Abstract
BACKGROUND: Clozapine is highly effective for treatment-resistant schizophrenia but has been associated with an increased risk of agranulocytosis. As a result, until 2025, the Food and Drug Administration required patients receiving clozapine to undergo regular blood testing to monitor for neutropenia as part of a Risk Evaluation and Mitigation Strategy (REMS) programme. OBJECTIVE: This study sought to compare the risk of neutropenia-related hospitalisations between clozapine and olanzapine initiators. METHODS: The study cohort was nested in claims data from Medicaid and two commercial health insurance databases and consisted of adults initiating clozapine or olanzapine who had a recorded diagnosis of schizophrenia or schizoaffective disorder and ≥1 dispensing of a different antipsychotic in the 6 months before initiation. Propensity score matching (1:1) was used to mitigate confounding. The primary outcome was hospitalisation with a neutropenia diagnosis in the primary position. Both as-treated and intention-to-treat analyses were implemented. FINDINGS: After propensity score matching, there were 16 873 initiators in each group. At 6 months postinitiation, there were 12 neutropenia-related hospitalisations among the clozapine cohort (incidence rate: 2.21 per 1000 person-years; 95% CI 1.25 to 3.89) and <11 among the olanzapine cohort (0.18; 95% CI 0.03 to 1.29), corresponding to an incidence rate ratio (IRR) of 12.18 (95% CI 1.58 to 93.71). The IRRs were 5.77 (95% CI 1.29 to 25.76) at 1 year, 5.50 (95% CI 1.23 to 24.55) at 2 years and 5.40 (95% CI 1.21 to 24.13) at 3 years postinitiation. Associations remained but were attenuated in intention-to-treat analyses. CONCLUSIONS: Clozapine initiators had an elevated risk of neutropenia-related hospitalisation, especially during the first 6 months of treatment, although the absolute risk was low. CLINICAL IMPLICATIONS: Despite removal of the REMS programme, it is important for prescribers to monitor patients for neutropenia after initiating clozapine.