How interchangeability and biosimilar substitution rules shape formulary placement and pharmacy coverage decisions
Interchangeability and biosimilar substitution are connected, but they do not do the same thing. Interchangeability is a regulatory designation tied to whether a pharmacist can substitute a biosimilar for the reference product under the applicable substitution rules. Formulary placement, by contrast, is a benefit design decision made by a plan or pharmacy benefit manager. One does not automatically determine the other.
Interchangeability vs. formulary status
| Issue | What it affects | Why it matters for coverage |
|---|---|---|
| Interchangeability | Whether substitution at the pharmacy counter is allowed under the governing substitution rules | It can make dispensing easier if the prescribed product and the covered product are different |
| Biosimilar formulary placement | Which product a plan prefers, covers, or puts on a lower or higher cost-sharing tier | It determines out-of-pocket cost, prior authorization, and whether step therapy or product-specific edits apply |
A plan can prefer a biosimilar on its formulary even when that product is not interchangeable, because coverage decisions turn on the formulary and the utilization management rules. Another plan might keep the reference product preferred and require extra review before covering a biosimilar. So the real question is not just whether substitution is allowed. It is also whether the member’s pharmacy benefit will pay for the substituted product.
When plans and pharmacies decide what gets covered
CMS explains that Medicare Part D provides outpatient prescription drug coverage through private plans approved by Medicare, and those plans use formularies and cost-sharing structures that vary by plan. As a result, biosimilar access can differ across Part D options even within the same clinical category. For a claim to adjudicate cleanly, the pharmacy has to line up the prescription, the plan’s formulary, and any utilization management rules. All of it.
If the prescribed biologic is not the product the plan prefers, several things can happen. The pharmacy can substitute if the governing substitution rules permit it and the claim is payable for that product. If substitution is not available, the prescriber may need to issue a new prescription or support an exception request. And if a product is covered but sits on a less favorable tier, the member can still face higher cost sharing even though the plan considers the drug clinically acceptable.
Start with the basics
Readers should first check which product is on formulary, whether prior authorization applies, and whether the pharmacy can dispense an alternative product without a new prescription. Pharmacists need to confirm both legal substitution authority and claim-level coverage. Benefits managers should look at formulary alignment, exception pathways, and member communications, since substitution rules by themselves do not guarantee affordable access.
Disclaimer: This information is for general educational purposes only and is not medical, legal, or insurance advice. Coverage, substitution authority, and formulary rules depend on the specific plan, benefit design, pharmacy system edits, and applicable law. Check plan documents and consult the prescribing clinician, pharmacist, or insurer for case-specific guidance.