The Purple Book isn’t a novel you read for fun-it’s the U.S. Food and Drug Administration’s official database for biological medicines, including biosimilars and interchangeable products. If you’re a pharmacist, doctor, or patient trying to understand what can be swapped for what, this is the single most important resource you need to know. Unlike a printed book, it’s online, searchable, and updated in real time. And as of 2026, it’s the only place you’ll find clear, federal-level answers about which biological drugs are interchangeable and which aren’t.
What Exactly Is the Purple Book?
The Purple Book is a searchable database maintained by the FDA that lists every approved biological product in the U.S. That includes the original brand-name biologics, their biosimilar copies, and the few that have been officially labeled as interchangeable. It was created under the Biologics Price Competition and Innovation Act (BPCIA) of 2010, but it wasn’t until 2020 that the FDA merged two separate lists-one for drugs under CDER and one for biologics under CBER-into one unified system. Before that, finding information was messy. Now, it’s all in one place.Each product in the Purple Book gets a color-coded card. Matching colors mean products are related-like a reference biologic and its biosimilar. You’ll see the brand name, the generic name, the date it was approved, and whether it’s a 351(a) reference product, a 351(k) biosimilar, or a 351(k) interchangeable. Icons show delivery methods too-autoinjectors, prefilled syringes, vials. It’s designed so pharmacists can quickly scan and know what’s available.
Biosimilars vs. Interchangeable: The Key Difference
This is where people get confused. All interchangeable products are biosimilars. But not all biosimilars are interchangeable. Think of it like this: a biosimilar is a very close copy of the original biologic. It’s not identical-no two biological molecules are-but it’s so similar that there are no meaningful differences in safety or effectiveness.An interchangeable product goes further. To earn that label, the FDA requires proof that switching back and forth between the biosimilar and the original doesn’t increase risk or reduce effectiveness. That means if a patient uses the reference drug for three months, then switches to the biosimilar for three months, then switches back-there’s no drop in results or spike in side effects. This isn’t just about being similar. It’s about being predictable in repeated use.
The FDA’s definition is clear: an interchangeable product must show that the risk of alternating between it and the reference product is no greater than using the reference product alone. That’s a higher bar than biosimilarity. And it’s why only a handful of biosimilars have made it to interchangeable status.
Why Does Interchangeability Matter?
Interchangeability isn’t just a technical label-it affects what happens at the pharmacy counter. In most states, pharmacists can swap an interchangeable biosimilar for the prescribed brand-name drug without asking the doctor. That’s not true for regular biosimilars. For those, the prescriber has to specifically write “do not substitute.”As of 2024, 47 states and Puerto Rico allow pharmacists to make this substitution without prior approval from the prescriber. But rules vary. Some states require the pharmacist to notify the doctor. Others require patient consent. A few still don’t allow substitution at all. That means even if a product is federally approved as interchangeable, whether you get it instead of the brand depends on where you live.
And here’s something important: the FDA stresses that interchangeable doesn’t mean better. It doesn’t mean safer. It doesn’t mean more effective. It just means you can switch between it and the original without added risk. The science behind both biosimilars and interchangeable products is solid. The difference is in the level of evidence needed for approval.
What’s in the Purple Book Right Now?
As of late 2023, only seven biosimilars had received interchangeable status from the FDA. That number is growing, but slowly. The approved interchangeable products include:- Two insulin products (used for diabetes)
- Three treatments for inflammatory conditions like rheumatoid arthritis and Crohn’s disease
- Two therapies for eye conditions affecting the retina
These are all high-cost, complex biologics. Their interchangeable versions are expected to bring down prices over time. But adoption isn’t automatic. Even with FDA approval, manufacturers still need to convince payers, hospitals, and pharmacies to switch. Many are still hesitant, partly because of state-level confusion and partly because of lingering misconceptions.
The Purple Book also lists every reference biologic and every biosimilar that’s been approved-even if it’s not interchangeable. So if you’re looking for a biosimilar version of Humira, Enbrel, or Herceptin, you’ll find it. The database shows which biosimilars have been approved, when, and against which reference product.
How to Use the Purple Book
It’s free, public, and easy to use. Go to the FDA’s website, search for a brand name like “Humira,” and you’ll see the original product and every biosimilar or interchangeable version that’s been approved to match it. The results are grouped under the reference product. That’s intentional. It helps you see the whole family of related products at once.Use the filters to narrow by product type-insulin, autoimmune, oncology-or by status: biosimilar, interchangeable, or reference. You can also check the date of approval and whether the product has exclusivity protection. That’s important because some reference products have a 12-year exclusivity period before biosimilars can even apply for approval.
Pharmacists use it daily to verify substitution rules. Patients can use it to check if their prescribed drug has a cheaper alternative. Providers use it to understand what’s available when writing prescriptions. It’s not a marketing tool. It’s a factual, regulatory resource.
What’s Not in the Purple Book
Don’t expect pricing info. You won’t find out how much a biosimilar costs compared to the brand. That’s up to insurers and pharmacies. You won’t find patient reviews or side effect reports either. Those come from other FDA systems like MedWatch.And don’t confuse “unbranded biologic” with “interchangeable.” An unbranded biologic is a version that’s not marketed under a brand name, but it hasn’t gone through the interchangeability process. The FDA considers it equivalent, but it’s not legally interchangeable under pharmacy substitution laws.
What’s Next for Biosimilars?
More applications for interchangeable status are coming. Companies are investing in the extra clinical studies needed to prove switching safety. The FDA has released draft guidance on labeling to make sure these products are clearly marked. That’s a big step-it means pharmacists and patients won’t be confused about what they’re getting.But progress is slow. The science is there. The regulatory path is clear. What’s holding things back is complexity: state laws, payer policies, provider education, and patient trust. The Purple Book gives you the facts. But changing how the system works takes time.
For now, if you want to know what’s approved, what’s interchangeable, and what can be substituted, the Purple Book is your starting point. It’s not perfect. But it’s the most accurate, up-to-date, and official source you’ve got.
Is the Purple Book the same as the Orange Book?
No. The Orange Book lists approved generic drugs-small-molecule medicines like ibuprofen or metformin. The Purple Book lists biological products-larger, complex molecules made from living cells, like insulin or antibodies. They’re two different systems for two different types of medicines.
Can a pharmacist substitute a biosimilar without a doctor’s permission?
Only if the biosimilar has been designated as interchangeable by the FDA and your state allows substitution. Regular biosimilars require a prescriber to write "dispense as written" or "no substitution." Interchangeable products can be swapped automatically in most states, but rules vary. Always check your state’s pharmacy laws.
Are interchangeable biosimilars safer than regular biosimilars?
No. The FDA says interchangeable biosimilars are not safer or more effective than other biosimilars. The only difference is that they’ve passed extra studies proving you can switch between them and the original drug multiple times without increased risk. Both types are equally safe and effective when used as directed.
Why are so few biosimilars interchangeable?
Because proving interchangeability requires additional clinical trials-specifically, studies that test switching back and forth between the biosimilar and the reference product. These studies are expensive and time-consuming. Many manufacturers choose to skip them and focus on getting biosimilar approval, which is enough to enter the market. Interchangeability is a bonus, not a requirement.
Where can I find the Purple Book?
Go to the FDA’s official website at fda.gov and search for "Purple Book." It’s a free, public database. No login required. You can search by brand name, generic name, or therapeutic category. It’s updated regularly as new products are approved.
Does the Purple Book include pricing information?
No. The Purple Book only shows regulatory status-approval date, reference product, interchangeability designation. It does not include prices, insurance coverage, or cost comparisons. Those details are managed by insurers, pharmacies, and pharmacy benefit managers.
Final Thoughts
The Purple Book is a quiet revolution in how we access biologic medicines. It brings transparency to a space that was once shrouded in confusion. For too long, patients and providers didn’t know which alternatives were legally allowed, which were scientifically sound, and which could be swapped without permission. Now, you can find the answers in minutes.It’s not a cure-all. State laws still create patchwork rules. Insurance companies still control access. But the Purple Book gives you the truth. And in a world full of marketing claims and misinformation, that’s powerful.
