BACKGROUND: Monopharmacy with antipsychotics and antidepressants is the first-line treatment for schizophrenia and major depressive disorder (MDD) in most clinical guidelines, while polypharmacy with psychotropic agents in the treatment of schizophrenia is common in clinical practice. There are no detailed data on the prescription patterns for inpatients with mental illness with reliable diagnoses made by treating psychiatrists. METHODS: We gathered prescription data at discharge from 2177 patients with schizophrenia and 1238 patients with MDD from October 2016 to March 2018. RESULTS: The patients with schizophrenia aged between 60 and 79 were prescribed lower doses of antipsychotics and hypnotics/anxiolytics than those aged between 40 and 59. There were significant differences between the prescription rate of antipsychotics in the patients with schizophrenia and that of antidepressants in the patients with MDD. The frequency of concomitant drugs such as anti-Parkinson drugs, anxiolytics/hypnotics and mood stabilizers in the subjects with schizophrenia prescribed antipsychotic polypharmacy was significantly higher than that with monotherapy. For the patients with schizophrenia, olanzapine, risperidone, aripiprazole, quetiapine, and blonanserin were the five most prescribed antipsychotics. For the patients with MDD, mirtazapine, duloxetine, escitalopram, trazodone and sertraline were the five most prescribed antidepressants. CONCLUSIONS: Our results showed the use of high doses of antipsychotics, high percentages of antipsychotic polypharmacy and concurrent use of hypnotics/anxiolytics in patients with schizophrenia. Notably, these data were collected before intensive instruction regarding the guidelines; therefore, we need to assess the change in the prescription pattern post guideline instruction.