Teen Marijuana Use Linked to Long-term Adverse Outcomes

Megan Brooks

September 09, 2014

Regular marijuana use during adolescence is strongly and independently associated with reduced educational attainment, mental health problems, and substance use in young adulthood, according to a new meta-analysis.

"The more frequently adolescents used cannabis, the more likely they were to experience adverse outcomes in young adult life," lead author Edmund Silins, PhD, of the National Drug and Alcohol Research Center, University of New South Wales, Sydney, Australia, said during a media briefing today.

Compared with nonusers, daily users of cannabis before age 17 are more than 60% less likely to complete high school or obtain a university degree, 7 times more likely to attempt suicide, have an 18 times' greater chance of cannabis dependence, and are 8 times as likely to use other illicit drugs down the road, the researchers found.

"The findings are particularly timely given the growing movement to decriminalize or legalize cannabis, raising the possibility that the drug might become more accessible to young people," said coauthor Richard Mattick, PhD, also from the National Drug and Alcohol Research Center.

The study was published online September 9 in Lancet Psychiatry.

Clear and Consistent Tie

The researchers combined individual-level data on up to 3765 young cannabis users from 3 large, long-running longitudinal studies from Australia and New Zealand to investigate the consequences of adolescent cannabis use on young adults.

They determined associations 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 7 developmental outcomes, which were assessed up to the age of 30 (completing high school, obtaining a university degree, cannabis dependence, use of other illicit drugs, suicide attempt, depression, and welfare dependence).

In adjusted analyses, "clear and consistent" associations between the frequency of adolescent cannabis use and most young-adult outcomes investigated emerged, Dr. Silins said. There was also a clear dose-response relationship, with daily cannabis users showing the strongest effects.

Outcome Adjusted Odds Ratio (95% Confidence Interval)
High-school diploma 0.37 (0.20 - 0.66)
College degree 0.38 (0.22 - 0.66)
Cannabis dependence 17.95 (9.44 - 34.12)
Other illicit drug use 7.80 (4.46 - 13.63)
Suicide attempt 6.83 (2.04 - 22.90)

 

"For most of the outcomes, the associations remained even after taking into account 53 factors which could potentially explain them, [including] sex, ethnicity, socioeconomic factors, use of other drugs, and mental health," Dr. Silins said. Depression and welfare dependence were not significantly associated with cannabis use after controlling for other factors.

The findings support the possibility that adolescent cannabis use is "a direct cause" of a range of problems in young adult life, Dr. Silins said.

The associations are "biologically plausible," he noted, "as some research suggests that heavy cannabis use in adolescence can have an effect on central nervous system development. For example, in relation to education, it's feasible that this could decrease cognitive function."

On the other hand, it is also possible that cannabis use is "simply a marker for developmental trajectories that place young people at risk of multiple adverse outcomes," he said.

Screen Teens for Cannabis Use

In a linked comment, Merete Nordentoft, MD, PhD, professor of psychiatry at the University of Copenhagen in Denmark, says the findings in this analysis are "convincing."

"The participation rate in the study is impressive, the attrition rates are modest, and the number of confounding factors, assessed across cohorts, is overwhelming and covers a broad range of factors that could affect outcome independently," Dr. Nordentoft notes.

Dr. Silins said preventing or delaying cannabis use is likely to have broad health and social benefits. "Screening for cannabis use in those under the age of 16 should be regarded as routine for GPs, for pediatricians, for child and adolescent psychiatrists, primary care nurses, and also for school counselors," he said.

This is an "important and informative" article with "high-quality longitudinal data," Niall Boyce, PhD, editor of The Lancet Psychiatry, told reporters, "and it comes at a time when the legal status of cannabis is changing in many countries."

Dr. Nordentoft warns that "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."

The study was funded by the Australian Government National Health and Medical Research Council. The authors report no relevant financial relationships.

Lancet Psychiatry. 2014;1:249-250, 286-293. Abstract

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