Objective This study aimed to estimate transition rates from substance-induced psychosis (SIP) to schizophrenia or bipolar disorder, evaluate the risk associated with dual use of methamphetamine and cannabis, and identify predictors of chronicity among Iranian men.

Methods A 25-year retrospective cohort study (1999–2024) was conducted using clinical data from Shafa Hospital, a tertiary psychiatric referral center in Guilan Province, Iran. The study included 258 male inpatients aged ≥17 years with SIP attributed to cannabis and/or methamphetamine use. Patients with a history of schizophrenia or bipolar disorder were excluded. Outcomes were validated using DSM-V criteria, and Kaplan-Meier survival curves and Cox proportional hazards models were employed for analysis.

Results Over a median follow-up of 33 months, 37.6% of participants transitioned to schizophrenia (25.2%) or bipolar disorder (12.4%). The median time to conversion was shorter for bipolar disorder (13.4 months) compared to schizophrenia (26.6 months). Dual cannabis-methamphetamine use significantly increased the risk of bipolar disorder (p=0.008). Familial psychiatric history doubled the risk of schizophrenia (HR=4.29, 95% CI: 2.48–7.41) and tripled the risk of bipolar disorder (HR=3.89, 95% CI: 1.87–8.11). Recurrent hospitalizations were associated with increased risks for both schizophrenia (HR=1.34) and bipolar disorder (HR=2.64). The cumulative incidence of schizophrenia rose linearly to 67.6% at 10 years.

Conclusion SIP poses a significant risk for the development of schizophrenia or bipolar disorder, particularly in cases involving dual-substance use, younger age, familial psychiatric history, and frequent hospitalizations. The findings underscore the need for targeted surveillance, early intervention, and long-term, risk-stratified care models in regions with rising methamphetamine use to reduce SIP-related morbidity

The authors have declared no competing interest.

No funding was received for this study. The authors declare no financial or material support from any organization.

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

Yes

The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

The study was approved by the Shafa Educational & Remedial Center Ethics Committee. The Research and Ethics Committee of Alborz University of Medical Sciences (ABZUMS) reviewed and approved the study proposal and waived the requirement for informed consent. All data were anonymized prior to analysis to preserve patient confidentiality.

I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals.

Yes

I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).

Yes

I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable.

Yes

All data produced in the present study are available upon reasonable request to the authors