February 9, 2012

Cannabis Causes Car Crashes

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Joseph V. Madia, MD By:

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Marijuana intake within three hours of driving nearly doubles risks of motor vehicle collision

(dailyRx News) Those abusing marijuana can no longer claim the drug does no harm. A report published today suggests that the use of cannabis nearly doubles a person’s chance of getting into a car accident.

Nova Scotian research coordinator Jennifer Cartwright, Ph.D., and her associate professors explain, “Driving under the influence of cannabis was associated with a significantly increased risk of motor vehicle collisions compared with unimpaired driving,” notably for fatal accidents.

"Don’t smoke marijuana and drive."

Dr. Cartwright and her colleagues work at Dalhousie University, where they gathered thousands of studies involving motor vehicle collisions. The team used a systematic approach to narrow the studies down to nine, focusing on control-based research that was observed firsthand in real-life settings,  All studies assessed marijuana use either via blood or from self-reports.

While traces of alcohol and other substances tarnish most data on marijuana’s link to car accidents, the authors’ primarily focused on research observing the sole presence of THC, or tetrahydrocannabinol, the psychoactive component of marijuana found in the blood.

Finally, Dr. Cartwright and her coworkers called in two independent reviewers to determine the biases of each study using the Newcastle–Ottawa scale, an internationally accredited measure for assessing the quality of nonrandomized studies.

Once the studies were given a stamp of approval, the team performed a meta-analysis on the studies and began drawing their conclusions.

“After a systematic review of the literature, this 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 wrote.

The meta-analysis revealed that those who operated a motor vehicle within three hours of ingesting marijuana increased their risks of getting into an accident 1.92 times.

Furthermore, the team found increasing risks when analyzing fatal car collisions, illustrating marijuana’s severe influence on our motor skills and ability to operate a vehicle.

Dr. Cartwright explains that these results are supported by several experimental studies assessing marijuana’s affects on driving in a lab or simulation. This study came available online through BMJ on February 9, 2012 and is funded by the Canadian Institute of Health Research.

No conflicts were reported during the research, and all incorporated studies scored a 7 or higher on a 10-scale of quality assessed by third-party reviewers.

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Reviewed by: 
Joseph V. Madia, MD
Review Date: 
February 9, 2012

Last Updated:
February 9, 2012