09/09/2014 06:30 EDT | Updated 11/09/2014 05:59 EST

Marijuana use by teens linked with problems in young adulthood

Frequent pot use by teens is linked to a greater likelihood of incomplete education, suicide attempts and other harmful effects, say researchers in Australia and New Zealand, who suggest their findings should be considered as countries move to decriminalize or legalize cannabis.

Marijuana is the most widely used illicit drug worldwide, and statistics suggest that adolescents in some countries are starting to use it at a younger age and more heavily.

In 2013, about 23 per cent of students surveyed in Ontario said they’d used cannabis at least once in the previous year, according to an annual report from the Centre for Addiction and Mental Health.

The prevalence of past-year cannabis use among Canadians aged 15 years and older was 10 per cent in 2012, the Canadian Alcohol and Drug Use Monitoring Survey suggested.

Cannabis use is more common among those with low educational attainment, but there’s a debate about whether marijuana use is a marker or a cause. A study published in the journal The Lancet Psychiatry helps to answer some questions.

Long-running studies

Researchers analyzed data on up to 3,765 participants who used marijuana from three long-running studies in Australia and New Zealand. The studies compared those who had never used pot with those who had and their developmental outcomes, which were assessed for the participants up to 30 years of age.

"Study findings suggest that adolescent cannabis use is linked to difficulties in successfully completing the tasks that mark the transition to adulthood," study author Richard Mattick, a professor of drug and alcohol studies at the University of New South Wales and his co-authors concluded.

"Prevention or delay of cannabis use in adolescence is likely to have broad health and social benefits."

The findings are relevant given the move in some countries to decriminalize or legalize cannabis, which raises the possibility that the drug might become more accessible to young people, the researchers said.

In the study, those who used marijuana daily before age 17 were less likely to complete high school or earn a degree compared with those who’d never used it.

Cannabis use was associated with increased risk of suicide attempts and later cannabis dependence and use of other illicit drugs, said Merete Nordentoft, a professor of psychiatry at the University of Copenhagen, in a journal commentary published with the study.

Harms of frequent cannabis use

Nordentoft said the "convincing results" are valuable and appropriate given several U.S. states and countries in Latin America and Europe have decriminalized or legalized cannabis and allow unrestricted marketing of the drug.

Increasing evidence shows that brain development during adolescence can be harmed by frequent cannabis use and cognitive functions can be permanently reduced, she said.

Young people need to develop and mature and prepare themselves to meet demands in adult life.

"Cannabis use, especially frequent use, impairs this development and reduces the likelihood that a young person will be able to establish a satisfactory adult life," Nordentoft concluded.

The researchers acknowledged that the measurements were based on self-reported data, which could lead to over-reporting or under-reporting of cannabis use. They said rates of cannabis use by young people in their study are similar to those in Canada, the U.S. and the U.K., but the social and legislative context of cannabis varies between regions.

The study was funded by the Australian Government National Health and Medical Research Council.