image 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.

It is an overview of nine previously-published papers which looked at more than 49,000 people. …