How Medicare Part D Plans Determine Formulary Tier Placement for Interchangeable Biosimilars Under CMS Substitution and Rebate Rules

Medicare Part D formularies balance clinical review with financial incentives set by federal regulation. When an interchangeable biosimilar enters the market, plans weigh the reference biologic’s negotiated rebate against the biosimilar’s net cost once manufacturer discounts apply. That rebate comparison largely decides whether the biosimilar ends up on a preferred or non‑preferred tier.

How CMS Rules Shape Substitution Decisions

The Centers for Medicare & Medicaid Services (CMS) permits Part D sponsors to treat FDA‑designated interchangeable biosimilars as therapeutically equivalent to their reference drugs. Plans can substitute these drugs on the formulary without prior CMS approval if the change leads to equal or lower member cost sharing. This flexibility is meant to promote market competition among similar biologics while keeping formulary reporting transparent.

CMS oversight also reaches into program integrity. Federal Register summaries note that the agency periodically renews data‑matching programs with the Department of Veterans Affairs, the Peace Corps, and other agencies to verify Medicare eligibility. These cross‑agency checks keep formulary changes aligned with enrollment and payment integrity rules.

Economic Incentives Under the IRA and Plan Behavior

Drug Channels analysis illustrates how plan decisions shift when the financial model changes. In prior years, brand drugs with high rebates often received better placement than lower‑cost generics. After the Inflation Reduction Act altered Part D’s cost structure, 2025 formularies displayed near‑universal generic coverage, evidence that net cost now outweighs list price in plan design. The same pattern applies to biosimilars: when their net cost drops below that of their reference product, plans tend to move them into preferred positions or lower cost‑sharing tiers.

This shift brings trade‑offs. Narrower formularies help control overall spending but may reduce product variety. Pharmacists and patients are likely to experience broader substitution between interchangeable biosimilars and their reference products as plan policies adjust to CMS substitution rules and the updated rebate framework.

This article is for informational purposes only and does not constitute medical, legal, or insurance advice. Individuals should consult plan documents or a licensed professional regarding specific coverage decisions.

Sources