Scientists at McGill University analyzed 11 worldwide longitudinal studies involving people aged 18 to 32 who smoked weekly or daily when they were 18 or younger.
Researchers estimate seven per cent of Canadians between the ages of 18 and 30 are diagnosed with depression that could be linked to prior cannabis use, or roughly 25,000 young people. Researchers found even stronger links to suicidal ideation and suicide attempt, but weaker links to anxiety.
The results were published Wednesday in the journal JAMA Psychiatry, and are based on a review of almost a dozen worldwide studies comprising more than 23,000 individuals. It was done in collaboration with Oxford University and Rutgers University-Camden and published Wednesday in the journal JAMA Psychiatry.
For individuals, this risk of depression may be small, Dr. Gobbi says. But given the prevalence of cannabis use among young Canadians (33 per cent of cannabis users are in the 15 to 24 age group, according to National Cannabis Survey data), this risk becomes "very important" at a population level, she says.
"We can not say for certain why this is, but a likely explanation is that adolescence is a period of rapid brain development in which grey matter volume is pruned", says Dr. Orr, who recently published a different study showing structural brain changes in teens who had used cannabis.
"Since we had legalization, young people continue to smoke as before", says Gobbi. "So legalization is not the only response".
"We know from studies of psychosis that the risk is much greater with daily use of modern, high-potency ... cannabis than old-fashioned ... varieties".
The study found only an association, and not a causal relationship, between cannabis use and later depression and suicidal behaviour. So in these studies, participants did not start using cannabis because they were depressed, she explains. Many also consider cannabis to be harmless, he says.
She also notes the subjects were teens during the 1980s, 1990s, and 2000s, when the amount of THC content was generally relatively low.
While the U.S. study suggested a link between early cannabis use and later issues, "we don't know if cannabis use as a teenager is causing these adult mental health problems".
"The findings of this [US] study further reinforce our concerns about the public health implications of any changes we may choose to make to cannabis laws in New Zealand", Boden told the Science Media Centre.
"This review both confirms and reinforces findings from the research literature on the adverse psychological effects of regular cannabis use by mid- to late adolescents", said Dr Joe Boden, the deputy director of the University of Otago's Christchurch long-term health and development study.