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Abstract Background Abnormal behavior after oseltamivir administration has been reported in the media; in 2007, the package insert for oseltamivir phosphate was revised to restrict its administration to individuals aged over 10 years. However, in 2018, the age limitation specified in the package insert was removed. Here, we evaluated the trends in anti-influenza drug prescription and adverse drug reactions (ADRs) reported in pediatric outpatients after revising the oseltamivir package insert as an ecological study. Methods Anti-influenza drug prescriptions for pediatric outpatients with influenza aged 0–19 years were downloaded from the acute Diagnosis Procedure Combination hospital databases using the MDV analyzer®. ADR reports on anti-influenza drug prescription among patients aged 0–20 years in the Japanese Adverse Drug Event Report database were downloaded from the Pharmaceutical and Medical Devices Agency website. Data were collected during the 2016/2017 and 2019/2020 influenza seasons. Results During the influenza epidemic season (January–March), the percentage of oseltamivir prescriptions for patients with influenza aged 10–19 years tripled after the revision of the oseltamivir package insert (9.3% during the 2016/2017 season and 29.2% during the 2019/2020 season); however, reports of abnormal behavior did not increase (two during the 2016/2017 season and none during the 2019/2020 season). Conclusions The number of oseltamivir-related ADR reports among minors over 10 years of age did not increase although the proportion of oseltamivir prescriptions increased after the revision of the oseltamivir package insert.