Medicare 2026

Biosimilars and Medicare: What "Interchangeable" Really Means for Your Prescriptions

If your doctor has prescribed a biologic medication — the kind made from living cells, often used for conditions like diabetes, rheumatoid arthritis, inflammatory bowel disease, psoriasis or certain cancers — you may have heard the word "biosimilar" come up at the pharmacy or in a letter from your Part D plan. And if you looked closely, you might have seen a second term: "interchangeable." Both words matter for your coverage, your copay, and sometimes for what actually ends up in your prescription bottle.

Here is a plain-English guide to how biosimilars fit into Medicare prescription coverage.

What a biosimilar actually is

According to the FDA, a biosimilar is a biologic medication that is highly similar to an already FDA-approved original biologic — called the "reference product" — with no clinically meaningful differences in safety, purity or potency. Think of it as the biologic world's version of a generic, with one important caveat: biologics are made from living systems, so no two batches are ever atom-for-atom identical. That is why the FDA uses the word "biosimilar" rather than "identical."

The FDA has been clear: biosimilars and interchangeable biosimilars are both as safe and effective as the reference product.

What "interchangeable" adds

Every interchangeable product is a biosimilar, but not every biosimilar is interchangeable. To earn the "interchangeable" label, a manufacturer has to specifically request that designation and meet extra FDA requirements.

The practical result shows up at the pharmacy counter. Depending on your state's pharmacy laws, an interchangeable biosimilar may be substituted for the reference product without the prescriber being involved — much the way a pharmacist can fill a brand-name prescription with its generic. A biosimilar that is not interchangeable, by contrast, typically has to be prescribed by name under state law.

So "interchangeable" isn't a statement that one biosimilar is better or safer than another. It is a regulatory and pharmacy-practice label about who gets to make the swap.

Insulin is now in this picture

Insulin is regulated as a biologic, which opened the door to biosimilar versions of brand-name insulins. Depending on your state's law, a pharmacist may be able to dispense an interchangeable biosimilar insulin in place of the brand-name product, much like a generic swap. If you take insulin, it is worth asking your pharmacist what version you're receiving and what your plan covers.

How biosimilars show up in Part D coverage

Part D drug lists (formularies) can include both the original biological product and one or more biosimilars. A plan that uses step therapy may require you to try a generic or a biosimilar first before it will cover a more expensive medication. That is legal under federal Part D rules, though your prescriber can request an exception if there is a medical reason you need a specific product.

Federal Part D rules also allow plans to update their drug lists when new biosimilars come to market:

• If a plan has given advance general notice, it may make an immediate change to an original biological product within 30 days of adding a newly marketed interchangeable biosimilar (or an unbranded version sold under the same license) on the same or a lower tier with the same or less restrictive coverage rules.
• If the new product is a biosimilar that is not interchangeable, the change is treated as a "maintenance change." The plan must give affected members written notice at least 30 days ahead or, at the next refill request, provide a month's supply under the old rules along with written notice.

Translation: your plan can bring biosimilars onto its drug list mid-year, and the kind of notice it has to give you depends on whether the new product is interchangeable. If you get a letter from your Part D plan about a formulary change, read it carefully — it may tell you about a new biosimilar option.

Why your copay might change

Here is the piece that catches many people off guard: your copayment or coinsurance may increase if you keep taking the original biological product after your plan adds a biosimilar and moves the original to a higher cost-sharing tier. The drug hasn't changed. Your coverage of it has.

If that happens, you have a few paths worth discussing with your prescriber: switching to the biosimilar the plan now prefers, asking your prescriber to request a formulary exception, or weighing the higher cost if you and your prescriber want to stay on the original. The Part D out-of-pocket cap — $2,100 in 2026 and $2,400 in 2027 — limits what you'll pay out of pocket for covered Part D drugs in a calendar year, which matters a lot for anyone on expensive biologics.

Checking whether your medication has a biosimilar

The FDA publishes the Purple Book, a searchable database of FDA-licensed biological products, including biosimilars and interchangeable products. If you want to see whether a biosimilar exists for your medication — and whether any version is designated interchangeable — the Purple Book is the official source.

Thinking about coverage during Annual Enrollment

Medicare's Annual Enrollment Period runs October 15 through December 7, 2026 for coverage that starts January 1, 2027. If you take a biologic, this is a good time to check how your options cover it, including whether the original product, the biosimilar, or both appear on the drug list and on what tier. The most any Part D plan may charge as a deductible in 2027 is $700, and the national Part D base beneficiary premium used in the late-enrollment penalty calculation is $41.33 for 2027; actual plan premiums are set plan by plan.

People with Medicaid or Extra Help have additional flexibility. Since 2025, there is a monthly Special Enrollment Period to move to Original Medicare with a stand-alone Part D plan, or to switch stand-alone Part D plans. Full-benefit dual-eligible individuals can also join an integrated Dual Eligible Special Needs Plan (D-SNP) in any month.

A broader choice some people weigh is Original Medicare paired with a Medigap policy and a stand-alone Part D plan versus a Medicare Advantage plan that includes drug coverage. Both routes can cover biologics; they differ in provider networks, cost-sharing structure, referral and prior authorization rules, and what extra benefits are included. Neither is "better" in the abstract — it depends on your medications, your doctors, and your budget.

CMS announces the 2027 Part B standard premium and deductible, and the 2027 IRMAA income brackets, in the fall (usually November). Those figures haven't been released as of this writing.

Where to go next

For official, up-to-date Medicare information — including drug list details, enrollment windows and coverage questions — visit Medicare.gov or call 1-800-MEDICARE (1-800-633-4227), TTY 1-877-486-2048, available 24 hours a day, 7 days a week. For questions about a specific biosimilar or whether it has been designated interchangeable, the FDA's Purple Book is the authoritative source.