Teens who use cannabis are at a higher risk of developing depression and suicidal behaviour in young adulthood, compared with those who do not use the drug, according to a new study led by researchers in Montreal.

The findings, published Wednesday in the journal JAMA Psychiatry, suggest a greater need for education about the mental-health risks associated with cannabis, says lead author Gabriella Gobbi, a researcher at the Research Institute of the McGill University Health Centre.

“A lot of young students and parents are not informed about the risks … of cannabis. They think it’s a light herb because it’s natural,” she says.

While previous research has linked cannabis to psychosis and schizophrenia, this new meta-analysis investigates the impact of the drug on young people’s risk of mood disorders and suicide. The findings provide further evidence to suggest cannabis may be particularly harmful to developing teenage brains.

The researchers conducted a systematic review and meta-analysis of 11 studies, involving a total of 23,317 participants. While they discovered the risk for anxiety was not statistically significant, they found daily-to-weekly cannabis use was related to a high risk for suicidal attempts, and a low to medium risk for developing depression.

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. It means earlier cannabis use may be linked to an estimated 7 per cent of young adults with depression, she says.

The study found only an association, and not a causal relationship, between cannabis use and later depression and suicidal behaviour. But Dr. Gobbi notes that only studies in which young participants were healthy prior to using cannabis were included in the analysis. So in these studies, participants did not start using cannabis because they were depressed, she explains. Rather, they developed depression after they started using the drug.

Among the limitations of the meta-analysis, the researchers noted not all of the studies they analyzed accounted for other drugs or psychosocial factors that may be linked to depression and early cannabis consumption, and they used different methods for detecting major depressive disorder. The researchers were also unable to evaluate the quantity or potency of cannabis that participants consumed.

Catherine Orr of Australia’s Swinburne University of Technology, who was not involved in the study, says there is a lot of research that suggests adolescents are more vulnerable to the potential brain effects of cannabis than adults.

“We cannot 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. This “pruning” refers to the natural elimination of unnecessary brain connections. Dr. Orr explains it is possible that cannabis consumption may disrupt this process.

At Ontario’s Pine River Institute, a treatment centre for young people ages 13 to 19, chief executive Vaughan Dowie says cannabis tends to be the drug of choice among those at the centre. Many of the teens he sees use cannabis to self-medicate for their mental-health issues. Many also consider cannabis to be harmless, he says.

“For a lot of kids, it is no big deal,” Mr. Dowie says, noting not everyone is adversely affected. “But it’s important for kids to know that it is not a benign substance, that there are dangers and risks associated with it.”

The Globe and Mail, February 13, 2019