Impact of China’s national volume-based procurement policy on insulin procurement, expenditure, and market concentration: an interrupted time series analysis in Shanghai, China

China implemented the National Volume-Based Procurement (NVBP) policy to lower drug prices and reduce medication costs. This study evaluated the effects of the insulin-specific NVBP policy on procurement volume, expenditure, defined daily dose cost (DDDc), and market concentration in Shanghai. Monthly insulin procurement data from Shanghai public medical institutions between January 2020 and December 2024 were analyzed, with July 2022 defined as the intervention point. Interrupted time-series analysis was used to assess changes in procurement volume, expenditure, and DDDc. Subgroup analyses were conducted by insulin generation, manufacturer origin, and bidding group. Market concentration was measured using concentration ratios for the largest one, three, and five corporate groups (CR1, CR3, and CR5) and the Herfindahl–Hirschman Index (HHI). Sensitivity analyses excluded the announcement-to-implementation transition period, incorporated calendar-month fixed effects, and restricted the sample to institutions with recorded insulin procurement in all 60 study months. After policy implementation, average monthly insulin procurement volume increased from 5.44 to 5.69 million DDDs, while average monthly expenditure decreased from CNY 52.43 million to CNY 28.75 million. DDDc decreased from CNY 9.65 to CNY 5.06, representing a 47.6% reduction. ITS analysis showed a significant immediate increase in procurement volume ( P < 0.001) and significant immediate decreases in DDDc ( P < 0.001) and expenditure ( P < 0.05). Sensitivity analyses yielded broadly consistent results. Subgroup analysis suggested a procurement shift toward insulin analogues and domestic insulin. HHI, CR1, CR3, and CR5 declined after policy implementation, indicating reduced market concentration. In Shanghai public medical institutions, the insulin-specific NVBP policy was associated with substantial reductions in insulin procurement cost and expenditure, a short-term upward level shift in procurement volume, and reduced market concentration. These findings suggest that NVBP may improve procurement-level affordability and reshape insulin procurement structure in the Shanghai public hospital sector.