ImportanceEvidence on the association of early intervention services (EISs) with self-harm and suicide among patients with first-episode schizophrenia (FES) at older than 25 years is lacking.ObjectiveTo examine changes in self-harm and suicide rates among patients with FES before and after the implementation of an EIS program.Design, Setting, and ParticipantsThis population-based cohort study conducted among 37 040 patients aged 15 to 64 years with FES between January 1, 2001, and March 31, 2020, used electronic medical records from the Hong Kong Clinical Data Analysis and Reporting System. All patients were followed up from the first diagnosis of schizophrenia (the index date) until the date of their death or the end of the study period (March 31, 2021), whichever came first. Statistical analysis was performed from July to November 2023.ExposureThe EIS extended the Early Assessment Service for Young People With Early Psychosis (EASY) program from patients aged 15 to 25 years to those aged 15 to 64 years (EASY Plus). The exposure was the implementation of the EASY Plus program in April 2011. The exposure period was defined as between April 2012 and March 2021 for the 1-year-time-lag analysis.Main Outcomes and MeasuresThe outcomes were monthly rates of self-harm and suicide among patients with FES before and after the implementation of the EASY Plus program. Interrupted time series analysis was used for the main analysis.ResultsThis study included 37 040 patients with FES (mean [SD] age at onset, 39 [12] years; 82.6% older than 25 years; 53.0% female patients). The 1-year-time-lag analysis found an immediate decrease in self-harm rates among patients aged 26 to 44 years (rate ratio [RR], 0.77 [95% CI, 0.59-1.00]) and 45 to 64 years (RR, 0.70 [95% CI, 0.49-1.00]) and among male patients (RR, 0.71 [95% CI, 0.56-0.91]). A significant long-term decrease in self-harm rates was found for all patients with FES (patients aged 15-25 years: RR, 0.98 [95% CI, 0.97-1.00]; patients aged 26-44 years: RR, 0.98 [95% CI, 0.97-0.99]; patients aged 45-64 years: RR, 0.97 [95% CI, 0.96-0.98]). Suicide rates decreased immediately after the implementation of the EASY Plus program among patients aged 15 to 25 years (RR, 0.33 [95% CI, 0.14-0.77]) and 26 to 44 years (RR, 0.38 [95% CI, 0.20-0.73]). Compared with the counterfactual scenario, the EASY Plus program might have led to 6302 fewer self-harm episodes among patients aged 26 to 44 years.Conclusions and RelevanceThis cohort study of the EASY Plus program suggests that the extended EIS was associated with reduced self-harm and suicide rates among all patients with FES, including those older than 25 years. These findings emphasize the importance of developing tailored interventions for patients across all age ranges to maximize the benefits of EISs.