Cannabis use predicts future psychotic symptoms, and vice versa


Robert F. Ferdinand
Erasmus Medical Center Rotterdam/Sophia Children's Hospital
Department of Child and Adolescent Psychiatry
Dr Molewaterplein 60
3015 GJ Rotterdam
the Netherlands
Tel: + 31 10 4636671
Fax: + 31 10 4636803


Aims  To assess if cannabis use is a risk factor for future psychotic symptoms, and vice versa, in adolescents and young adults from the general population.

Design  Cohort study.

Setting/participants  ‘Zuid Holland’ study, a 14-year follow-up study of 1580 initially 4–16-year-olds who were drawn randomly from the Dutch general population. Because cannabis use is generally condoned in the Netherlands, false-negative reports of cannabis use may occur less frequently than in countries with stricter drug policies, which supports the value of the present study.

Measurements  Life-time cannabis use and psychotic symptoms, assessed with the Composite International Diagnostic Interview (CIDI).

Findings  Cannabis use, in individuals who did not have psychotic symptoms before they began using cannabis, predicted future psychotic symptoms (hazard ratio = 2.81; 95% confidence interval = 1.79–4.43). However, psychotic symptoms in those who had never used cannabis before the onset of psychotic symptoms also predicted future cannabis use (hazard ratio = 1.70; 95% confidence interval = 1.13–2.57).

Conclusions  The results imply either a common vulnerability with varying order of onset or a bi-directional causal relationship between cannabis use and psychosis. More research on patterns and timings of these relationships is needed to narrow down the possibilities.