Teenagers who use cannabis daily before the age of 17 are more than 60% less likely to complete high school or university compared with those who have never used, an Australian-led study found.



Daily teenage users were seven times more likely to attempt suicide and eight times as likely to use other illicit drugs compared with non-users, the research published in journal the Lancet Psychiatry found.

The researchers have called for their findings to be considered in any cannabis legalisation reform.

There has been much debate over the impact of heavy cannabis use in teenage years, with existing studies too small to draw strong conclusions.

To address this gap, researchers led by the National Drug and Alcohol Research Centre at the University of New South Wales combined the results from three large, long-running studies to examine data from more than 3,700 people.

They linked frequency of use with seven developmental outcomes up to the age of 30: completing high school; obtaining a university degree; cannabis dependence; use of other illicit drugs; suicide attempts; depression, and welfare dependence.

They found a clear association between frequency of cannabis use in adolescence and poor outcomes across most measures, even after controlling for factors such as socioeconomic status and mental illness.

Risks increased as the amount of the drug taken also increased.

“The findings are timely given movement in some states in the US and Latin America to decriminalise marijuana, and there is also a movement here in Australia to decriminalise and legalise the drug for medicinal use,” the lead author of the study, Dr Edmund Silins, said.

“Because our study has shown the potential harms of adolescent use, particularly heavy use, policy makers must be aware of this and reform efforts should be carefully considered to protect against this.

“We can’t say what would happen if people were able to access the drug for medical use but I would say those products would have to be heavily tested and regulated to ensure it didn’t make it easier for teenagers to access the drug.”

Cannabis is the most widely used illicit drug in the world.

In Australia, 1% of 14- to 19-year-olds are daily users, while 4% use weekly. Studies from England show 4% of 11- to 15-year-olds there reported cannabis use within the past month, while in the US, roughly 7% of senior high school students are daily, or near-daily, cannabis users.

The president of the Australian Drug Law Reform Foundation, Dr Alex Wodak, said the evidence for the harms of frequent use presented in the study was”compelling” but added daily use before age 17 would be “pretty uncommon”.

“Daily use of any strong, psychoactive drug at any age, particularly at a young age, is not a good thing so I’m not surprised by the finding,” he said.

“And I don’t have a problem with the claim that efforts to reform cannabis legislation should be carefully evaluated to ensure they reduce adolescent cannabis use, but these studies were carried out in countries that continue to have a fairly punitive approach to cannabis which clearly hasn’t worked, and that’s now become a widely shared view.”

In a comment piece written for the Lancet Psychiatry to accompany the study, a professor of psychiatry at the University of Copenhagen in Denmark said when countries decriminalised or legalised cannabis: “It will probably be followed by decreased prices and increased use, which will lead to more young people having difficulties with school completion and social and personal maturation, and will increase the risk of psychosis.”

But many studies had shown that simply prohibiting cannabis did not make it any less easy for young people to get hold of it, Wodak said.

“A lot of papers that have studied more moderate approaches to cannabis legislation have not found that it results in increased cannabis use,” he said.

“You might intuitively expect that it would increase use, but we have learned many times that empirical evidence trumps intuition. And the actual data, from the US and other countries, has not shown cannabis reform policies that legalise the drug causes increased use.”

Wodak said it was important to recognise that it was not just frequent use of cannabis that was associated with risk of using other illicit drugs later in life.

“The much closer association has been found between earlier use of tobacco and alcohol and later use of illicit drugs, so its not just cannabis we should be looking out for but also rates of tobacco and alcohol use in young people,” he said.

The study was funded by the government’s National Health and Medical Research Council.