Substance-Induced Psychoses Converting Into Schizophrenia: A Register-Based Study of 18,478 Finnish Inpatient Cases

Background: Despite the clinical importance of substance-induced psychosis (SIP), few studies have examined the course of this condition after its acute manifestation.

Objective: To investigate the rate of SIP conversion to a schizophrenia spectrum disorder and the length of follow-up needed to catch the majority of these patients whose diagnoses change. In addition to the conversion rate and pattern, we wanted to look for possible related factors.

Method: Using the nationwide Finnish Hospital Discharge Register, we followed all patients (N = 18,478) since their first inpatient hospital admission with a diagnosis of SIP (codes 2921 and 2928 in DSM-III-R and codes F10-F19 in ICD-10 with a third digit of 4, 5, or 7) between January 1987 and December 2003 in Finland. Patients (mean age = 43.7 years, standard deviation = 13.5 years) were followed until first occurrence of schizophrenia spectrum disorder, death, or the end of December 2003, whichever took place first. Conversions of discharge diagnoses into schizophrenia spectrum disorders (codes 2951-2959 and 2971 in DSM-III-R and codes F20, F22, and F23 in ICD-10) were recorded at follow-up.

Results: Eight-year cumulative risk to receive a schizophrenia spectrum diagnosis was 46% (95% CI, 35%-57%) for persons with a diagnosis of cannabis-induced psychosis and 30% (95% CI, 14%-46%) for those with an amphetamine-induced psychosis. Although alcohol-induced psychosis was the most common type of SIP, 8-year cumulative risk for subsequent schizophrenia spectrum diagnosis was only 5.0% (95% CI, 4.6%-5.5%). No differences were detected with regard to gender, except for amphetamine-induced psychosis, which converted into a schizophrenia spectrum disorder significantly more often in men (P = .04). The majority of conversions to a schizophrenia spectrum diagnosis occurred during the first 3 years following the index treatment period, especially for cannabis-induced psychosis.

Conclusion: Substance-induced psychotic disorders predict schizophrenia spectrum disorders to a greater extent than previously thought. The intensity of clinical attention focused on substance-induced psychotic disorders should be increased.

J Clin Psychiatry 2013;74(1):e94-e99

© Copyright 2013 Physicians Postgraduate Press, Inc.

Submitted: April 2, 2012; accepted September 17, 2012 (doi:10.4088/JCP.12m07822).

Corresponding author: Jussi A. Niemi-Pynttäri, MD, Department of Psychiatry, Helsinki City Health Centre, PO Box 6560, 00099 Helsinki, Finland (jussi.niemi-pynttari@hel.fi).