Male Cannabis Users Developed Schizophrenia Nearly 7 Years Earlier Than Male Non-Users | Research

The takeaway

In a population-based study of 133 schizophrenia patients, cannabis use independently predicted earlier onset of psychosis, with male cannabis users developing their first psychotic episode 6.9 years younger than male non-users.

Read this if you want to understand the research linking cannabis use to earlier onset of schizophrenia, especially in men.

Male cannabis users were 6.9 years younger at first psychotic episode than male non-users

What the researchers found

Researchers conducted a population-based, first-contact incidence study in The Hague, Netherlands, examining 133 schizophrenia patients. They looked at three milestones: first social/occupational dysfunction, first psychotic episode, and first negative symptoms.

Male patients reached all three milestones at significantly younger ages than female patients. Cannabis-using patients also reached these milestones significantly younger than non-users.

In multivariate analysis, cannabis use, but not gender, independently predicted the age of first psychotic episode. Male cannabis users were a mean of 6.9 years younger at illness onset than male non-users. However, gender rather than cannabis use predicted the age of first social/occupational dysfunction and the risk of developing negative symptoms before treatment.

Why it matters

The finding that cannabis use independently predicted earlier onset of psychosis, even after controlling for gender differences, adds to evidence of a specific relationship between cannabis and the timing of psychotic episodes. The 6.9-year difference is clinically substantial, as earlier onset is generally associated with worse long-term outcomes.

The numbers in context

133 schizophrenia patients studied. Male cannabis users were 6.9 years younger at first psychotic episode than male non-users. Cannabis use, but not gender, independently predicted age at first psychotic episode in multivariate analysis.

How the study worked

Population-based, first-contact incidence study conducted in The Hague, Netherlands. One hundred thirty-three patients were interviewed using the Comprehensive Assessment of Symptoms and History. Key informants were interviewed with the Instrument for the Retrospective Assessment of the Onset of Schizophrenia. Multivariate analyses controlled for the independent effects of gender and cannabis use on three illness milestones.

What this study cannot tell us

The cross-sectional design cannot establish causation. Cannabis use was assessed retrospectively, which introduces recall bias. The study cannot determine whether cannabis use precipitated psychosis in individuals who would not have developed it otherwise or simply accelerated onset in those already predisposed.

How to read the evidence

Population-based incidence study with multivariate analysis, providing stronger evidence than clinic-based samples, though still unable to establish causation.

When this study was published

Published in 2004. Subsequent longitudinal studies have further examined the cannabis-psychosis relationship, generally supporting the association with earlier onset.

The bigger picture

This study is part of a growing body of evidence suggesting cannabis use is associated with earlier onset of schizophrenia, particularly in males. The population-based design strengthens the finding compared to clinic-based samples. Whether this represents a causal effect of cannabis or reflects shared vulnerability factors continues to be debated.

Questions still open

Does the earlier onset associated with cannabis use lead to worse long-term outcomes? Would earlier intervention in cannabis-using individuals at risk for psychosis delay or prevent the first psychotic episode?

Common questions

Does cannabis cause schizophrenia?

This study found cannabis use was associated with earlier onset of schizophrenia but cannot prove causation. It showed male cannabis users developed psychosis 6.9 years earlier than non-users, but whether cannabis triggered the illness or other factors explain the link remains uncertain.

Were women also affected by cannabis use?

The study focused heavily on male patients, who showed the clearest association. Gender independently predicted some milestones (social dysfunction, negative symptoms) while cannabis use independently predicted the timing of the first psychotic episode.

Read the original research

Cannabis use and age at onset of schizophrenia.

The American journal of psychiatry, 161(3), 501-6

View on PubMed ↗

Citation

Veen, Natalie D; Selten, Jean-Paul; van der Tweel, Ingeborg; Feller, Wilma G; Hoek, Hans W; Kahn, René S. (2004). Cannabis use and age at onset of schizophrenia.. The American journal of psychiatry, 161(3), 501-6.