Increasing formulary adoption of adalimumab biosimilars and differential cost-sharing in Medicare Part D plans.

OBJECTIVESAdalimumab was among the first biologics to face widespread biosimilar competition following US market entry of multiple adalimumab biosimilars in 2023. However, Medicare Part D formulary adoption of these products and their associated specialty-tier cost-sharing requirements remain unknown.

METHODSTo evaluate adalimumab biosimilar adoption and associated cost-sharing patterns across Medicare Part D formularies, Centers for Medicare & Medicaid Services (CMS) Medicare Part D Formulary and Plan Benefit Package files were analyzed. Plans were categorized as covering originator adalimumab-only, both originator and biosimilar products (dual coverage), or biosimilars-only.

RESULTSIn 2023, all Medicare Part D plans covering adalimumab listed only the originator product. By 2024, 52.5% of plans provided dual coverage, increasing to 79.9% in 2025. Biosimilar-exclusive coverage increased from 8.6% of plans in 2025 to 45.9% in 2026, while originator-only coverage declined to 0.8%. Median specialty-tier coinsurance differed according to coverage strategy. In 2026, median specialty-tier coinsurance was 33% among originator-only plans, 27% among dual-coverage plans, and 25% among biosimilar-only plans.

CONCLUSIONSMedicare Part D formularies rapidly included adalimumab biosimilars following market entry, with substantial growth in biosimilar-exclusive coverage by 2026. Specialty tier coinsurance varied by medication coverage type, suggesting that evolving formulary strategies may influence beneficiary cost-sharing requirements.

Abstract from PubMed