The changing Part D landscape.

Introduction: Differences in how Medicare advantage prescription drug plans (MA-PDs) and stand-alone prescription drug plans (PDPs) are financed may contribute to fewer coverage options for traditional Medicare beneficiaries. The Inflation Reduction Act (IRA) capped annual out-of-pocket spending and reduced government reinsurance, thereby placing more cost responsibility on plans and manufacturers and potentially further limiting coverage. Methods: This study draws on 2020-2025 data from the CMS public use files to compare coverage options by cost-sharing categories (defined by mean premium and deductible) across plan types and plan sponsors. Results: A majority of MA-PDs maintained low-premium and low-deductible designs (2020 = 66.0%, 2025 = 62.4%), reflecting cross-subsidies from integrated medical benefits, while enhanced PDPs increasingly pursued low-premium and high-deductible structures (38.2%, 51.5%), and basic/actuarially equivalent (AE) PDPs maintained high-premium and high-deductible structures (66.0%, 65.1%). In 2025, more than half of brand-only drugs in unprotected classes were excluded from Part D formularies (enhanced MA-PDs = 51.8%, enhanced PDPs = 57.3%, basic/AE PDPs = 60.0%), especially in high-spending therapeutic areas. Conclusions: Results reflect increasing consolidation and reduced plan diversity, with potential consequences for access and affordability among traditional Medicare beneficiaries. Policymakers may need to reconsider subsidy and risk-adjustment mechanisms to preserve competition and equitable access across plan types in the evolving Part D marketplace.