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Purpose: Antipsychotic monotherapy is often recommended over antipsychotic polypharmacy because of fewer adverse events, reduced treatment complexity, and lower medication cost. This study compared the rate and the duration of antipsychotic monotherapy following initiation of olanzapine or risperidone in the treatment of outpatients with schizophrenia in Japan. Methods: Outpatients diagnosed with schizophrenia in the Japan Medical Data Center database were identified using International Statistical Classification of Diseases and Related Health Problems, 10th Revision, diagnosis codes. Patients were between 20 and 65 years old, initiated on olanzapine or risperidone therapy between August 2003 and July 2008, and continuously enrolled during the 6 months prior to and the 12 months following the initiation date. Antipsychotic polypharmacy was defined as concurrent use of two or more antipsychotics. The probability of monotherapy during the 12-month follow-up period was assessed using a propensity score-adjusted generalized estimating equation model. Duration of monotherapy was contrasted using a propensity score-adjusted bootstrapping model. Results: After applying all inclusion and exclusion criteria, the final analytic sample consisted of 332 olanzapine- and 496 risperidone-treated outpatients. At treatment initiation, 61.5% of the olanzapine-treated patients and 45.6% of the risperidone-treated patients received antipsychotic monotherapy (P < 0.001). After correcting for background differences, monotherapy was more common among olanzapine-treated patients (P = 0.001). In addition, olanzapine was used as monotherapy for a longer duration (P = 0.006). Conclusion: Consistent with prior global research, this retrospective naturalistic study of schizophrenia outpatients in Japan found that olanzapine is more likely to be used as monotherapy and to be used as monotherapy for a longer duration than risperidone. [ABSTRACT FROM AUTHOR]