When people talk about a future "generic Ozempic" or "generic Zepbound," they're often using the word generic loosely. Depending on the specific molecule, what eventually reaches the market may legally be a generic, a biosimilar, or something else entirely, and the difference affects both the approval pathway and, potentially, whether a pharmacist can substitute it automatically.
Generic vs. Biosimilar: The Basic Split
A generic drug, approved through an Abbreviated New Drug Application (ANDA), must be shown to be chemically identical to its reference product. That standard works well for small, simple molecules that can be manufactured to an exact, verifiable structure every time. A biosimilar, approved through a different pathway under the Public Health Service Act (often called a 351(k) application), is reviewed against a different standard: highly similar to its reference biologic, with no clinically meaningful differences, rather than identical. That distinction exists because large, structurally complex biologic molecules, including many therapeutic proteins, can have minor, expected batch-to-batch variation even from the original manufacturer, making a strict "identical" standard impractical to enforce.
Where Peptides Fall
Peptides sit in an awkward middle zone, and the FDA has not applied one single rule to all of them. Teriparatide, a relatively short 34-amino-acid peptide, got a true ANDA generic: the FDA approved Apotex's generic teriparatide injection as chemically equivalent to Forteo, not as a biosimilar. Liraglutide followed a similar path, with the FDA approving the first generic liraglutide injection as an ANDA product rather than a biosimilar. Larger, more structurally modified peptides are a different story. Semaglutide and tirzepatide carry fatty-acid side chains and other modifications specifically designed to extend their half-life, discussed in more detail in this site's guide to how peptide half-life gets extended, and most industry and regulatory analysts expect these more complex, modified molecules will need to go through FDA workshops and guidance specifically addressing a generic pathway for GLP-1 drugs, a process covered elsewhere on this site, precisely because it isn't yet fully settled which pathway, or what specific testing, will apply.
Why the Distinction Matters Practically
The generic-versus-biosimilar distinction isn't just academic. An ANDA generic is typically automatically substitutable by a pharmacist, the same way a generic for a small-molecule pill is, once it's rated therapeutically equivalent. A biosimilar is not automatically substitutable unless the FDA separately designates it "interchangeable," a higher bar that requires additional switching studies. For a patient, that can mean the difference between a pharmacist swapping products without a new prescription versus needing the prescriber to specifically authorize the switch.
What the Evidence Standard Looks Like
Both pathways still require real clinical and analytical evidence, just structured differently. A proposed biosimilar typically needs analytical characterization showing structural similarity, at least one clinical pharmacokinetics study, and often a clinical efficacy and immunogenicity study in at least one approved indication, with the goal of showing no clinically meaningful difference from the reference product rather than re-proving the drug works from scratch. A generic peptide under an ANDA needs to demonstrate the same active ingredient, strength, and bioequivalence, similar in spirit to how small-molecule generics are evaluated, but can require additional peptide-specific analytical work given how structurally complex even a "simple" peptide can be compared to a typical small-molecule pill.
The Bottom Line
Whether a future lower-cost semaglutide or tirzepatide product arrives as a generic or a biosimilar isn't settled, and the two paths carry real practical differences in both approval evidence and pharmacy substitution rules. The clearest takeaway from how teriparatide and liraglutide were actually handled is that peptide size and structural complexity, not just the fact that something is called a "peptide," is what determines which pathway applies.