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Background: Schizophrenia in adolescence disrupts neurodevelopment and long-term functioning. While symptom reduction remains a primary treatment goal, quality of life (QoL) represents a critical, patient-centered outcome. Pharmacogenetic variability, particularly in CYP2D6 metabolism of second-generation antipsychotics, may influence tolerability and subjective well-being beyond symptom control. Materials and Methods: Forty-seven adolescents (aged 14-18 years) diagnosed with schizophrenia (DSM-5) were followed in routine clinical care. CYP2D6 genotyping classified patients as normal metabolizers (NM, n = 27) or reduced-function metabolizers (RFM, including intermediate/poor, n = 20). Symptom severity was assessed with PANSS, QoL was assessed with the Pediatric Quality of Life Enjoyment and Satisfaction Questionnaire (PQ-LES-Q), and adverse effects (hyperprolactinemia, extrapyramidal symptoms, sedation, metabolic changes) were monitored. Non-parametric tests and multiple linear regression were applied. Results: At 12 months, RFM patients showed significantly higher PANSS scores, markedly more adverse reactions (95% vs. 48.1%), and lower PQ-LES-Q total and domain scores (all p < 0.0001) compared to NM patients. A regression analysis identified the metabolizer status (β = -0.410, p = 0.001), extrapyramidal symptoms (β = -0.248, p = 0.003), sedation (β = -0.193, p = 0.029), and hyperprolactinemia (β = -0.190, p = 0.012) as independent predictors of a reduced QoL, explaining 84% of the variance. The residual symptom severity was not independently associated. Conclusions: In adolescent schizophrenia, the CYP2D6-reduced metabolizer status is the strongest independent predictor of long-term QoL impairment, associated primarily through a substantially higher burden of treatment-related adverse effects (metabolic, endocrine, neurological, and sedative) rather than through persistence of psychotic symptoms alone. These findings support early pharmacogenetic testing to guide individualized dosing and improve tolerability and patient-reported outcomes.