Individuals who are daily users of cannabis before age 17 are over 60% less likely to complete high school or obtain a degree compared to those who have never used the drug, new research published in The Lancet Psychiatry journal shows. The large meta-analysis also indicates that daily users of cannabis during adolescence are seven times more likely to attempt suicide, have an 18 times greater chance of cannabis dependence, and are eight times as likely to use other illicit drugs in later life.
“Our findings are particularly timely given that several US states and countries in Latin America have made moves to decriminalise or legalise cannabis, raising the possibility that the drug might become more accessible to young people”, says Richard Mattick, study author and Professor of Drug and Alcohol Studies at the National Drug and Alcohol Research Centre, University of New South Wales, in Australia.
Cannabis is the most widely used illicit drug globally and recent statistics have shown that in some countries adolescents are starting cannabis use at a younger age and more adolescents are using cannabis heavily. In England, 4% of 11–15 year olds report cannabis use in the past month, roughly 7% of US high-school seniors are daily or near-daily cannabis users, and in Australia, around 1% of 14–19 year olds are daily users of the drug, whilst 4% use weekly.
In this study, a team of Australian and New Zealand researchers combined individual-level data on up to 3765 participants who used cannabis from three large, long-running longitudinal studies to find out more about the link between the frequency of cannabis use before the age of 17 years (never, less than monthly, monthly or more, weekly or more, or daily) and seven developmental outcomes up to the age of 30 years (completing high school, obtaining a university degree, cannabis dependence, use of other illicit drugs, suicide attempt, depression, and welfare dependence).
The researchers recorded clear and consistent associations between frequency of cannabis use during adolescence and most young adult outcomes investigated, even after controlling for 53 potential confounding factors including age, sex, ethnicity, socioeconomic status, use of other drugs, and mental illness. Importantly, they also noted that the risks increased relative to dose, with daily cannabis users showing the strongest effects.
According to the study’s lead author, Dr Edmund Silins, “Our results provide strong evidence that the prevention or delay of cannabis use is likely to have broad health and social benefits. Efforts to reform cannabis legislation should be carefully assessed to ensure they reduce adolescent cannabis use and prevent potentially adverse effects on adolescent development.”
Writing in a linked Comment, Merete Nordentoft, Professor of Psychiatry at the University of Copenhagen in Denmark says, “The convincing results presented by Silins and colleagues are very valuable and highly appropriate at a time when several American states and countries in Latin America and Europe have decriminalised or legalised cannabis and allow unrestricted marketing of various formulations of the drug. Such changes in legislation 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.”
NOTES TO EDITORS:
The study was funded by the Australian Government National Health and Medical Research Council.
For interviews with Dr Edmund Silins or Professor Richard Mattick, please phone Marion Downey on +61 02 9385 0333 / 0401 713 850.
This media release was prepared by The Lancet Psychiatry.