Stoned drivers were almost twice as likely to be involved in a fatal car crashes than abstemious drivers, according to a study of 10,748 fatal car crashes in France between 2001 and 2003. More than half of the drivers in the study themselves died as a result of their accidents and all the subjects were tested for drug and alcohol use after crashing.

Even after accounting for factors such as the age of the drivers and the condition of the vehicle, the researchers conclude that cannabis caused a significant number of the fatalities, with 2.5% of the crashes directly attributed to cannabis use. Alcohol was the direct cause of about 29%.

Using cannabis and alcohol together was 16 times more risky than driving with neither drug in their body.

“You are more likely to be involved in a crash, probably because of the drug’s effect on your reaction times and concentration,” says Jean-Louis Martin who carried out the research at the Université Claude Bernard in Lyon, France. “But the drug also makes you more vulnerable to the effects of the crash, so you are more likely to die.” The study did not explore why cannabis smokers fair less well in a crash.

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