Smoking cannabis within three hours of driving doubles the chance of causing a serious accident, claims a paper published today in the British Medical Journal (BMJ).
The risk of collision is substantially higher if the driver is aged under 35, it said.
The study seeks to filter out factors such as alcohol use that can skew inquiries into cannabis and road accidents.
These investigations were deemed to be of high quality because the driver had given a blood sample after the accident or admitted to smoking cannabis in the runup to the event.
The nine studies also matched cannabis users against counterparts who did not take the drug to get an idea of the level of risk.
The overview by researchers from Dalhousie University in Canada's Nova Scotia, was unable to investigate a key question -- how much of cannabis's active ingredient, tetrahydrocannabinol (THC), is needed to impair driving or worsen the risk of a smash.
Cannabis may also be a risk factor for minor collisions, which were not part of the study's scope, the authors added.