Introduction and Objective: The high cost of insulin hinders access and utilization of insulin, a life-sustaining medication, for U. S. adults with diabetes. By 2025, 27 states and DC have passed and implemented insulin out-of-pocket (OOP) caps, but the reach of these caps is uncertain. In this study, we estimate the population being protected by state OOP caps among commercially insured insulin users, identify unprotected populations, and project the protected population across different implementation scales. Methods: Policy surveillance across 50 states and DC (2019-2025) identified insulin OOP cap implementation and specifications. We linked data from CDC USDSS diabetes prevalence, Census HICR private coverage, KFF self-funded penetration, and NHIS insulin use (via IPUMS) to estimate the impacted populations per state-year. Results: In 2025, an estimated 774, 116 commercially insured insulin users were protected (734, 061 with 35 per insulin per month prescription cap; 40, 055 with 100 for all monthly insulin cap). Conversely, 3, 162, 338 remained unprotected: 1, 043, 797 in states that implemented caps (due to enrollment in a noneligible plan) and 2, 118, 541 in states without caps. If all states adopted caps for state-regulated plans, 906, 139 additional people would gain protection. Protected proportions were highest in Washington (55. 9%) and lowest in Connecticut, Kentucky, and West Virginia (30. 6%). California, with the upcoming 2026 implementation, would newly protect ~249, 846, reducing its unprotected population from 445, 353 to 195, 510. Conclusion: State OOP caps substantially extend financial protection but still leave many commercially insured insulin users uncovered due to ERISA exemptions for self-funded plans and non-adopting states. Broader adoption and federal action can expand reach. Future work should assess effects on patient spending, utilization of insulin, and health outcomes, as well as monitoring potential shifts in premiums and pricing. Disclosure R. Wang: None. U. F. Naeem: None. A. Everhart: None. S. Dusetzina: None. P. Karaca Mandic: Research Support; Current; Mayo Clinic. Other - CEO and holds equity; Current; XanthosHealth. K. Lipska: None. R. McCoy: None. Funding American Diabetes Association (1-26-PDF-0552), NIDDK (R01DK141666)
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