Outpatient Dispensing Patterns and Changes in Tirzepatide Strength Mix in Japan: A Study Using the National Open Claims Data

BACKGROUND: Tirzepatide is a once-weekly injectable dual agonist of the glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) receptors; in Japan, it is approved for the treatment of type 2 diabetes and marketed under the brand name Mounjaro, which is available in six strengths. Japanese studies of tirzepatide have primarily evaluated efficacy, real-world effectiveness, safety, persistence, and patient-level maintenance doses. National patterns in dispensing settings and longitudinal changes in the product-strength mix have not been described. We examined whether increases in claim-recorded quantities were accompanied by structural changes in outpatient dispensing and strength mix. METHODS: We analyzed injectable-drug quantity tables for the fiscal year 2024 from the 11th National Database of Health Insurance Claims and Specific Health Checkups of Japan Open Data. Full National Health Insurance drug codes were used to identify all six Mounjaro strengths. The primary tables excluded public-expense claims, and public-expense-inclusive tables were used in a sensitivity analysis. Monthly analyses required numeric values for every strength in both the within-institution and outside-institution outpatient prescription worksheets. Outcomes included annual quantities by dispensing category and strength, monthly total quantity, the share dispensed outside medical institutions, strength-mix indicators, and each strength's contribution to the increase from June 2024 to March 2025. RESULTS: The primary outpatient total was 7,829,974.7 kit units. Of this total, 6,748,743.5 kit units (86.2%) were recorded as outpatient prescriptions dispensed outside medical institutions. Complete monthly data were available from June 2024 through March 2025. During this interval, monthly quantity increased from 392,354 to 1,027,955 kit units; outside-institution quantity increased from 329,431 kit units (84.0%) to 899,994 kit units (87.6%); and the quantity represented by strengths of 7.5 mg or higher increased from 50,902 kit units (13.0%) to 246,316 kit units (24.0%). The quantity of the 2.5-mg product increased from 159,318 kit units (40.6%) to 278,540 kit units (27.1%), while its share decreased. The 5-mg product accounted for 320,965 kit units (50.5%) of the absolute increase, whereas strengths of 7.5 mg or higher collectively accounted for 195,414 kit units (30.7%). The public-expense-inclusive sensitivity analysis preserved the ranking of strengths and the predominance of outside-institution dispensing. CONCLUSIONS: Tirzepatide dispensing in Japan was concentrated in the outside-institution outpatient category, and the share in this category increased over the fully observable period. Growth was also accompanied by a measurable shift in the product-strength mix. This study provides an auditable national description of dispensing-setting and product-mix dynamics rather than an analysis of patient uptake, adherence, clinical outcomes, or individual titration.