Young people who regularly smoke pot do less well at school than their abstinent peers and those who drive under the influence of cannabis double their risk of crashing, Wayne Hall, professor of addiction policy at London's King's College, said.
Hall's research, published in the journal Addiction, explodes the myth that marijuana smoking is benign - a safer alternative to cigarettes or harder drugs.
Hall, an Australian who also leads of Queensland University's centre for research into substance abuse among young people, said one in 10 regular pot smokers is likely to go on to take harder drugs.
"The risk goes up to one in six among young people who begin using it in their teens," he said.
And there was a warning for older users: middle-aged pot smokers raise their risk of bronchitis and also, probably, of a heart attack.