Biosimilars and interchangeable insulins, explained
Biosimilars are the closest thing biologic medicines such as insulin and adalimumab have to generics. Only some are designated interchangeable, which is what lets a pharmacist substitute one at the counter. Here is what the terms mean, which insulins are which, how substitution works by state, and what to check when you switch.
Updated October 4, 2026
What happened
- 2021Semglee (insulin glargine-yfgn) became the first biosimilar the FDA designated interchangeable, so pharmacies can substitute it for Lantus where state law allows.
- 2023Biosimilars of adalimumab (Humira) reached US pharmacies, several of them designated interchangeable.
- 2024The FDA proposed that switching studies should generally no longer be needed to show interchangeability, which could make more biosimilars substitutable.
- 2025The FDA approved the first biosimilars of rapid-acting insulin aspart (NovoLog), including one designated interchangeable, and ustekinumab (Stelara) biosimilars launched.
What a biosimilar is
Biologic medicines are made in living cells rather than by chemical synthesis, so a copy can never be molecule-for-molecule identical the way a generic tablet is. A biosimilar is a biologic the FDA has found highly similar to an approved original, with no clinically meaningful differences in safety, purity or effectiveness. Insulins, adalimumab, ustekinumab and many cancer and immune treatments now have biosimilars.
Biosimilar or interchangeable: the difference at the pharmacy
Every biosimilar can be prescribed by name, but only one the FDA has also designated interchangeable can be substituted by the pharmacist for the original without asking the prescriber first, where state law permits. Most states then require the pharmacist to tell the prescriber about the substitution within a set time and to record it. The FDA’s Purple Book lists which biosimilars are interchangeable with which original.
Interchangeability is a legal designation about substitution, not a sign that other biosimilars are less safe or effective. The FDA has proposed simplifying the requirements, so more biosimilars may gain the designation.
Insulin options in plain terms
For long-acting insulin glargine, the original is Lantus. Semglee and Rezvoglar are interchangeable biosimilars, and Basaglar is a related glargine approved before insulin moved to the biosimilar pathway; it is not interchangeable, so it needs its own prescription. For rapid-acting insulin aspart, the original is NovoLog, and the first biosimilars were approved in 2025.
Some insulins are also sold under their generic name by the original maker, such as Insulin Lispro (the same insulin as Humalog) and Insulin Aspart (the same as NovoLog). These are the brand product without the brand name, at a lower list price, not biosimilars, and they are another way to pay less.
Cost and insurance
Biosimilars and unbranded insulins usually have lower list prices than the originals, but what you pay depends on your plan: many formularies prefer one product and charge more for the others. Ask your plan which version it prefers, and if you pay cash, compare pharmacy prices for each version your prescriber is comfortable with. State insulin caps and Medicare’s monthly insulin cap apply to covered insulins whatever the brand.
If you switch
Unit for unit, an interchangeable insulin is dosed the same as the original, but the pen or device may work differently, so ask the pharmacist to show you the new pen. Check your blood sugar a little more often for the first weeks after any switch, and tell your prescriber if your numbers change. For adalimumab and other injected biologics, ask for injection training on the new device and keep to your usual schedule.
What to do
- Ask your pharmacy which version of your biologic or insulin it will dispense, and whether it is interchangeable.
- Ask your plan which version it prefers; the copay can differ between them.
- If you switch insulin, learn the new pen and check your blood sugar more often at first.
- Look up interchangeability in the FDA Purple Book if you are unsure.
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Related
Sources
- FDA, biosimilars
- FDA Purple Book, biosimilar and interchangeable products
- FDA, first interchangeable biosimilar insulin approved (2021)
Written by the Script Unlock editorial team from the reporting and official sources above, as of October 4, 2026. Policies and prices in this area change often; check the linked sources for the latest.
Script Unlock is a marketplace that lets licensed pharmacies quote a cash price on your prescription. It does not prescribe, dispense or sell medication, does not process patient payments, and does not give medical advice. Always follow the instructions of your prescriber and pharmacist and read the medication guide supplied with your medicine.