Objective: In October 2024, the Selected Medical Care System, a patient cost-sharing scheme for off-patent brand-name drugs, was introduced to promote the use of generic drugs (GEs). To clarify the impact of introducing this system on the rate of GE usage, we examined the degree of change following its implementation. Methods: We analyzed the volume share of GEs (September 2019 to March 2025) using an interrupted time-series design with a linear mixed model, and assessed the deviations of individual insurers from historical trends by calculating 95% prediction intervals using linear regression. Results: The median GE volume share of all 1,877 organizations increased from 0.854 in September 2024, immediately prior to introduction of the Selected Medical Care System, to 0.902 in March 2025, revealing that the median half-year-on-half-year rate of change increased from +0.021 to +0.057. Linear mixed modeling confirmed the positive impact of the system on GE volume share among professional subgroups, even after controlling for pre-existing trends, whereas trend analysis revealed that 92.4% (146/158) of the analyzed organizations showed deviations from their historical trends during the post-implementation period. Conclusions: The system for selected medical care coverage has had a notable effect regarding the promotion of GE use and can be considered an effective measure for encouraging behavioral change, even in groups for which the rate of GE usage is generally relatively low.