Inappropriate polypharmacy increases the risks of adverse drug reactions and hospitalization. Thus, it is important to evaluate the appropriateness of prescriptions in polypharmacy. We designed a checklist based on previous studies and guidelines for pharmacists in our hospital to evaluate the appropriateness of a prescription of multiple medications. The efficacy of checklist-based standardization was evaluated by investigating inpatient medical records and prescriptions. We designed a checklist for pharmacists in our hospital to evaluate the appropriateness of a prescription and reduce the prescription of medications with multiple administrations for all age groups. When patients using more than six medications were admitted, pharmacists assessed the prescriptions of these patients using the checklist. We examined 729 inpatients over the course of 4 months before and after the standardization. The research protocol was approved by the Human Ethical Committee of Showa University, School of Pharmacy. For prescriptions with six or more medications, the total number of suggestions for all patients significantly increased upon implementation of the checklist (50 vs. 21, P < 0.05). Additionally, the number of changes in prescriptions by doctors increased while using the checklist (44 vs. 17, P < 0.05), whereas the rate of changes per suggestions did not change. The most common reason for the increase in prescription suggestions after the standardization was a medication was prescribed to patients despite the absence of symptoms. Our checklist was effective in reducing the prescription of inappropriate medications in patients of all ages.