Does accumulating exposure to illicit drugs bring forward the age at onset in schizophrenia?

Whilst cannabis has been associated with an earlier age at onset in schizophrenia, the impact of amphetamine and/or cocaine plus cannabis consumption on age at onset remains unclear. The present study was designed to test the hypothesis that consumption of amphetamine and/or cocaine in addition to cannabis would lead to an earlier age at onset of schizophrenia than that seen for cannabis consumption alone. A secondary objective was to determine what kind of effect additional substance use exerted (e.g. additive, multiplicative).

Patients with a diagnosis of schizophrenia were recruited from consecutive admissions to the inpatient and outpatient services of a large psychiatric hospital in Perth, Australia and 167 participants were assessed using the Diagnostic Interview for Psychosis, which included detailed inquiry into illicit drug use in the 12 months prior to the onset of psychiatric symptoms. Participants were categorized into four groups: no illicit substance use (n = 65), cannabis use (n = 68), cannabis plus amphetamine use (n = 25), and cocaine plus cannabis/cocaine plus cannabis plus amphetamine use (n = 9). Analysis of variance was performed to detect trends, and linear regression used to analyze the consumption of each additional substance as a predictor of age at onset.

We observed a linear trend for mean age at onset: 23.34 (SD = 6.91) years for no illicit substance use, 22.51 (SD = 5.27) years for cannabis use, 20.84 (SD = 3.48) years for cannabis plus amphetamine use, and 19.56 (SD = 3.54) years for cocaine plus cannabis/cocaine plus cannabis plus amphetamine use; the variation in the means between groups was statistically significant: F(1,163) = 5.66, p = 0.008, Cohen's d = 0.38. For the consumption of each additional substance, age at onset was earlier by 1.2 years: R (2) = 0.034, F(1,165) = 5.72, p = 0.018.

Whilst preliminary, these findings suggest that additional consumption of each substance predicted an earlier age at onset by approximately 1 additional year.