Delay in clozapine initiation and subsequent treatment response in schizophrenia spectrum disorders: a systematic...

Clozapine remains the gold standard treatment for treatment-resistant schizophrenia (TRS), yet its initiation is frequently delayed for years, potentially contributing to irreversible clinical and functional deterioration. A previous systematic review suggested that longer delays in clozapine initiation are associated with poorer outcomes; however, more recent longitudinal and prospective studies warrant an updated synthesis of the evidence. This review evaluates whether delayed clozapine initiation is consistently associated with poorer clinical, symptomatic, and functional outcomes in adults with schizophrenia spectrum disorders. A systematic search of PubMed, Embase, Scopus, and Web of Science identified observational studies and randomized controlled trials examining the association between delay in clozapine initiation and treatment outcomes. Two reviewers independently conducted study selection, data extraction, and quality assessment according to PRISMA guidelines, using the Newcastle-Ottawa Scale and GRADE framework. Any discrepancies, methodological inconsistencies, or potential errors were assessed and resolved by two senior reviewers. Findings were synthesized descriptively. Sixteen studies involving up to 2,380 participants with TRS met inclusion criteria. Across studies, longer delays in clozapine initiation were consistently associated with poorer treatment response and reduced symptomatic improvement. Several studies also identified delayed initiation as a predictor of ultra-treatment-resistant schizophrenia, lower rates of remission, and poorer long-term functional recovery. These findings consistently support the potential benefits of earlier clozapine initiation in TRS, despite methodological limitations related to the predominantly observational nature of the evidence. Thus, adopting a timely-intervention framework may help prevent stable ultra-treatment resistance, improve symptomatic outcomes, and preserve long-term functioning.