The Purple Book: Biosimilars and Interchangeability Explained

The Purple Book isn’t a book you can hold. It’s a living, online database run by the U.S. Food and Drug Administration (FDA), and it’s one of the most important tools for anyone involved in prescribing, dispensing, or using biological medicines. If you’ve ever wondered which biosimilars can be swapped for brand-name drugs at the pharmacy, or why some biosimilars are treated differently than others, the Purple Book holds the answers.

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 products handled by the Center for Drug Evaluation and Research (CDER) and another for those managed by the Center for Biologics Evaluation and Research (CBER). That changed in 2020 when the FDA merged them into a single, searchable database. Today, it includes every FDA-approved biological product, including reference products, biosimilars, and interchangeable biosimilars.

It’s not just a list. It’s a decision-making tool. Pharmacists use it to check if a biosimilar can be substituted without a doctor’s approval. Prescribers use it to understand what alternatives are available. Patients can look up whether their medication has a cheaper, equally effective version.

Each product in the database has a color-coded card. Products that are biosimilar or interchangeable to the same reference drug share the same color. You’ll also see icons showing delivery methods-like autoinjectors or prefilled syringes-and clear labels indicating whether a product is licensed under section 351(a) (reference), 351(k) biosimilar, or 351(k) interchangeable.

Biosimilar vs. Interchangeable: The Key Difference

Not all biosimilars are created equal. All interchangeable products are biosimilars, but only a small number of biosimilars earn the interchangeable label. Why? Because interchangeability comes with a higher bar.

A biosimilar is a biological product that’s highly similar to a reference product. Minor differences in inactive ingredients are allowed, but there must be no clinically meaningful differences in safety, purity, or potency. That means if you switch from the brand-name drug to a biosimilar, your treatment won’t change in effectiveness or risk.

An interchangeable biosimilar goes further. To earn this designation, the FDA requires additional studies-called switching studies. These show that a patient can switch back and forth between the biosimilar and the reference product multiple times without any increase in safety risks or loss of effectiveness. Imagine alternating between two identical-looking pens: one is the original, the other is the copy. If you can use them interchangeably without noticing a difference in how they write, that’s the standard for interchangeability.

As of November 2023, only seven biosimilars in the U.S. had received this higher designation. Two are insulins, three treat inflammatory conditions like rheumatoid arthritis, and two are used for eye diseases. All of them are listed in the Purple Book with a clear “Interchangeable” tag.

Why Does Interchangeability Matter?

The difference between biosimilar and interchangeable isn’t just scientific-it’s legal and practical. In most states, pharmacists can substitute a biosimilar for a brand-name drug only if it’s been labeled as interchangeable by the FDA. Even then, state laws control what happens next.

Forty-seven states and Puerto Rico allow pharmacists to substitute an interchangeable biosimilar without contacting the prescriber. But some states require the pharmacist to notify the doctor, inform the patient, or document the swap. That means two patients with the same prescription in different states might get different versions of the same drug, simply because of where they live.

The FDA makes it clear: an interchangeable designation doesn’t mean the product is safer or more effective than a non-interchangeable biosimilar. It only means it’s been proven safe to switch between repeatedly. This is important because many biosimilars are already being used successfully-just not automatically substituted.

How to Use the Purple Book

The Purple Book is free, public, and searchable. You can look up a brand-name biologic-say, Humira-and see every biosimilar and interchangeable product linked to it. The results show the product name, manufacturer, approval date, and status. You can filter by product type, manufacturer, or therapeutic area.

For example, if you search for “adalimumab,” the reference product for Humira, you’ll see a list of biosimilars like Amjevita, Cyltezo, and Hyrimoz. Some will have an “Interchangeable” tag next to them. That’s your signal: in states that allow automatic substitution, a pharmacist can legally swap your prescription for one of those without asking your doctor.

The database also shows when a product lost its exclusivity period. That’s important because manufacturers can’t submit biosimilar applications until after the reference product’s exclusivity expires. The Purple Book tracks this, so you can predict when new biosimilars might enter the market.

What’s Missing from the Purple Book?

It’s easy to assume that if a product isn’t in the Purple Book, it doesn’t exist. But that’s not true. The database only includes FDA-approved biological products. It doesn’t list products still under review. It doesn’t include off-label uses. And it doesn’t track real-world usage patterns or cost differences between products.

Also, the FDA doesn’t use the term “unbranded biologic” in the Purple Book-but some people confuse it with interchangeable biosimilars. An unbranded biologic is a version of a drug that’s chemically identical but sold without a brand name. The FDA considers it equivalent to the brand, but it’s not necessarily a biosimilar. These aren’t listed in the Purple Book because they’re not approved under the 351(k) pathway.

Another limitation: state laws change. The Purple Book tells you what the FDA says. But whether a pharmacist can substitute it depends on your state’s pharmacy rules. That means the database gives you the federal answer-but you still need to check local regulations.

What’s Next for Biosimilars?

The number of biosimilars approved each year is rising. In 2024, more applications for interchangeability were submitted than ever before. Companies are investing heavily in switching studies because the market reward is big: automatic substitution at the pharmacy level means faster adoption and lower costs.

But challenges remain. Some prescribers still hesitate to use biosimilars due to misinformation. Pharmacists in states with strict notification laws face administrative burdens. And patients often don’t know their medication has been switched.

The Purple Book is helping fix that. By making the data clear, public, and searchable, it empowers everyone-from doctors to patients-to make informed choices. As more biosimilars gain interchangeability status, the Purple Book will become even more critical. It’s not just a reference. It’s becoming the backbone of how biological medicines are prescribed and dispensed across the U.S.

What to Do Next

If you’re a patient, ask your pharmacist if your biologic has an interchangeable biosimilar option. If you’re a prescriber, review the Purple Book before writing prescriptions for expensive biologics. If you’re a pharmacist, bookmark the site-it’s your go-to source for substitution rules. The Purple Book isn’t just data. It’s a tool that’s lowering costs and increasing access to life-changing treatments.