This study aimed to evaluate the relationship between gene polymorphisms of metabolic enzymes, particularly the CYP2D6 gene, and the plasma concentration of olanzapine, as well as treatment response in patients with chronic schizophrenia. We recruited olanzapine-treated patients and examined their plasma olanzapine levels. Additionally, a common mutation site within each of the nine exons of the full-length CYP2D6 sequence was assayed. The Positive and Negative Syndrome Scale, Brief Psychiatric Rating Scale, and Overall Clinical Impression were used to assess schizophrenic symptoms, whereas the Barnes Akathisia Scale and Extrapyramidal Symptom Rating Scale were used to evaluate adverse effects. The results showed no significant differences in plasma olanzapine concentrations, treatment response, or the occurrence of adverse effects among different CYP2D6 genotypes. However, an association between olanzapine concentrations and improvement in clinical symptoms and adverse reactions was observed. In conclusion, the CYP2D6 genotype did not significantly impact plasma olanzapine concentrations, treatment response, or the occurrence of adverse effects.