Young people who use cannabis face a greater long-term risk of developing depression and thinking of suicide, a McGill University-led review of published studies suggests.

Dr. Gabriella Gobbi, a psychiatry professor at McGill, and her team in Montreal and London, U.K., analyzed international studies on more than 23,000 people under the age of 18 who were followed for developing depression into young adulthood.

In the long term, those who used cannabis were 1.5 times more likely to have depression and three times more likely to say they had suicidal thoughts compared with nonusers, the researchers said in Wednesday's issue of the journal JAMA Psychiatry.

In Canada, people aged 15 to 25 make up the majority of cannabis users of all ages at an estimated 20 to 33 per cent, the authors said. That's higher than among adolescent users in other developed countries.

In the meta-analysis, Gobbi estimated about seven per cent of depression diagnoses among Canadians aged 18 to 35 are linked to cannabis use. That amounts to about 25,000 Canadians.

"Cannabis is a double-edged sword. It's true that adolescents like cannabis because of its euphoric effect," Gobbi said. "But in the long term, the cannabis produces an opposite effect, so increased the risk of having depression."

Gobbi said they were surprised about suicide behaviour rates attributed to cannabis use, particularly having the thought of suicide cross one's mind.

Previous studies reported cannabis use is associated with lack of motivation.

'Bad consequences years later'

"The impact of suicide was less known. In fact, we were very surprised," Gobbi said.

Suicidal ideation — thinking about one's own demise — is a proxy for suicidal behaviour, said Dr. Anthony Levitt, chief of the Hurvitz Brain Sciences Program at Toronto's Sunnybrook Health Sciences Centre. He's an expert on treatment-resistant depression and wasn't involved in the new review, which he called powerful.

But it doesn't mean you have a plan to do it or the desire to act on the plan. So the findings don't mean that cannabis users are more suicidal, Levitt said.

Cannabis compounds can be therapeutic as well as dangerous, said Toronto psychiatrist Dr. Anthony Levitt. (Craig Chivers/CBC News)

"If somebody decides to use marijuana because it's socially acceptable and not considered that dangerous when they're 14 or 15 and uses it for four, five years and then decides, 'You know what? It's actually interfering with my capacity to function at the level I like' and stops, they don't necessarily go back to zero-risk level. They have an elevated risk of depression … and other bad consequences years later."

But Levitt said when he speaks to people about how cannabis compounds can be therapeutic as well as dangerous, both young people and their parents often struggle to see how cannabis isn't all good or all bad. The truth is somewhere in the middle, he said.

Levitt attributed part of the difficult conversation to how ubiquitous cannabis is in Canadian society.

Prevention and treatment

"Marijuana is one of those substances you can become addicted to, and the behaviours that come with addiction are dangerous. Right now, because it's been legalized, a lot families and a lot of youth are using the argument, 'It can't be that bad.' And so trying to get help sometimes is problematic."

Alcohol takes the edge off in the short term, but it may increase depression in the long term. The same seems to be true for cannabis, Levitt said.

Both Levitt and Gobbi called on medical researchers to intensify the search for ways to prevent the substance use disorder, as well as easier access to treatment programs for the small percentage of cannabis users who become addicted to it.

In terms of prevention, Gobbi pointed to studies from Iceland, where educating parents to improve their dialogue with their children about drugs and programs to involve young people more in activities such as sports seemed to prevent cannabis use among adolescents.

The studies included in the review took place when tetrahydrocannabinol or THC, the active ingredient in marijuana thought to give users a high, were lower than in today's cannabis, Gobbi said.

One of the main limitations of the meta-analysis is the studies included can't be used to determine cause and effect in terms of whether cannabis use produces depression. The studies that were analyzed excluded those where cannabis was used for self-medication, Gobbi said.

There was no association with anxiety.

The study was funded by the Canadian Institutes of Health Research (CIHR) and the Quebec Network on Suicide, Mood Disorders and Related Disorders.

Where to get help

If you're worried someone you know may be at risk of suicide, you should talk to them about it, says the Canadian Association for Suicide Prevention. Here are some warning signs: