Biosimilar Interchangeability Checker
Check Product Status
Enter a biological product name to check if it's FDA-designated as interchangeable and view substitution rules.
Enter a product name and click "Check Status" to see if it's interchangeable.
The Purple Book isn’t a book you buy at a pharmacy. It’s a living, online database maintained by the U.S. Food and Drug Administration (FDA) that tells you exactly which biological medicines are approved, which ones are biosimilars, and which ones can be swapped out like generic pills. If you’re a pharmacist, a doctor, or even a patient trying to understand why your insulin prescription changed, this is the single most important resource you need to know about.
What Exactly Is the Purple Book?
Launched in 2012 as part of the Biologics Price Competition and Innovation Act (BPCIA), the Purple Book started as two separate lists-one for drugs handled by CDER and another for biologics managed by CBER. That changed in 2020 when the FDA merged them into one searchable database. Today, it includes every FDA-approved biological product: vaccines, insulin, arthritis drugs, cancer treatments, gene therapies, and more. You’ll find the original brand-name products (called reference products), the biosimilars that copy them, and the rare few that have been officially labeled as interchangeable.
Each product card in the Purple Book shows 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. The colors match up: if a biosimilar has the same color card as the reference product, it means they’re linked. You can click on any product and see exactly which reference it was tested against. This isn’t just paperwork-it’s a tool that helps pharmacies know what they can substitute and when.
Biosimilars vs. Interchangeable: The Big Difference
Not all biosimilars are created equal. Every interchangeable product is a biosimilar, but only a handful of biosimilars ever become interchangeable. Why? Because the FDA sets a higher bar.
A biosimilar is a biological product that’s highly similar to its reference product-no clinically meaningful differences in safety, purity, or potency. That means it works the same way, has the same side effects, and delivers the same results. Think of it like a generic version of a complex drug. But an interchangeable product has to prove something extra: that you can switch back and forth between it and the original brand without any added risk.
That’s where switching studies come in. For a product to be labeled interchangeable, the manufacturer must show that alternating between the biosimilar and the original doesn’t reduce effectiveness or increase side effects. For example, if a patient takes the reference insulin for three months, then switches to the biosimilar for three months, then switches back-there should be no change in blood sugar control or safety signals. Only then does the FDA grant the interchangeable designation.
And here’s the catch: being interchangeable doesn’t mean it’s better. The FDA is clear-interchangeable doesn’t mean safer or more effective. It just means you can swap it in without needing a new prescription every time.
Why Does Interchangeability Matter?
For patients, interchangeability means easier access and lower costs. For pharmacists, it means they can substitute the biosimilar without calling the doctor. But here’s the twist: federal approval doesn’t automatically mean state law allows substitution.
As of 2023, 47 states and Puerto Rico let pharmacists substitute an interchangeable biosimilar without asking the prescriber. But in those states, there are still rules. Some require the pharmacist to notify the doctor. Others require the patient to be informed. A few even require documentation in the patient’s record. In the other three states, substitution is blocked unless the prescriber specifically allows it.
This patchwork of state laws makes it messy. A patient in California might get a biosimilar substituted automatically. A patient in New York might need a new prescription just to get the same drug. That’s why the Purple Book doesn’t just list products-it helps you understand the legal landscape too.
What’s in the Purple Book Right Now?
As of late 2023, only seven biosimilars had received the interchangeable designation from the FDA. That’s out of dozens of approved biosimilars. The interchangeable ones include:
- Two insulin products (used for diabetes)
- Three drugs for inflammatory conditions like rheumatoid arthritis and Crohn’s disease
- Two treatments for eye conditions affecting the retina
These are all high-cost biologics where savings from biosimilars could be massive. Insulin, for example, can cost over $1,000 a month without insurance. Biosimilars cut that by 30% to 50%. Interchangeable versions could push those savings even further by removing the need for paperwork and delays.
The Purple Book is updated regularly. New products are added as they’re approved. You can search by brand name, active ingredient, or even by manufacturer. If you’re looking for a specific drug, just type it in. The results show you every version-original, biosimilar, and interchangeable-all grouped together under the reference product. No more flipping through outdated paper guides.
How to Use the Purple Book
It’s free. It’s online. And it’s designed to be used by anyone-not just experts.
- Go to the FDA’s Purple Book page on fda.gov.
- Use the search bar to look up a brand name like Humira or Enbrel.
- Scroll down. You’ll see the reference product at the top.
- Below it, you’ll find all the biosimilars and interchangeable versions that match.
- Check the designation: 351(a), 351(k) Biosimilar, or 351(k) Interchangeable.
- Click on any product to see its full profile, including approved uses and delivery methods (like autoinjectors or pre-filled syringes).
The site even has a video tutorial showing how to navigate the database. If you’re confused, watch it. It’s under 15 minutes and walks you through real examples.
What the Purple Book Doesn’t Tell You
The Purple Book doesn’t tell you what your insurance will cover. It doesn’t say which biosimilar your pharmacy stocks. And it doesn’t explain state-specific substitution rules in detail. You still need to check with your pharmacist or insurer for those details.
Also, the FDA doesn’t list unbranded biologics as interchangeable-even if they’re chemically identical. That’s a legal distinction, not a scientific one. An unbranded biologic might be just as safe, but without the interchangeability label, pharmacists can’t substitute it without a new prescription.
And while the database is powerful, it’s not perfect. Sometimes new products appear with delays. Occasionally, labeling changes aren’t updated immediately. Always cross-check with the latest FDA press releases if you’re making critical clinical decisions.
What’s Next for Biosimilars?
More are coming. Companies are actively applying for interchangeable status for drugs treating multiple sclerosis, lupus, and even rare genetic disorders. The FDA has signaled it’s streamlining the review process, especially for insulin and autoimmune treatments where cost savings are urgent.
As more interchangeable biosimilars hit the market, the hope is that patients will get the same quality care at a fraction of the cost. But that only works if everyone-from doctors to pharmacists to patients-understands how to use the Purple Book correctly.
It’s not just a database. It’s the foundation of a new, more affordable future for biologic medicines.
Is the Purple Book only for U.S. patients?
Yes. The Purple Book is maintained by the U.S. FDA and only includes biological products approved for sale in the United States. Other countries have their own databases-for example, the European Medicines Agency (EMA) publishes a list of biosimilars approved in the EU, but it’s not called the Purple Book. If you’re outside the U.S., check your country’s regulatory agency for similar resources.
Can my pharmacist substitute a biosimilar without telling me?
It depends on your state. In 47 states and Puerto Rico, if the biosimilar is FDA-designated as interchangeable, the pharmacist can substitute it without a new prescription. But they’re often required by law to notify you and sometimes the prescriber. In the remaining three states, substitution is only allowed if the doctor specifically permits it. Always ask your pharmacist if a substitution was made and why.
Are biosimilars as safe as the original brand?
Yes. The FDA requires that biosimilars show no clinically meaningful differences in safety, purity, or potency compared to the reference product. Thousands of patients have used biosimilars for years with the same outcomes as the original. Interchangeable products go through even more testing to prove that switching back and forth doesn’t cause harm. The science is solid.
Why aren’t all biosimilars interchangeable?
Because the FDA requires additional clinical data to prove interchangeability. Not every manufacturer chooses to pay for those extra studies. It’s expensive and time-consuming. So many biosimilars are approved as just that-biosimilars-and can still be prescribed directly. But they can’t be swapped at the pharmacy unless they have the interchangeable label.
How often is the Purple Book updated?
The FDA updates the Purple Book regularly-usually within days of a new approval or designation. The database is live and searchable, so changes appear in real time. However, there can be slight delays between a product’s official approval date and when it shows up in the search results. For the most accurate info, always check the FDA’s official press releases when a new biosimilar is announced.
So the Purple Book is basically the FDA’s version of a Netflix queue for biologics? 😏 I just thought it was a typo for ‘Purple Haze’ until I clicked it. Now I know why my insulin costs less but still gives me the same existential dread.