Orange Book vs Purple Book: A Practical Guide

If you're reviewing a competitor's approval or a proposed reference product, you need to know what FDA's record supports. The Orange Book lists drugs approved through new drug applications (NDAs) and abbreviated new drug applications (ANDAs). The Purple Book lists licensed biologics, including biosimilars and interchangeable biosimilars.

This guide explains which book to open for which question, what a record does and doesn't tell you, and where teams get tripped up. The three usual traps are the reference standard, blank exclusivity fields, and treating either patent list as complete. We use insulin as the example because it's one of the products that moved between the two books.

When to use the Orange Book

Open the Orange Book when the product was approved as a drug under an NDA or ANDA. Its full name is Approved Drug Products with Therapeutic Equivalence Evaluations, and it does three jobs: it tells you which approved product a generic relies on, how FDA has rated products against each other, and which patents and exclusivities the NDA holder has on file.

The first job is the one most people come for. Every generic application names a reference listed drug, the approved product the generic is compared against, and FDA marks RLDs in the Orange Book. FDA also picks a reference standard, the specific product a generic applicant has to test against in its bioequivalence study. Most of the time these are the same product. Sometimes they aren't, and a brief that names only one of them is missing half the answer.

The second job is the therapeutic equivalence code. An A code (AB is the common one) means FDA considers the product therapeutically equivalent to other pharmaceutically equivalent products. A B code means there's an actual or potential bioequivalence problem that hasn't been resolved. Pharmacists and payers read the codes for substitution. Regulatory teams read them to understand what a competitor's product has been rated against. The letter isn't a quality grade, and it only means something within a product's own grouping, which the Orange Book Preface explains.

The third job, patents and exclusivity, gets its own section below because it's where both books mislead people.

When to use the Purple Book

Open the Purple Book when the product is a biologic licensed under a BLA. It covers originator products licensed under section 351(a) and biosimilars and interchangeable products licensed under 351(k), across both CDER and CBER. For each product it shows the license application, the reference product where there is one, and FDA's determination of whether the product is biosimilar or interchangeable.

That last distinction matters in practice. A biosimilar has been shown to be highly similar to its reference product with no clinically meaningful differences. An interchangeable biosimilar has met an additional standard, and the payoff is that a pharmacist can substitute it for the reference product without going back to the prescriber, subject to state pharmacy law.

Compare Orange Book vs Purple Book records

FDA refreshes the Orange Book data files monthly. The Purple Book covers products regulated by both CDER and CBER. Both are free, and both are the authoritative record. Anything else, including the free tools on this site, is a convenience layered on top of the same data.

What insulin shows

Insulin is the case that shows the pathway matters more than the molecule. Lantus (insulin glargine) was approved as a drug under an NDA in April 2000 and sat in the Orange Book for twenty years. On March 23, 2020, FDA deemed that NDA and the other protein products on its transition list to be BLAs, and they moved to the Purple Book. If someone on your team remembers looking up an insulin in the Orange Book, that memory is from before the transition. The transition did not restart the reference-product exclusivity clock: the law doesn't count a deemed license as a first licensure, and FDA's transition guidance says these products aren't eligible for the reference product exclusivity a new biologic would get.

Semglee (insulin glargine-yfgn) is what a follow-on insulin looks like after the transition. Its Purple Book record names Lantus as the reference product and lists the license type as 351(k) Interchangeable. That one field is what lets a pharmacist substitute Semglee for Lantus when the prescription says Lantus, without going back to the prescriber, subject to state pharmacy law.

Two fields on the Lantus record look like answers and aren't. Its first licensure and exclusivity expiration fields are blank, and a search of the Purple Book patent list turns up no Lantus entry. For Lantus those blanks are consistent with the transition: deeming created no new first licensure and no new reference-product exclusivity. The missing patent entry needs a different explanation, and the next section is about how to read both kinds of gap. Records checked September 10, 2026.

The check itself is four steps.

1. Open Semglee's record in the Purple Book and confirm the license type reads "351(k) Interchangeable" and the reference product is Lantus.
2. Follow the reference product to the Lantus record: BLA 021081, original approval April 20, 2000.
3. Read the exclusivity fields. Date of First Licensure and Ref. Product Excl. Exp. Date are blank, consistent with the transition's no-new-exclusivity rule.
4. Search the patent list for Lantus: no entry, because the list publishes only the sponsor exchanges reported since FDA began posting them in 2021.

View full-size FDA record

That is the whole check, and it is what the brief to your VP should say: Semglee is an interchangeable biosimilar to Lantus (BLA 761201). Lantus (BLA 021081, approved April 20, 2000) shows blank first-licensure and exclusivity fields consistent with the 2020 transition and no patent-list entry. Anything further, including labeling, patents and launch timing, needs the approval documents and patent counsel, not the databases.

What to check before relying on a record

The RLD and the reference standard can be different products

The Orange Book marks both designations, and for an ANDA the distinction is binding: the reference standard is the specific product the applicant has to use in the in vivo bioequivalence study required for approval (21 CFR 314.3). When the RLD and the reference standard differ, a brief that names only the RLD leaves the study design unsupported. Name both, every time. The ANDA entry covers the mechanics. A 505(b)(2) has its own reliance rules, so the same record fields matter without the ANDA requirement carrying over.

A blank exclusivity field is a question, not an answer

FDA hasn't made a first-licensure determination for every product in the Purple Book, so the exclusivity fields for many older 351(a) products are empty. For a biosimilar developer, that date drives the whole plan: under section 351(k)(7), a 351(k) application generally can't be submitted until four years after the reference product's first licensure and can't be approved until twelve, with a possible six-month pediatric extension on top. Treat a blank as unresolved. Don't calculate a date by adding twelve years to an approval date. And keep reference product exclusivity separate from first-interchangeable exclusivity, which is a different clock that FDA's Purple Book FAQ explains.

Neither patent list is complete

The Orange Book lists the patents NDA holders are required to submit under 21 CFR 314.53, and the rules exclude some patent types, process patents among them. Generic and 505(b)(2) applicants certify against what's listed, and a Paragraph IV certification is what can start patent litigation. The Purple Book only carries the patent lists exchanged between a reference product sponsor and a biosimilar applicant during the application process. That reporting requirement came from the Consolidated Appropriations Act, 2021, and FDA began publishing the exchanged lists that year, so a reference product whose exchanges predate the requirement, or that has none, can show no patents at all. FDA publishes what it's given and doesn't assess validity or infringement. Use the listings to find the questions, then have patent counsel answer them for the product and uses you actually have in mind.

How Arca helps with FDA research

The Orange Book and Purple Book are two of the regulatory and scientific sources Arca indexes, alongside Drugs@FDA, FDA guidance, PubMed and the clinical trial registries. Your team connects its own documents, the filings, protocols, SOPs and prior correspondence, and Arca answers questions across both, with every answer traced back to the source it came from.

In practice, that means the research in this guide runs end to end: ask what the Lantus record shows and how it lines up with your draft 351(k) strategy, and you get the FDA record, the relevant guidance and the matching passage from your own filing, cited.