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Study: Marijuana Use Doubles Risk of Fatal Car Accidents

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Smoking marijuanaA new study has found that driving within three hours of smoking marijuana doubles the risk of serious car accidents, especially those resulting in fatalities. The study was done at Canada’s Dalhousie University and was published on Thursday in the British Medical Journal.

Researchers came to their conclusions after examining the results of nine studies involving 49,411 people, including drivers of cars, vans, sport utility vehicles, light or heavy trucks, buses, motorcycles or scooters, all-terrain vehicles, and snowmobiles. The research process, called a meta-analysis, looks for patterns in the various studies.

As Steven Reinberg writes for, marijuana makes it harder for drivers to judge distance, and drivers under its influence often tailgate and swerve from lane to lane, which cuts down on their reaction time and leads to crashes, according to lead researcher Mark Asbridge, an associate professor in Dalhousie University’s community health and epidemiology department.

Reinberg reports:

Studies on the effect of driving under the influence of marijuana have had mixed results, [Asbridge] said.

‘There were some studies finding that cannabis actually had a negative association with crash risk, so people were actually safer using cannabis driving than when they weren’t, but these were poorly designed studies,’ Asbridge said.

‘So our study gives some clarity to the issue in showing a doubling of the risk in the very best studies that are out there and adds some level of justification to existing policies that restrict drug-impaired driving,’ he said.

The research found that fatally injured drivers had higher amounts of tetrahydrocannabinol, or THC (the principal psychoactive constituent of the marijuana plant) in their blood than non-fatally injured drivers, either from heavier consumption of marijuana, or because there was a shorter amount of time between when they consumed marijuana and when their blood was tested than for drivers with non-fatal injuries.

But the study points out that blood testing of drivers killed in crashes is done sooner than blood testing of drivers with non-fatal injuries. Drivers in non-fatal crashes are likely to refuse drug testing, the study says. Netdoctor notes that the study only looked at collisions involving serious injuries or deaths, and thus did not assess marijuana as a risk factor for minor collisions.

The study, funded by the Canadian Institutes of Health Research, shows that although alcohol is the substance most often found in drivers involved in vehicle crashes, “[s]urveys of young drivers have also shown that rates of driving under the influence of cannabis have surpassed rates of drinking and driving in some jurisdictions.”

Reinberg reports:

These findings reaffirm many of our accepted understandings regarding acute cannabis intoxication and psychomotor performance,’ said Paul Armentano, deputy director of NORML (the National Organization for the Reform of Marijuana Laws). ‘That is why operating a motor vehicle while acutely impaired by cannabis is presently a criminal offense in all 50 states.’

While many experts say that roadside testing could be a useful way to identify drivers under the influence of marijuana, there are no handy devices like breathalyzers to tell if someone has recently used marijuana. Reinberg quotes Dr. Guohua Li, a Columbia University professor of epidemiology, as saying that it is hard to determine with certainty if a driver testing positive for marijuana is indeed impaired at the time of testing, because THC, the active ingredient in marijuana, can be detected for several weeks after use. “This issue is especially urgent and important in light of the ongoing epidemic of drugged driving and increased permissibility and availability of marijuana worldwide,” Li said.

Jennifer LaRue Huget writes in The Washington Post that this study comports with a similar study published in October in the journal Epidemiological Reviews, in which Dr. Li was one of the researchers.

Image by arturo ponciarelli (Gigi Galiazzo), used under its Creative Commons license.


2 Responses to “Study: Marijuana Use Doubles Risk of Fatal Car Accidents”

  • Thank you Judy Pokras for that very important factoid
    about driving safety. I am beginning to think the DEA should
    regulate medicinal use of THC more stringently.
    As always it probably is a case of “follow the money”.
    Lots of money in growing marijuana for legal sale. LOTS.

  • Judy Pokras says:

    You’re welcome, Dr. Dupont. Thanks for reading this blog and sharing your opinion.


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