Introduction: Globally, 10.5-25.9 million cases of pyelonephritis have been reported, and the nature of the disease is changing over time. In Mongolia, as of 2022 diseases of the genitourinary system has increased from the average of the last 10 years, and account for 9% of all diseases. The incidence rate by age group increased in all age groups in 2022 compared to 2013, with the largest increase in the age group over 65 with 2,104 cases. The basis of this study is to determine the use of PIM drugs based on the Beers criteria for the treatment of patients diagnosed with pyelonephritis in our country. To study the use of drugs by inpatients diagnosed with pyelonephritis (N10), (N11) in referral hospitals and the use of PIM drugs used in the treatment of elderly people. Methods: A retrospective descriptive study was conducted on the medical history of a total of 150 patients diagnosed with pyelonephritis (year 2023) at The State Secondary Central Hospital, The State Third Central Hospital, and Hovd Regional Medical Centers. Among the 150 inpatients who participated in our study, 14.3% (n=18) of 126 types of drugs were used for the treatment of pyelonephritis, and 85.7% (n=108) for the treatment of multimorbidity and supportive treatment. Inpatients used 11 types of PIM drugs according to Beers criteria. Nitrofurantoin 39% was used the most and amitriptyline 9.8% were the most used drugs affecting the central nervous system. Conclusion 11 types of drugs included in the inappropriate category for use in the elderly are risky and should not be selected. Among them, nitrofurantoin 39%, from NSAIDs ketoprofen 19.5%, ibuprofen 7.3% were most used. PIM drugs used for treatment in 2014 are still used in 2024, indicating the need for improved drug monitoring in the elderly