The effect of drug use on the age at onset of psychotic disorders in an Australian cohort.
BACKGROUND
We aimed to examine the association between illicit substance use and age at onset in psychotic disorders in an Australian cohort.


METHODS
Retrospectively acquired information on substance use during the year prior to illness onset was collected from 1642 participants enrolled in the Australian National 2010 Survey of High Impact Psychosis study (SHIP), with an ICD-10 diagnosis of schizophrenia spectrum or affective psychosis. Latent class analysis was performed according to illicit substance use, using age as an active covariate; identified classes were subsequently validated. Cox regression was used to examine the independent contribution of the identified substance use classes and several confounding variables to the prediction of age at onset of psychosis.


RESULTS
Three classes according to substance use were identified: non-users (n=803), cannabis predominant users (n=582), and polysubstance users (n=257). For participants with schizophrenia spectrum disorders, cannabis predominant users had a higher hazard of earlier age at onset than for non-users (adjusted HR=1.38, 95% CI=1.2-1.6); polysubstance users had an even higher hazard (adjusted HR=1.95, 95% CI=1.5-2.4). In contrast, for participants with affective psychosis, cannabis predominant users (adjusted HR=1.10, 95% CI=0.8-1.4) and polysubstance users (adjusted HR=0.87, 95% CI=0.6-1.3) did not have a higher hazard of earlier age at onset compared with non-users.


CONCLUSIONS
Illicit substance use in the 12 months prior to psychosis onset has a differential effect on age at onset in schizophrenia spectrum and affective psychotic disorders. Our findings are compatible with the notion that illicit drugs bring forward age at onset in schizophrenia spectrum disorders but not affective psychotic disorders.
School of Psychiatry and Clinical Neurosciences, The University of Western Australia, WA; Clinical Research Centre, North Metropolitan Health Service - Mental Health, Perth, WA. Electronic address: nikos.stefanis@uwa.edu.au.
Nikos C
NC
Stefanis
School of Psychiatry and Clinical Neurosciences, The University of Western Australia, WA; Clinical Research Centre, North Metropolitan Health Service - Mental Health, Perth, WA. Electronic address: milan.dragovic@health.wa.gov.au.
Milan
M
Dragovic
School of Psychiatry and Clinical Neurosciences, The University of Western Australia, WA; Clinical Research Centre, North Metropolitan Health Service - Mental Health, Perth, WA; Peel and Rockingham Kwinana Mental Health Service, South Metropolitan Area Health Service, Perth, WA. Electronic address: brian.power@health.wa.gov.au.
Brian D
BD
Power
School of Psychiatry and Clinical Neurosciences, The University of Western Australia, WA. Electronic address: assen.jablensky@health.wa.gov.au.
Assen
A
Jablensky
St. Vincent's Hospital, University of Melbourne, Australia. Electronic address: David.Castle@svhm.org.au.
David
D
Castle
School of Psychiatry and Clinical Neurosciences, The University of Western Australia, WA. Electronic address: vera.morgan@health.wa.gov.au.
Vera A
VA
Morgan
D000328:Adult; D017668:Age of Onset; D000662:Amphetamines / Q000009:adverse effects; D001315:Australia / Q000453:epidemiology; D002188:Cannabis / Q000009:adverse effects; D015331:Cohort Studies; D005260:Female; D006801:Humans; D013287:Illicit Drugs / Q000009:adverse effects; D008297:Male; D011618:Psychotic Disorders / Q000453:epidemiology*; D012044:Regression Analysis; D012189:Retrospective Studies; D012559:Schizophrenia / Q000453:epidemiology*; D019966:Substance-Related Disorders / Q000453:epidemiology*
D000662:Amphetamines; D013287:Illicit Drugs
Amphetamine; Cannabis; Causality; Psychosis; Schizophrenia