The Purple Book: Understanding Biosimilars and Interchangeability from the FDA

The Purple Book isn’t a book you read cover to cover. It’s a living, online database run 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 or rheumatoid arthritis drug changed, the Purple Book is the official source you need to check.

What Exactly Is the Purple Book?

The Purple Book started as a response to the Biologics Price Competition and Innovation Act (BPCIA) of 2010. Before that, there was no clear path for cheaper versions of complex biologic drugs-like those made from living cells-to enter the market. The BPCIA created a new approval pathway called 351(k), which lets companies develop biosimilars. The FDA built the Purple Book to track all of them.

In 2020, the FDA merged two separate lists-one for drugs under CDER (like insulin and antibodies) and one for biologics under CBER (like vaccines and gene therapies)-into one searchable database. That made it way easier to find what you’re looking for. Now, you can search by brand name, active ingredient, or manufacturer and instantly see the status of each product.

Each entry shows:

• The date the product was first licensed
• Whether it’s a reference product (the original), a biosimilar, or an interchangeable product
• Whether it’s protected by exclusivity (meaning no biosimilar can be approved yet)
• Icons showing how it’s delivered-autoinjector, prefilled syringe, vial

The color-coding is simple: matching colors mean products are linked. If you see a blue card for Humira and another blue card for its biosimilar, you know they’re connected. No guesswork.

Biosimilars vs. Interchangeable: What’s the Difference?

This is where things get tricky-and why the Purple Book matters so much.

All interchangeable products are biosimilars. But not all biosimilars are interchangeable. Think of it like this: a biosimilar is a very close copy. An interchangeable is a copy that’s been proven to work just as well even if you switch back and forth between it and the original.

The FDA defines a biosimilar as a product that’s “highly similar” to the reference medicine, with no clinically meaningful differences in safety, purity, or potency. That means it works the same way. You won’t see different side effects or lower effectiveness.

But to be labeled “interchangeable,” the company has to prove one more thing: if you switch between the biosimilar and the original product multiple times, your body doesn’t react worse. That’s called a “switching study.” It’s not just about being similar-it’s about being predictable in real-world use.

The FDA is clear: an interchangeable designation doesn’t mean the product is better. It just means you can swap it in without needing a new prescription.

Why Does Interchangeability Matter?

It’s all about access and cost.

Biosimilars are already cheaper than the original biologics-sometimes by 30% to 50%. But if a pharmacist can’t substitute it without asking the doctor, many patients never get the lower-cost option. Interchangeability removes that barrier.

In practice, this means:

• A patient gets prescribed Humira
• The pharmacy has an interchangeable biosimilar on the shelf
• The pharmacist can give them the biosimilar without calling the doctor
• The patient pays less, and the healthcare system saves money

But here’s the catch: federal rules say it’s allowed. State laws decide if it happens.

As of 2023, 47 states and Puerto Rico let pharmacists substitute interchangeable biosimilars without prior approval from the prescriber. But some states require the pharmacist to notify the doctor, document the swap, or even get patient consent. That patchwork of rules means a patient in California might get a substitution automatically, while someone in New York might have to ask for it.

The FDA doesn’t control pharmacy-level substitution. That’s why the Purple Book is so important-it tells you what’s federally approved, but you still need to know your state’s rules.

What Products Are Actually Interchangeable?

As of late 2023, only seven biosimilars have earned the interchangeable label from the FDA. That’s not a lot, but it’s growing.

Here’s what’s on the list:

• Two insulin products (for diabetes)
• Three drugs for inflammatory conditions like rheumatoid arthritis and Crohn’s disease
• Two treatments for eye conditions like macular degeneration

All of them are copies of well-known brand-name drugs. For example, the first interchangeable insulin was approved in 2021, and since then, more have followed. Each one had to pass the extra switching studies to prove safety over multiple switches.

The Purple Book shows you exactly which ones are interchangeable. You can’t assume a biosimilar is interchangeable just because it’s on the market. You have to check.

How to Use the Purple Book

It’s free. It’s online. And it’s designed to be used.

Go to the FDA’s website and search “Purple Book.” You’ll land on the official database. You can search by:

• Brand name (like Enbrel or Remicade)
• Generic name (like adalimumab or infliximab)
• Manufacturer

Once you find a product, click on it. You’ll see all its versions-original, biosimilars, and interchangeable ones-all grouped together. The interface is clean. The data is updated weekly.

If you’re a pharmacist, you can use this to confirm whether you can legally substitute. If you’re a doctor, you can check if the biosimilar your patient is taking is interchangeable. If you’re a patient, you can verify whether your drug is the original or a copy-and whether it’s been approved for swapping.

The FDA even has a short video walkthrough showing how to navigate it. No jargon. No fluff. Just step-by-step.

What’s Next for the Purple Book?

More products are coming. Companies are actively applying for interchangeability status. The FDA has released draft guidance on labeling for interchangeable biosimilars to make sure packaging and instructions are clear.

There’s also growing pressure to make substitution easier. Some states are updating their laws to reduce paperwork. Others are considering automatic substitution without any notification.

Meanwhile, the Purple Book continues to evolve. It’s no longer two separate lists. It’s not a static document. It’s a dynamic tool that reflects the real-time state of biologic medicine in the U.S.

The goal? To bring down costs without compromising safety. To give patients more choices. And to make sure everyone-doctors, pharmacists, patients-has the same, accurate information.

The Purple Book doesn’t make decisions for you. But it gives you the facts you need to make the right one.