Biosimilars and interchangeable biologics explained

Updated 2026-10-17 · Reviewed for accuracy by the EKO Labs team. Educational only; not medical advice.

Some medicines are not made from simple chemicals but from living cells. These are called biologics, and the lower-cost versions of them are called biosimilars. If your doctor or pharmacist mentions a name with four odd letters at the end, such as "insulinInsulin the hormone that moves sugar from the blood into the body's cells. glargine-aaaa", this guide explains what that means, how FDA reviews biosimilars, what "interchangeable" really means, and what to ask. It is general information, not medical advice.

What is a biologic?

FDA explains that biological products, also called biologics, can be made of sugars, proteins, living cells, tissues, or a combination of these. They come from natural and living sources such as animal and plant cells and microorganisms like bacteria or yeast. Biologics are usually more complex than other drugs and are often harder to purify, process and manufacture. Biologics are used for many chronic and serious conditions, including diabetes, arthritis, chronic skin and bowel diseases, kidney conditions, multiple sclerosis, osteoporosis, macular degeneration and some cancers.

For legal purposes, the federal definition of a "protein" in the biologics rules is an amino acid polymer with a specific, defined sequence that is greater than 40 amino acids in size (21 CFR 600.3(h)(6)). Smaller peptides are generally handled under the drug rules, not the biologic rules. Whether your own medicine is a biologic is something your pharmacist or the FDA labelPrescribing information (label) the official FDA document for a medicine, with its uses, warnings and side effects. can tell you.

What is a biosimilar?

According to FDA, a biosimilar is a biologic that is highly similar to an FDA-approvedFDA-approved the FDA reviewed the medicine's safety and effectiveness for a specific use. biologic, called the reference product, and has no clinically meaningful differences in safety, purity and potency (safety and effectiveness). FDA says you can expect the same safety and effectiveness from a biosimilar over the course of treatment as you would from the reference product. A biosimilar and its reference product:

• are made from the same types of sources (for example living cells or microorganisms);
• provide the same benefits when treating a disease or condition;
• are given the same way and at the same strength and dosage;
• have the same potential side effects, and are not expected to cause new or worse ones.

A biosimilar is not identical to the original. Because biologics come from living sources, FDA notes that even the original product has small differences from batch to batch, so a biologic cannot be copied exactly. FDA reviews any differences to be sure they do not matter for safety or effectiveness. FDA also says a biosimilar is not safer or more effective than the original biologic.

How does FDA approve a biosimilar?

FDA approves biosimilars through an abbreviated pathway created by the Biologics Price Competition and Innovation Act of 2009. The goal of a biosimilar program is to show that the product is biosimilar to its reference product, not to prove the safety and effectiveness of the new product all over again. FDA evaluates the evidence in the context of its earlier finding that the reference product is safe and effective. An application generally includes:

• Analytical studies comparing structure and function, which are the foundation of the application;
• Animal studies, which may provide toxicology or pharmacology information;
• Clinical studies, such as how the body handles the product, how the immune system responds (immunogenicity), and sometimes comparative studies in patients.

FDA evaluates each proposed biosimilar individually and has discretion to decide that an element is unnecessary. A biosimilar can also be approved for a use of the reference product it was not directly studied in, if the manufacturer gives adequate scientific justification. After approval, FDA says it keeps monitoring safety and effectiveness, checks product quality during production and reviews patient safety reports.

What does "interchangeable" mean?

An interchangeable biosimilar is a biosimilar that, depending on state pharmacy law, may be substituted for the reference product without the prescriber stepping in, much like a generic drug is substituted for a brand-name drug. FDA calls this "pharmacy-level substitution." A manufacturer must specifically ask FDA for this status and meet additional requirements, which can include a switching study in which patients alternate between the reference product and the biosimilar.

Two points matter for patients. First, FDA states that being interchangeable does not mean it is safer or more effective than other biosimilars: both kinds meet the same high standards for quality and similarity. Second, whether a pharmacist may actually substitute depends on your state's law, so ask your pharmacy how it works where you live.

Why the odd four-letter ending?

FDA's naming guidance (final, January 2017) says the nonproprietary name of each originator biologic, related biologic and biosimilar is a core name plus a distinguishing suffix of four lowercase letters that is devoid of meaning, for example the "-aaaa" in a made-up "insulin glargine-aaaa". The suffix helps tell products apart when prescribing, dispensing and reporting side effects. Always use the full name, suffix included, when you report a problem.

Biosimilars versus generics

FDA says biosimilars are like generics in some ways: both are compared with an original product, both go through abbreviated pathways that avoid repeating certain costly clinical trials, and both may offer more affordable options. But generics are often made from chemical ingredients and can generally be copied exactly, while a biologic cannot be copied exactly, so the information needed to show that a biologic is biosimilar can be much more extensive. See also our guide on generic versus brand-name medicines.

Cost and insurance

FDA says a patient might switch to a biosimilar because of a change in insurance coverage or to save money. The lower cost is not a sign of lower safety or effectiveness: as with generics, the maker relies on FDA's finding that the original biologic is safe and effective. FDA does not control drug prices or decide whether insurers cover a biosimilar. Check your plan's drug list (formulary) and ask your pharmacy or insurer about your own cost.

Insulin as an example

FDA explains that insulin was historically regulated as a drug made from chemicals, and insulin and other products that meet the criteria for a biologic are now regulated as biologics. This does not change the ingredients of insulin or how you get it at the pharmacy. It does allow several companies to make biosimilar versions of "brand name" insulins, similar to how generics are versions of brand-name drugs.

How to look one up

FDA publishes a list of approved biosimilars on its Biosimilar Product Information page, and the Purple Book (purplebooksearch.fda.gov) is its database of licensed biological products, including biosimilars and interchangeable products. Lists change as new products are approved, so check the FDA pages for the current list rather than relying on an old copy.

Questions to ask your doctor or pharmacist

• Is my medicine a biologic? Is there a biosimilar or an interchangeable biosimilar for it?
• If I switch, does anything change in how I take or store it?
• Is the biosimilar on my plan's formulary, and what would I pay?
• Will the pharmacy substitute a product without calling you, and will I be told?
• What is the full name of the product I am getting, including the suffix?

Do not stop or switch a prescribed medicine on your own. At EKO Labs, our team can help you in Portuguese or English by phone to prepare questions for your consultation.

Sources: FDA, "Biosimilars: What Patients Need to Know" (fda.gov/drugs/biosimilars/biosimilar-basics-patients); FDA, "Biosimilars: Review and Approval" (content current as of 12/13/2022); FDA, "Biosimilar Product Information"; FDA, "Nonproprietary Naming of Biological Products, Guidance for Industry" (January 2017); FDA Purple Book; 21 CFR 600.3(h)(6) (eCFR, consulted October 2026). This guide is general information, not legal or medical advice.