Effectiveness of a formulary system on prophylactic oral proton pump inhibitors for drug-induced peptic ulcer in a Japanese tertiary hospital: an interrupted time series analysis

Abstract Background Pharmacy formulary systems have been established recently in various fields of pharmacotherapies. We evaluated the effectiveness of pharmacy formulary interventions on proton pump inhibitors (PPI) used to prevent drug-induced peptic ulcers in a Japanese tertiary hospital. Methods A retrospective cohort study was conducted in Kurashiki Central Hospital. A pharmacy formulary system of PPIs was implemented on October 1, 2020. Between April 2020 and March 2021, six months before and after the implementation date of the formulary system, inpatients aged ≥ 18 years were included if they newly received drugs (low-dose aspirin, anti-platelets, or non-steroidal anti-inflammatory drugs) recommended for prophylactic PPI use for peptic ulcers within seven days from hospital admission. Eligible patients were divided into two groups based on the implementation date, and changes in PPI prescription patterns were evaluated by interrupted time series analysis, along with the risk of drug-induced peptic ulcers and drug costs. Results In total, 2,449 inpatients were included. The median age of the pre- and post-formulary group was 60 and 58 years, respectively. The proportion of the targeted PPI prescription increased by 8.75% (95% confidence interval (CI); 0.12, 17.38) in level change, without increased risk of drug-induced peptic ulcers (risk difference -0.41%, 95% CI; -1.38, 0.55). The distribution of medication days in the two groups was similar, and $1,000 per 90 patient days was saved on drug costs. Conclusion The formulary system on oral PPIs in a Japanese tertiary hospital contributed to a positive level change in the prescription patterns, without increased risk of drug-induced peptic ulcers. Although slight, the drug costs were saved.