Cannabis Use From Ages 14-21: What Happened to Mental Health, Crime, and Other Drug Use | Research

The takeaway

A 21-year longitudinal study of 1,265 New Zealand children found that cannabis use was associated with increased crime, depression, suicidal behavior, and other drug use, even after controlling for confounding factors, with younger users affected more than older users.

Read this if you want to understand the long-term risks of cannabis use that begins in the teenage years.

Younger users (14-15) showed stronger adverse associations than older users (20-21)

What the researchers found

Across annual assessments from ages 14 to 21, more frequent cannabis use was significantly associated with property and violent crime, depression, suicidal ideation, suicide attempts, and other illicit drug use. Statistical controls for both fixed and time-varying confounders substantially reduced but did not eliminate these associations.

A key finding was age-related variation: younger cannabis users (14-15 years old) showed stronger associations with crime, suicidal behaviors, and other drug use than older users (20-21 years old). However, the association between cannabis use and depression did not vary by age. The strongest and most persistent association was between cannabis use and other illicit drug use.

Why it matters

This study was influential because of its long follow-up period, population-based design, and sophisticated approach to confounding. The finding that younger users were disproportionately affected provided empirical support for age-specific drug policies and reinforced concerns about adolescent cannabis exposure during brain development.

The numbers in context

The cohort included 1,265 children followed for 21 years. Cannabis use was assessed annually from ages 14-21. Associations remained significant (P < 0.05) for all outcomes after adjustment. Younger users (14-15) showed stronger associations than older users (20-21) for crime, suicidal behavior, and other drug use.

How the study worked

This was a 21-year longitudinal birth cohort study following 1,265 children born in Christchurch, New Zealand. Annual assessments of cannabis use frequency were obtained for ages 14-21, alongside measures of crime, depression, suicidal behaviors, and other drug use. Statistical models controlled for both fixed confounders (childhood factors) and time-dynamic confounders (changing circumstances).

What this study cannot tell us

Observational studies cannot definitively establish causation, even with sophisticated confound adjustment. Unmeasured confounders such as genetic vulnerability or peer network effects may partly explain the associations. Self-reported cannabis use may be underreported. The cohort was from a single city in New Zealand and may not generalize to all populations.

How to read the evidence

This is a 21-year population-based longitudinal study with sophisticated confound adjustment, providing strong evidence from long-term follow-up.

When this study was published

Published in 2002. The Christchurch cohort has continued to be followed and has produced additional findings on cannabis and mental health.

The bigger picture

The Christchurch Health and Development Study has been one of the most cited longitudinal studies in cannabis research. Its findings contributed to the scientific consensus that adolescent cannabis use carries greater risks than adult use, a principle reflected in cannabis legalization frameworks that typically set minimum ages of 18 or 21.

Questions still open

Would the age-related differences in risk persist if cannabis potency were controlled for? Why did the cannabis-depression association not vary by age when other associations did? How much of the association with other drug use reflects shared risk factors versus a causal pathway?

Common questions

Is cannabis more dangerous for teenagers than adults?

This study found that the associations between cannabis use and negative outcomes (crime, suicidal behavior, other drug use) were stronger for younger users (14-15) than for older users (20-21), supporting the view that adolescent use carries greater risks.

Does cannabis cause depression?

This study found an association between cannabis use and depression that persisted after controlling for confounding factors, but the observational design cannot prove causation. The relationship was consistent across age groups, unlike other outcomes.

Read the original research

Cannabis use and psychosocial adjustment in adolescence and young adulthood.

Addiction (Abingdon, England), 97(9), 1123-35

View on PubMed ↗

Citation

Fergusson, David M; Horwood, L John; Swain-Campbell, Nicola. (2002). Cannabis use and psychosocial adjustment in adolescence and young adulthood.. Addiction (Abingdon, England), 97(9), 1123-35.