Biosimilars are biologic drugs shown to be highly similar to an already-approved reference biologic, with no clinically meaningful differences in safety or effectiveness. Interchangeability is a separate, additional designation a biosimilar can earn: it means the product has met a higher evidentiary bar allowing it to be substituted for the reference product at the pharmacy counter, similar in spirit to how a generic can be substituted for a brand-name small-molecule drug.
Not every biosimilar carries an interchangeable designation, and even when one does, whether and how a pharmacist may act on it — what notification to the prescriber is expected, what recordkeeping applies, whether the patient must be informed — is shaped by state pharmacy practice law layered on top of the federal designation.
Why it matters
Biosimilars occupy a growing share of specialty and infused-drug therapy, and pharmacists working in specialty pharmacy, infusion, or health-system settings increasingly need to know whether a given biosimilar can be substituted the way a generic drug can, or whether it requires a new prescription. Because interchangeability status is a federal designation but substitution mechanics are governed at the state level, this is a two-layer question: is the product interchangeable, and does the state permit acting on that status the way the pharmacist intends.
This entry explains the concept generally and is not a determination about any specific product or transaction. RxByState tracks how each state handles biosimilar substitution authority — see https://app.rxbystate.com/signup.