Comparative effectiveness of antipsychotic monotherapy and polypharmacy in schizophrenia patients with clozapine treatment: A nationwide, health insurance data-based study

Clozapine is the most effective antipsychotic for treatment-resistant schizophrenia (TRS). However, it remains uncertain whether antipsychotic augmentation to clozapine has the superior effectiveness over clozapine alone and the effect size of clozapine compared to other antipsychotic drugs in TRS. Therefore, we examined the comparative effectiveness of antipsychotic monotherapy and polypharmacy on the risk of psychiatric admission and treatment discontinuation in TRS. Data were collected from the Health Insurance Review Agency database between January 2010 and December 2019 in South Korea. Among prevalent patients with schizophrenia, we defined 22,327 patients with TRS as those who had been prescribed with clozapine at least once during the entire observation period. Stratified Cox proportional hazards regressions were performed using data on all antipsychotic prescriptions of patients with TRS to investigate the risk of psychiatric hospitalization and treatment discontinuation associated with antipsychotic treatment. In individual comparisons, clozapine monotherapy was the most effective for the risk of psychiatric hospitalization compared to no use (hazard ratio [HR] = 0.23, 95% confidence interval [CI] = 0.22-0.25). In group comparisons, clozapine with long-acting injectable (LAI) second-generation antipsychotics (SGA) was superior to clozapine monotherapy for the risk of psychiatric hospitalization (HR = 0.60, 95%CI = 0.41-0.88). Clozapine monotherapy was associated with the lowest risk of treatment discontinuation in the individual and group comparisons. This retrospective observational population-based study reports that clozapine with LAI SGA is more effective in lowering the risk of psychiatric hospitalization in antipsychotic group comparison with the reference of clozapine monotherapy.