Linking substance use with symptoms of subclinical psychosis in a community cohort over 30 years

Aims The aim of the study was to examine the temporal associations between substance use and subclinical psychosis symptoms. Design Data from a prospective community study sampled within a single cohort over 30 years (1978–2008) were analysed with discrete‐time hazard models. Setting General population‐based sample. Participants At initial sampling in 1978 males (n = 292) were 19 and females (n = 299) were 20 years old. Measurements Two psychosis syndromes representing ‘schizotypal signs’ and ‘schizophrenia nuclear symptoms’ and various substance use variables including cannabis, alcohol, tobacco and multiple‐drug use (i.e. cannabis combined with other drugs). Findings In bivariate analyses, schizotypal signs were predominantly associated with regular cannabis use in adolescence (OR = 2.29, 95% CI 1.32–3.97). Schizophrenia nuclear symptoms were mainly related to alcohol (OR = 1.84, 95% CI 1.00–3.38) and multiple‐drug use (OR = 2.35, 95% CI 1.38–4.02) during adolescence. Multivariate analyses showed that, in particular, regular cannabis use during adolescence was associated with the occurrence of subsequent schizotypal symptoms over a 30‐year period (OR = 2.60, 95% CI; 1.59–4.23), whereas multiple‐drug use in adolescence was associated predominantly with schizophrenia nuclear symptoms (OR = 1.75, 95% CI 1.01–3.03). Alcohol misuse was only slightly associated with the onset of such symptoms. Conclusions A significant portion of the occurrence of subclinical psychosis symptoms in adulthood can be attributed to excessive cannabis and multiple‐drug use during adolescence. This is in line with the hypothesis that long‐term sensitization of dopaminergic brain receptors plays a role in developing psychotic symptoms.