Background: Ipilimumab, a monoclonal antibody that targets cytotoxic T-lymphocyte antigen-4 (CTLA-4), is effective against malignant tumors, including non-small-cell lung cancer. In Japan, ipilimumab was launched in 2015 with only a 50-mg formulation, followed by a 20-mg formulation in November 2021. However, its effects on clinical practice remain unclear. This study evaluated changes in ipilimumab prescription patterns and drug costs following the introduction of the 20-mg formulation. Methods: Clinical data were analyzed using the MDV analyzer^® (Medical Data Vision Co., Ltd.). The study population included patients who received ipilimumab between November 2020 and October 2023. The investigated parameters included the number of patients, number of prescriptions, prescribed dose (potency), drug costs, and prescribed formulations. Results: Data from 245 medical institutions included 4,366 patients and 13,805 prescriptions. The prescription rate of the 20-mg formulation increased over 12 months after introduction and stabilized at approximately 40% in the following year. The average prescribed dose per prescription decreased from the 80-mg range before the introduction to the 70-mg range 12 months after the introduction. The average drug cost per prescription decreased from the 800,000-yen range to the 600,000-yen range. Conclusion: The addition of the 20-mg formulation increased its usage and reduced the prescribed dose and drug costs. This reduction was likely due to decreased drug waste. These findings suggest that the introduction of low-dose formulations can contribute to cost savings in clinical practice.