Heavy cannabis users are 10 times more likely to be injured, or to injure others, in car accidents, researchers have found.
The study confirms the link between cannabis and crashes
The scientists from the University of Auckland, New Zealand, say their study is the first proof that there is a link between using cannabis and accidents.
Previously, there was only laboratory research and post mortem evidence to suggest a connection.
UK experts said the Addiction study showed more education was needed.
Doctors had believed there was a link between cannabis use and accidents for some time, but had been unable to prove a connection.
'Target heavy users'
The New Zealand team surveyed 571 drivers of cars involved in crashes in which at least one occupant was hospitalised or killed and a control group of 588 drivers randomly selected from cars driving in Auckland.
They were asked if they had taken cannabis in the three hours prior to the crash or survey and were also asked about their use of cannabis over the previous 12 months.
It was found that habitual cannabis users were 9.5 times more likely to be involved in crashes, with 5.6% of people who crashed having taken the drug compared to 0.5% of the control group.
Their risk of an accident was increased whether or not they had used cannabis immediately before the accident.
However the researchers said more research was needed to understand how cannabis use was linked to risk-taking behaviour, and how other factors - such as the person also using alcoho - might affect.
The researchers, led by Dr Stephanie Blows, said: "The prevalence of self-reported recent cannabis use in the Auckland driving population was less than 1%, and those who did use marijuana prior to driving were highly likely to be habitual users.
"This suggests that interventions targeting high risk marijuana use groups may be more cost-effective than random roadside testing."
Professor Robert West, editor of Addiction, said: "People have been saying for some time that drugs increase your risk of an accident, but there was no good evidence to show that.
"It shows public information campaigns would be a good idea."
Martin Barnes, chief executive of Drugscope, said: "Cannabis can impact on your perception and your responses."
He added: "Obviously if you have been taking drugs you should really not be driving."