A 35-year New Zealand study found regular cannabis use linked to lower education, welfare dependence, and psychotic symptoms

What the researchers found

The Christchurch Health and Development Study tracked 1,265 New Zealanders from birth to age 35, providing one of the most comprehensive longitudinal datasets on cannabis outcomes. Findings showed that individuals who used cannabis regularly or began using at earlier ages were at increased risk for multiple adverse outcomes.

These included lower levels of educational attainment, greater welfare dependence and unemployment, progression to other illicit drugs, and psychotic symptomatology. The associations were observed after adjusting for confounding factors.

However, the authors made an important qualification: a substantial proportion of regular adult cannabis users did not experience harmful consequences. The study concluded that cannabis policy needs further development to regulate a substance that is widely used and increasingly legal.

Why it matters

Birth cohort studies following people for decades provide the strongest observational evidence for long-term cannabis outcomes. This study's finding that risk is real but not universal for regular users directly informs the policy challenge of regulating cannabis in a proportionate way.

The numbers in context

1,265 participants followed from birth to age 35. Regular users showed increased risk for: lower education, welfare dependence, unemployment, other illicit drug use, psychotic symptoms. A substantial proportion of regular users experienced no harmful outcomes.

How the study worked

Birth cohort study of 1,265 children born in Christchurch, New Zealand in 1977, followed from birth to age 35 with multiple assessment waves. Comprehensive data on cannabis use patterns, educational outcomes, employment, other drug use, and psychiatric symptoms were analyzed using methods that adjusted for confounders.

What this study cannot tell us

Single cohort from one New Zealand city. Born in 1977, so their cannabis use occurred with lower-potency products than available today. Observational design, even with confounder adjustment, cannot fully establish causation. Attrition over 35 years may bias results.

How to read the evidence

Birth cohort study with 35 years of follow-up. Among the strongest observational evidence available for cannabis outcomes.

When this study was published

Published in 2015 with data through age 35 for the 1977 cohort.

The bigger picture

This study captures the complexity of cannabis policy. The harms are real for some users but absent for others, making both total prohibition and unrestricted access difficult to justify. The challenge is finding regulatory approaches that reduce harm to vulnerable users while not over-criminalizing those who use without consequence.

Questions still open

• What distinguishes regular users who experience harm from those who do not? Would the findings be more severe with today's higher-potency cannabis? How should the finding that many regular users are unharmed influence policy?

Common questions

Read the original research

Psychosocial sequelae of cannabis use and implications for policy: findings from the Christchurch Health and Development Study.

Social psychiatry and psychiatric epidemiology, 50(9), 1317-26

Citation

Fergusson, David M; Boden, Joseph M; Horwood, L John. (2015). Psychosocial sequelae of cannabis use and implications for policy: findings from the Christchurch Health and Development Study.. Social psychiatry and psychiatric epidemiology, 50(9), 1317-26. https://doi.org/10.1007/s00127-015-1070-x

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