Aim: In 2010, the maximum dosage of metformin was increased in Japan to the maximum dosage approved in the United States. This study aimed to evaluate the impact of the revised dosage in the package insert on metformin-associated lactic acidosis (MALA) reports in Japanese patients using the Japanese Adverse Drug Event Report (JADER) database. Methods: Adjusted reporting odds ratios of MALA was evaluated using multiple logistic regression models. Trends were analyzed using the Mann–Kendall test and Pettitt's test. Receiver operating characteristic (ROC) curve analysis was used to estimate the metformin dose that influenced MALA development. Results: The JADER database contains 845,956 reports submitted between April 2004 and March 2023. The number of adverse event reports and mean dose increased after the 2010 revision of the package insert. According to the ROC curve for lactic acidosis, the cutoff value was 1000.0 mg/day. Conclusions: Increase in metformin doses owing to regulatory actions and the recommendation of the Japan Diabetes Society may have influenced the increased MALA reporting rate, suggesting a dose-dependent relationship between the drug and MALA. The cutoff for the daily dose of metformin associated with MALA was the current maintenance dose listed in the package insert. We believe that MALA development must be carefully monitored in Japan even at the dosage specified in the package insert.