, “Most studies in our meta-analysis consistently showed an increased effect of cannabis use on the risk of motor vehicle collisions.”

They also found fatal crashes were more likely from higher doses of tetrahydrocannabinol in the blood, compared to non-fatal injuries.

The researchers say the results could be limited by other cofounders that were “probably” not controlled for in the medium to high quality studies used in the analysis.

The finding could be used for public education and by policy makers.to control drug use while driving. The authors concluded marijuana use is associated with increased risk of motor vehicle crash, especially fatal accidents.

BMJ 2012;344:e536

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Wikimedia commons



The headline is not supported by the content of the article. What the article states is that drivers who are under the influence of marijuana are more likely to be in a fatal vehicle crash than drivers who are not. However, being a marijuana consumer does not mean that one drives under the influence of marijuana, just as being an alcohol consumer does not mean one drives drunk. This can be confirmed by comparing state-level rates of marijuana use, collected by the National Survey on Drug Use and Health, with state-level motor vehicle fatality rates collected by the Fatality Analysis Reporting System. Marijuana use and motor vehicle fatality rates actually show a negative statistical correlation, meaning that states with higher rates of marijuana use tend to have lower motor vehicle fatality rates than states with lower rates of marijuana use. Impaired drivers, regardless of the cause of impairment, are a danger to other members of the public and must be punished accordingly. People who consume marijuana and do not drive under the influence should be left in peace just as alcohol consumers are.
Thanks - I've edited the title.
"Talking on a cell phone while driving can quadruple your chances of a crash ending with a hospital visit, according to a new study released by the Insurance Institute of Highway Safety (IIHS)." Cannabis is less dangerous to drivers than cell phones use, so why aren't we talking about cell phones instead of cannabis.
There is much talk and many studies about cell phones and distraction and accidents when driving. In some States it's illegal to text or talk on a cell phone when driving. But this isn't that topic.
Very true, however I was attempting to bring this stat into perspective. "Marijuana users are found in a new study to be more than twice as likely to be involved in a vehicle crash; especially a fatal collision."
Right - but remember, the study authors are ONLY looking at marijuana influence and driving. There is no doubt that prescription drugs, alcohol and cell phones also are safety risks when behind the wheel (or on a bike, motorcycle and even walking) They tried to remove other confounders in the review, but admit the studies they looked at may have had shortcomings. I think the issues is just to keep people safe. There isn't any legislation about driving under the influence of marijuana - but there is for alcohol, cell phone, and prescription drugs. It's a new arena that should be explored as marijuana use is increasingly used medicinally. I mean, why not, right? We don't know the effect on driving, so they're looking at it. It's not something that needs to raise hackles (not yours, but it's rampant in comments on other sites).