Objective: In the treatment of patients with schizophrenia, pro re nata (PRN) drugs are commonly prescribed for medical indications such as agitation, acute psychiatric symptoms, insomnia, and anxiety. However, high-quality evidence supporting the use of PRN medications is lacking, and these drugs are administered on the basis of clinical experience and habits. Therefore, the actual use of psychotropic PRN drugs and its influence on the patients’ outcomes need to be investigated. Methods: This study included 205 patients who underwent inpatient treatment for schizophrenia. We investigated the prescription of psychotropic drugs before admission and at discharge, as well as the dosing frequency of PRN drugs during hospitalization. We also examined the influence of psychotropic PRN drug use on hospitalization days, antipsychotic polypharmacy, and readmission rates. Results: Patients who used psychotropic PRN drugs during hospitalization had significantly longer hospitalization days (p = 7.5 × 10−4 ) and significantly higher rates of antipsychotic polypharmacy (p = 2.4 × 10−4 ) at discharge than those who did not use psychotropic PRN drugs. Moreover, a higher number of psychotropic PRN drugs used per day was associated with higher readmission rates within 3 months of discharge (p = 4.4 × 10−3 ). Conclusion Psychotropic PRN drug use is associated with prolonged hospitalization, antipsychotic polypharmacy, and increased readmission rates in inpatients with schizophrenia. Therefore, psychiatric symptoms should be stabilized with regularly prescribed medications without the extensive use of psychotropic PRN drugs. Moreover, a system for monitoring and reexamining PRN drug use needs to be established.