Driving after taking even small amounts of cannabis almost doubles risk of a fatal road accident, research suggests.
Drugs can slow reaction times
The French National Institute for Transport and Safety Research found evidence of cannabis use among 7% of drivers involved in fatal crashes.
However, the figure was dwarfed by the 21.4% who tested positive for alcohol consumption.
The British Medical Journal study was based on 10,748 drivers involved in fatal crashes between 2001 and 2003.
All of the drivers had compulsory tests for drugs and alcohol.
The researchers found the risk of being responsible for a fatal crash increased as the blood concentration of cannabis increased.
While even small amounts of cannabis could double the chance of a driver suffering an accident, larger doses could more than triple the risk.
The findings also showed 2.9% of drivers tested positive for both cannabis and alcohol use.
Men bigger offenders
Men were more often involved in crashes than women, and were also more often positive for both cannabis and alcohol.
The same was true of young drivers, and users of mopeds and motorcycles.
The study showed that the prevalence of cannabis in the driving population at 2.9% was similar to that for alcohol at 2.7%.
The researchers estimated that at least 2.5% of fatal crashes were directly attributable to cannabis use.
However, alcohol was estimated to be responsible for 28.6%.
Writing in the BMJ, the researchers concluded: "Driving under the influence of cannabis increases the risk of involvement in a crash."
Roger Vincent, of the Royal Society for the Prevention of Accidents, told the BBC News website public education campaigns were needed.
He said: "Research like this proves just how dangerous it is to take drugs, and then get behind the wheel of a car.
"It is totally irresponsible, as taking drugs such as cannabis does affect your reactions."
Dr John Heyworth, past president of the British Association of Accident and Emergency Medicine, said increasing numbers of people who had taken various types of drugs were being treated following road traffic accidents, and other incidents.
He said few people had taken cannabis alone - often it was combined with alcohol or other types of drugs, such as cocaine or speed.
"Sometimes it is difficult to know whether a person's behaviour is linked to their injuries, or to the drugs they have taken," he said.