The transition to Schizophrenia spectrum disorder from a first psychotic episode that did or did not appear to be induced by substance use.

The first years following a first-episode psychosis (FEP) are crucial. This retrospective cohort study investigates the evolution of first-episode psychosis (FEP), including substance-induced psychosis (SIP), and explores factors influencing the diagnostic conversion to Schizophrenia or Schizophrenia Spectrum Disorders (SSD). Diagnoses of patients discharged from Basurto University Hospital's inpatient psychiatry unit between January 2002 and December 2016 were reviewed. Sociodemographic, clinical, and substance use data, including cannabinoids, opioids, amphetamines, cocaine, and alcohol, were collected. The analysis utilized descriptive statistics, Kaplan-Meier survival curves, and Cox regression. Among 341 patients, 64.8% were male, with a mean age of 33.8 years. Psychiatric family history was present in 33.4% of cases, and cannabis was the most commonly used substance (78.9%). Of the patients, 52.8% received subsequent diagnoses of Schizophrenia or SSD, with 86.9% of these cases occurring within the first five years. No significant differences were observed between patients diagnosed with SIP and other diagnoses in terms of sociodemographic, clinical characteristics, or progression to Schizophrenia or SSD. However, use of cannabis (compared to use of another substance or polysubstance use) was associated with a higher risk to conversion (HR 1.96; p = 0.001). These findings underscore the importance of addressing substance use and treatment adherence in FEP.