A 35-year New Zealand study found regular cannabis use linked to lower education, welfare dependence, and psychotic symptoms
Following 1,265 people from birth to age 35, regular cannabis use was associated with lower educational attainment, unemployment, other drug use, and psychotic symptoms, though many regular users experienced no harmful outcomes.
Quick Facts
What This Study 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.
Key Numbers
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 They Did This
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.
Why This Research 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 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.
What This Study Doesn't 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.
Questions This Raises
- ?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?
Trust & Context
- Key Stat:
- 35 years of follow-up from birth; 1,265 participants
- Evidence Grade:
- Birth cohort study with 35 years of follow-up. Among the strongest observational evidence available for cannabis outcomes.
- Study Age:
- Published in 2015 with data through age 35 for the 1977 cohort.
- Original Title:
- Psychosocial sequelae of cannabis use and implications for policy: findings from the Christchurch Health and Development Study.
- Published In:
- Social psychiatry and psychiatric epidemiology, 50(9), 1317-26 (2015)
- Authors:
- Fergusson, David M(2), Boden, Joseph M(4), Horwood, L John(4)
- Database ID:
- RTHC-00955
Evidence Hierarchy
Follows a group of people over time to track how outcomes develop.
What do these levels mean? →Frequently Asked Questions
Does regular cannabis use lead to bad outcomes?
This 35-year study found associations between regular use and lower education, unemployment, other drug use, and psychotic symptoms. However, a substantial proportion of regular users experienced none of these outcomes.
Is starting cannabis use younger more risky?
Yes. Earlier age of first use was associated with increased risk across multiple outcomes in this study, consistent with research on adolescent brain vulnerability to cannabis effects.
Read More on RethinkTHC
Cite This Study
https://rethinkthc.com/research/RTHC-00955APA
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
MLA
Fergusson, David M, et al. "Psychosocial sequelae of cannabis use and implications for policy: findings from the Christchurch Health and Development Study.." Social psychiatry and psychiatric epidemiology, 2015. https://doi.org/10.1007/s00127-015-1070-x
RethinkTHC
RethinkTHC Research Database. "Psychosocial sequelae of cannabis use and implications for p..." RTHC-00955. Retrieved from https://rethinkthc.com/research/fergusson-2015-psychosocial-sequelae-of-cannabis
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.