A biosimilar is a biological medication that is highly similar to an already approved biologic drug and has no clinically meaningful differences from it in terms of safety or effectiveness. Think of it as the biologic equivalent of a generic, but with an important distinction: biologics are far too complex to copy exactly, so biosimilars go through a more rigorous approval process than traditional generics do. The FDA has approved 76 biosimilars to date, and they typically cost about 50% less than the brand-name biologic they reference.
Biologics vs. Traditional Drugs
To understand biosimilars, you first need to understand what makes biologic drugs different from the pills most people are familiar with. A conventional drug like ibuprofen is a small, simple molecule that can be precisely replicated through chemical synthesis. Its molecular weight is around 500 Daltons, a unit scientists use to measure molecular size. A biologic, by contrast, is a large protein produced by living cells. A monoclonal antibody, one of the most common types, has a molecular weight exceeding 150,000 Daltons. That’s roughly 300 times larger than a typical small-molecule drug.
This size difference matters because biologics have a layered, four-level structure that determines how the drug works in your body. Each molecule also undergoes a process called glycosylation, where sugar molecules attach to the protein in patterns that affect its function. These patterns aren’t identical even between molecules in the same manufacturing batch. So a batch of a biologic is really a pool of closely related but not perfectly identical molecules, rather than a collection of exact copies. This complexity is why there’s a saying in the field: “the process is the product.”
Why Biosimilars Aren’t Just “Generic Biologics”
When a traditional generic drug is approved, the manufacturer simply proves its product has the same active ingredient and is absorbed into the body at the same rate. The molecule is small enough to be chemically identical, and confirming that identity is straightforward.
Biosimilars can’t work this way. Because biologics are grown in living cells rather than synthesized chemically, and because even small changes in the manufacturing environment can alter the final protein, no two manufacturers will produce a perfectly identical product. Minor differences in clinically inactive components, like stabilizers or buffers, are acceptable. But the core protein must be shown to be highly similar through extensive structural and functional testing using state-of-the-art analytical technology. The manufacturer compares characteristics like purity, chemical identity, and bioactivity between its product and the original.
How Biosimilars Get Approved
The original biologic goes through a full, standalone approval process. The manufacturer must generate all the evidence from scratch to prove the drug is safe and effective. This is expensive and time-consuming, which is a major reason brand-name biologics cost so much.
Biosimilars follow an abbreviated pathway. Instead of proving safety and effectiveness from the ground up, the biosimilar manufacturer demonstrates that its product is highly similar to the already approved reference product. It can rely in part on the FDA’s previous determination that the reference product is safe and effective. The application still requires analytical studies comparing the two products, animal toxicity studies, and clinical studies assessing how the drug behaves in the human body and whether the immune system reacts to it. However, the FDA has the discretion to waive any of these elements if the analytical data is strong enough on its own.
Interchangeable Biosimilars
Some biosimilars carry an additional designation: interchangeable. Both standard biosimilars and interchangeable biosimilars meet the same high standard of safety and effectiveness. The difference is a practical one. An interchangeable biosimilar can be substituted for the reference product at the pharmacy level, similar to how a pharmacist can swap a brand-name pill for its generic without calling your doctor.
To earn this designation, the biosimilar must show it can be expected to produce the same clinical result as the reference product in any given patient. For drugs given more than once, the manufacturer must also demonstrate that switching back and forth between the biosimilar and the reference product carries no greater risk than staying on the reference product alone. The FDA has approved 13 interchangeable biosimilars so far, and notably, 9 of those were approved without requiring a dedicated switching study. The agency is moving toward making this the norm, signaling growing confidence in the data from standard biosimilarity testing.
Safety of Switching
If your doctor recommends switching from a brand-name biologic to a biosimilar, the clinical evidence is reassuring. A systematic review covering more than 5,200 patients who switched between biosimilars and their reference biologics found no statistical difference in safety outcomes compared to patients who stayed on a single product. Rates of serious adverse events, treatment discontinuation, and deaths were essentially identical between the two groups. Immune-related reactions, including injection site reactions, allergic responses, and the development of antibodies against the drug, were also comparable whether or not patients switched. This held true for patients who switched once and for those who switched multiple times.
Common Biosimilar Categories
Biosimilars are available for some of the most widely prescribed (and expensive) biologic drugs. Several biosimilars reference adalimumab, originally sold as Humira, which treats conditions like rheumatoid arthritis and Crohn’s disease. Others target biologics used in cancer treatment, such as pertuzumab (originally Perjeta) and pegfilgrastim (originally Neulasta), a drug that helps prevent infection during chemotherapy. Eye conditions are another active area, with biosimilars now available for ranibizumab (Lucentis) and aflibercept (Eylea), both used to treat age-related vision loss. Multiple biosimilars for denosumab, used to treat osteoporosis and bone complications from cancer, were approved in 2025 alone.
Cost Savings
The financial case for biosimilars is significant. On average, their sales price is 50% lower than what the reference biologic cost at the time the biosimilar launched. For patients on biologics that can run thousands of dollars per dose, this translates into real savings on copays and coinsurance, since those costs are often calculated as a percentage of the drug’s price. Still, the 76 approved biosimilars correspond to only a small fraction of all approved biologics, meaning there’s considerable room for the market to expand and bring prices down further across more treatment areas.