Medicare Part D insulin coverage: formulary strategies amid policy headwinds.

In recent years, multiple insulin-related policies took effect for Medicare Part D plans, including the Part D Senior Savings Model test (2021-2023) and the 2022 Inflation Reduction Act, provisions of which were implemented between 2023 and 2025. These policies created a variety of financial incentives for plans that may influence insulin formulary design in Part D, potentially affecting beneficiary access to insulin. For example, while limiting cost-sharing lowers out-of-pocket costs for beneficiaries, it may also reduce plans' ability to use tiering to steer beneficiaries toward preferred insulins. Using Part D formulary files from 2020-2025 we examined plan insulin coverage separately for Medicare Advantage Prescription Drug Plans and stand-alone Prescription Drug Plans. We found that plans are moving insulins to a single tier but not broadly changing their utilization management tool strategies. Combination agents (insulins paired with GLP-1s) account for the majority of insulins with quantity limits, and concentrated insulins account for the bulk of insulins with prior authorization requirements. This study demonstrates strategic adjustments that insurers may be making in response to policy changes; ongoing study will be needed to monitor access to and spending on insulin in Medicare as the market and policy context continue to evolve.