BackgroundThe stability of the diagnostic distinction between a substance-induced psychosis and a primary psychotic disorder co-occurring with substance use is not established.AimsTo describe DSM – IV diagnostic changes over 1 year and determine the predictive validity of baseline indicators of the substance-induced psychosis v. primary psychosis distinction.MethodWe conducted a 1-year follow-up study of 319 psychiatric emergency department admissions with diagnoses of early-phase psychosis and substance use comorbidity.ResultsOf those with a baseline DSM—IV diagnosis of substance-induced psychosis, 25% had a diagnosis of primary psychosis at follow-up. These patients had poorer premorbid functioning, less insight into psychosis and greater family mental illness than patients with a stable diagnosis of substance-induced psychosis. Reclassifying change cases to primary psychoses on follow-up, key baseline predictors of the primary/substance-induced distinction at 1 year also included greater family history of mental illness in the primary psychosis group.ConclusionsFurther study of substance-induced psychoses should employ neuroscientific and behavioural approaches. Study findings can guide more accurate diagnoses at first treatment.