Background . The study aimed to evaluate inpatient antibiotic use in both the State Second Hospital and State Third Hospital in Mongolia, using the WHO developed and standardized ATC/DDD methodology. Methods. Data were collected from the State Second Hospital and State Third Hospital which are major public hospitals that provide health care for approximately one fifth of the Mongolian population. Antibiotic utilization was monitored retrospectively for five years (2013–2017) using the ATC/DDD methodology and data were presented in DDD/ admission and DDD/100 bed days. Statistical analysis was performed using a Student’s t-test for parametric data. A P value of ≤0.05 was considered to be statistically significant. Results. The annual consumption rates in the State Second Hospital were stable over time while in the State Third Hospital consumption rates varied considerately between years. Overall, the total antibiotic consumption rate was very high, but has decreased in both hospitals. The rate of consumption of all antibiotics was approximately twice that in the State Third Hospital (421.7 DDD/100 bed days) between 2013 and 2017 when compared with the State Second Hospital (199.7 DDD/ 100 bed days), P<0.001). The seven most frequently used antibiotics comprised approximately 75% of all DDDs in both hospitals, in the period 2013–2017; being: amoxicillin, cefazolin, cefotaxime, ceftriaxone, clarithromycin, ciprofloxacin, and nitroxoline. However, this was not consistent when considering the individual years, since in 2015 and 2016, these seven active agents represented approximately 50%. Conclusion . This is the first hospital-based study of antibiotic consumption rates reported in Mongolia. In addition to very high consumption rates, large differences occurred between the hospitals investigated. Inappropriate and high levels of antibiotic use lead to increased costs and also increased nosocomial infection rates with potentially resistant species. The Government and health professionals need to take more active roles in improving and promoting quality antibiotic use among inpatients.

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