In a New Zealand birth cohort, cannabis use before age 15 was associated with 2.7 to 30.8 times higher odds of mental health and behavioral problems, but most of this risk was explained by pre-existing family disadvantage and adjustment difficulties.
Read this if you want to understand whether early cannabis use causes problems or whether at-risk youth are simply more likely to use cannabis.
Most elevated risks disappeared after controlling for pre-existing risk factors
What the researchers found
Researchers followed a New Zealand birth cohort to examine what happened to children who began using cannabis before age 15. Early users showed dramatically elevated risks across multiple domains.
Compared to non-users, early cannabis users had higher rates of subsequent substance use, conduct and oppositional disorders, juvenile offending, severe truancy, school dropout, anxiety, depression, and suicidal ideation. The odds ratios ranged from 2.7 to 30.8 times higher.
However, a critical nuance emerged: most of these elevated risks were explained by pre-existing characteristics. Early cannabis users already had family disadvantages, early childhood adjustment problems, and strong connections to substance-using or delinquent peers before they started using cannabis.
After statistically adjusting for these confounding factors, one association remained: early onset cannabis use independently predicted increased risk of later cannabis use, suggesting a self-reinforcing pattern.
Why it matters
This study delivered an important methodological lesson: associations between early cannabis use and negative outcomes can be misleading if pre-existing risk factors are not accounted for. The finding that most associations disappeared after controlling for confounders challenged simplistic narratives about cannabis causing these problems.
The numbers in context
Odds ratios: 2.7 to 30.8 for various outcomes before adjustment. Most associations attenuated after controlling for family disadvantage, early adjustment problems, and delinquent peer affiliation. Cannabis use before age 15 independently predicted later cannabis use even after adjustment.
How the study worked
Longitudinal birth cohort study following children from birth through ages 15-16 in New Zealand. Early onset cannabis use (before age 15) was the exposure variable. Outcomes included mental health disorders, substance use, behavioral problems, and educational outcomes. Extensive confounders were controlled including family disadvantage, early adjustment problems, and peer affiliations.
What this study cannot tell us
Observational cohort study cannot fully establish causation even with extensive confounder adjustment. Some confounders may be unmeasured. The study covered outcomes only through age 15-16, missing longer-term consequences. The New Zealand cohort may not generalize to all populations.
How to read the evidence
A well-designed longitudinal birth cohort with extensive confounder adjustment. Strong methodology, though observational studies cannot fully eliminate confounding.
When this study was published
Published in 1996. Subsequent longitudinal studies have built on these findings with larger samples and longer follow-up periods.
The bigger picture
This study is a cornerstone in the ongoing debate about whether cannabis use in adolescence causes harm or whether at-risk adolescents are simply more likely to use cannabis. The truth appears to be both: at-risk youth are drawn to early use, and early use may compound existing vulnerabilities.
Questions still open
- Does early cannabis use create additional risk beyond what pre-existing factors explain, particularly when outcomes are tracked further into adulthood? Would preventing early cannabis use actually reduce these negative outcomes, or would at-risk youth experience them regardless? What drives the independent association with continued cannabis use?
Common questions
Does early cannabis use cause mental health problems?
Was there any independent effect?
Read the original research
The short-term consequences of early onset cannabis use.
Journal of abnormal child psychology, 24(4), 499-512
Citation
Fergusson, D M; Lynskey, M T; Horwood, L J. (1996). The short-term consequences of early onset cannabis use.. Journal of abnormal child psychology, 24(4), 499-512.
Explore the wider topic
- How to Talk to Your Teenager About Weed (Without Losing Them)
- My Kid's Friends Smoke Weed: A Parent's Guide
- I Smoke Weed But Don't Want My Kids To: Navigating the Hypocrisy
- Quitting Weed as a Parent: Why It Is the Hardest and Most Important Quit
- Quitting Weed as a Teen or Young Adult
- My Kid Is Smoking Weed: A Parent's Guide