Smoking Marijuana Doubles Risk of Fatal Accidents

Larger Doses Can Triple the Risk, Study Finds

Smoking Driver
Marijuana Impairs Driving Skills. © Getty Images

Driving after smoking even a small amount of marijuana almost doubles the risk of a fatal highway accident, and heavier doses of marijuana can more than triple the risk of dying behind the wheel.

A study by the French National Institute for Transport and Safety Research of more than 10,000 drivers involved in fatal crashes between 2001 and 2003 found that seven percent of drivers involved in a fatal highway crash used marijuana.

All the drivers in the study were involved in auto crashes in which someone died and the drivers were taken by police to hospitals where they were given a urine or blood test for cannabis, amphetamines, opiates, and cocaine as well as alcohol.

The researchers estimated that at least 2.5 percent of the 10,748 fatal crashes studied were directly caused by the use of marijuana.

Small Amounts Can Cause Impairment

The researchers concluded that the risk of being responsible for a fatal crash increased as the blood concentration of THC, the active ingredient in marijuana, increased. Even small amounts of marijuana could double the chance of a driver suffering an accident, researchers said, and larger doses could more than triple the risk.

The number of highway deaths contributed the smoking weed were significant, even though they were dwarfed by the number caused by drinking alcohol. Of the drivers involved in fatal accidents, 21.4 percent tested positive for alcohol consumption.

Alcohol was estimated to be responsible for 28.6 percent of all fatal highway accidents.

The French research found that 2.9 percent of drivers involved in fatal crashes tested positive for both marijuana and alcohol. Men were more often involved in fatal crashes than women and men more often tested positive for both marijuana and alcohol.

Drugged Driving Is 'Totally Irresponsible'

Young drivers and drivers of motorcycles and mopeds were also more likely to test positive for both substances.

"Research like this proves just how dangerous it is to take drugs, and then get behind the wheel of a car," Roger Vincent, of the Royal Society for the Prevention of Accidents, told the BBC. "It is totally irresponsible, as taking drugs such as cannabis does affect your reactions."


Laumon, B, et al. "Cannabis intoxication and fatal road crashes in France: population based case-control study." British Medical Journal December 2005.

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