Driving Under the Influence of Marijuana Raises Motor Vehicle Crash Risk, Study Finds

Driving under the influence of marijuana is associated with an increased risk of a motor vehicle crash, especially for fatal collisions, an analysis of nine studies concludes.

The analysis found driving under the influence of marijuana was associated with almost twice the risk of a motor vehicle crash compared with unimpaired driving, CNN reports. The studies in the analysis included nearly 50,000 people.

The results are published in the British Medical Journal. According to a press release issued by the journal, this is the first review to look at observational studies concerned with the risk of vehicle collision after the use of marijuana. “Previous studies have failed to separate the effects of alcohol and other substances from the use of cannabis, resulting in a lack of agreement,” the release notes.

Lead researcher Mark Asbridge of Dalhousie University in Halifax, Nova Scotia, said while alcohol impairs drivers’ speed and reaction time, marijuana affects spatial location. He said drivers who have recently smoked marijuana may follow cars too closely, and swerve in and out of lanes. He added that while people who are drunk often recognize they are impaired by alcohol, those under the influence of marijuana often deny they are impaired.

A 2009 report from the National Highway Traffic Safety Administration (NHTSA), based on blood, breath and saliva tests collected on weekends from drivers in 300 locations nationally, found that 16.3 percent of drivers at night were impaired from legal or illegal drugs, including 9 percent of drivers who had detectable traces of marijuana in their system.

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    February 24, 2013 at 10:45 AM

    “first review to look at observational studies concerned with the risk of vehicle collision after the use of marijuana….”

    observational study.. just lost all scientific credibility there.

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    Dan R. Gray, ICRAADC, MARS

    November 16, 2012 at 7:06 PM

    THC is easily detectable in the system for as long as 30+ days. THC screens are grossly unreliable. The report has been seriously “spun” in the press release. The study was done on immediate heavy use of high THC content marijuana and on “driving simulators” and not out in the traffic. I am not sure where the real source of funding for this “research” came from, however; I suspect that it came from some source that had a direct money interest in showing how bad THC could under extreme circumstances cause highway problems. The beer and malt liquor industry in the British Isles and related British populated countries is huge. This report has so much “spin” in it that someone that is a long-time professional in the addictions field would spot it immediately. I personally discount it as being mostly “junk science”. That is simply a personal and addictions professional opinion. I would urge all well qualified addictions professionals to read the entire abstract and arrive at their own conclusions.

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    February 15, 2012 at 8:43 PM

    Present-day accident statistics may be inflated from fact that prohibition drives many users to confine their cannabis dosage to a place where it is least likely to be observed– in the car. To @Scott’s reasonable concerns: the first hour(s) after cannabis use should be reserved for Creative activities where one can perform and improvise and be exempted from the requirment of sustained closely focused watch or management over situations such as car driving. However I admit having experienced driving a pallet and cardboard recycling truck “in the first hour” without incident– because in fact the travel route of such a vehicle (industrial streets, loading-docks) never requires driving over 20 miles an hour.

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    janet long

    February 11, 2012 at 3:10 PM

    This would be one of my major concerns over rx marijuana. A physician issuing dxes for such was in our state this week.

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    February 11, 2012 at 1:56 PM

    I agree with the general thrust of this article. I’ve never thought that it was reasonable to assume that driving under the acute influence of marijuana would not cause impairment sufficient to lead to impaired driving and accidents. And, I am pro-legalization and pro medical marijuana. The profound damage done by incarceration and allowing needless suffering are quite consequential.

    However, I depart company with those who wrote this study because there has never been a test that can provide an acute marijuana level equivalent to that used for alcohol. Alcohol is excreted rapidly from the system in such a way that the breath alcohol tests used provide a reasonable measure to the level of acute intoxication. The tests cited by the authors of this study do not do that. They measure only the presence of marijuana without regard to the level of acute intoxication. Until such a test is devised, these results fall only into the realm of reasonable assumptions and cannot be considered to be sound science that would support appropriate social policy.

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