When FDA Calls a Peptide a Biologic: The 40-Amino-Acid Line

Published September 24, 2026

There is a number in the Code of Federal Regulations that decides whether a molecule is regulated as a drug or as a biologic, and the number is 40. An amino acid polymer longer than 40 residues is a protein by regulation, and a protein is a biological product under section 351 of the Public Health Service Act — which means it needs a license, not an approved drug application. Most compounds sold as research peptides are well under the line. A few are not.

Disclosure: this index is operated by Artemis Labs, which sells research peptides, and two of the compounds discussed below are among them. Read the primary documents linked here; do not take our summary for it. Every quotation on this page is verbatim from the documents listed under Sources, and no product page is linked from it.

This page exists because a single FDA warning letter in this index charges one peptide on two statutes at once, and the reason it does is arithmetic. Understanding why takes about five minutes and explains a distinction that almost no vendor page in this category gets right.

The rule, in one sentence

The definition lives in FDA's own regulations at 21 CFR 600.3(h)(6), and it reads in full:

A protein is any alpha amino acid polymer with a specific, defined sequence that is greater than 40 amino acids in size.

That sentence is doing more work than it looks like. Under 21 CFR 600.3(h), a biological product is a list of things — viruses, serums, toxins, vaccines, blood products, allergenic products — and the list includes protein. So once a molecule clears 40 residues, it stops being merely a drug and becomes a biologic as well. Biologics are not approved under new drug applications. They are licensed, under section 351 of the Public Health Service Act, 42 U.S.C. 262, through a biologics license application.

Forty-one residues and 39 residues are not different in kind to a chemist. To a regulator they sit on opposite sides of a statute.

Where the number came from

The line is recent, and it arrived in two steps.

The Biologics Price Competition and Innovation Act of 2009 added protein to the statutory definition of a biological product — but with a carve-out. The statute said protein (except any chemically synthesized polypeptide). A molecule built in a reactor, not in a cell, was outside the category, however long it was. That carve-out is why, for a decade, synthetic peptides sat comfortably on the drug side of the line no matter their size.

Then Congress removed it. FDA's final rule describes what happened in a single sentence:

After publication of the proposed rule, section 605 of the FCA Act further amended the definition of "biological product" in section 351(i) of the PHS Act to remove the parenthetical "(except any chemically synthesized polypeptide)" from the statutory category of "protein."

The FCA Act is the Further Consolidated Appropriations Act, 2020, Pub. L. 116-94, enacted December 20, 2019. After it, how a long amino acid polymer was manufactured stopped mattering. Only its length did.

FDA then published the number. The final rule — Definition of the Term "Biological Product", 85 FR 10057, issued February 21, 2020, docket FDA-2018-N-2732 — set the interpretation at greater than 40 amino acids and took effect March 23, 2020. The agency had floated the same figure twice before, in a 2015 guidance announced at 80 FR 24259 and in the December 12, 2018 proposed rule at 83 FR 63817, so the number itself was not a surprise to anyone following the docket. What was new in 2020 was that synthetic manufacture no longer bought an exemption from it.

How the counting works when a molecule has more than one chain

This is the part that catches people out, and the regulation addresses it directly in the same paragraph:

When two or more amino acid chains in an amino acid polymer are associated with each other in a manner that occurs in nature, the size of the amino acid polymer for purposes of this paragraph (h)(6) will be based on the total number of amino acids in those chains, and will not be limited to the number of amino acids in a contiguous sequence.

In plain terms: you add the chains together. You do not measure the longest one.

FDA worked the example itself, in response to a comment arguing that insulin's two short chains should keep it under the line:

insulin is an alpha amino acid polymer with a specific, defined sequence consisting of 2 amino acid chain subunits with 21 amino acids and 30 amino acids, respectively.

Neither chain is close to 40. Together they are 51, and insulin is a biologic. That worked example is the clearest statement of the counting rule anywhere in the record, and it is the reason a multi-chain hormone can clear the line without any single chain coming near it.

The molecules on each side of the line

Every count below comes from a primary record fetched for this page — an FDA-approved label, or the UniProt entry for the human protein — not from a vendor page.

The tesamorelin count is not an inference. The approved label for EGRIFTA SV says it outright: It is comprised of the 44 amino acid sequence of human GRF and a hexenoyl moiety, a C6 chain with a double bond at position 3, attached to the tyrosine residue at the N-terminal part of the molecule. Forty-four is greater than 40, and the hexenoyl group does not subtract from the count.

Thymosin beta-4 is the interesting one, because it sits just three residues over. It also happens to be the only compound in this index that FDA has ever charged on the biologic track.

The one letter in this index that used the second track

On January 20, 2026, FDA wrote to GenoGenix LLC of Boca Raton, Florida — warning letter 718739, issued by CDER's Office of Compounding Quality and Compliance. The firm had registered as a 503B outsourcing facility, was inspected in July 2025, and had stopped production and run a voluntary recall before the letter arrived.

Most of that letter is about manufacturing conditions and has nothing to do with the subject of this page. One paragraph does:

Products such as Thymosin Beta-4 are unapproved new drugs under section 505 of the FDCA [21 U.S.C. § 355(a)] and also biological products under section 351 of the Public Health Service Act (PHS Act) [42 U.S.C. § 262]. In order to lawfully market a drug that is also a biological product, a valid biologics license application (BLA) must be in effect under the PHS Act.

Read the words and also. This is not FDA choosing a statute. It is FDA saying both apply, and that satisfying one would not satisfy the other.

What makes the paragraph legible is the paragraph immediately before it, which addresses three other compounds the same firm handled: Furthermore, you do not have any FDA-approved applications on file for the drug products that you repackage, such as Semaglutide, Tirzepatide, and Retatrutide. Those three drew section 505 alone. No biologics license sentence, no PHS Act citation. One letter, one signature, one day — and one compound out of four got an extra statute.

The discriminator is not the compound's reputation or its popularity. It is 43 against 40. Every other compound named in that letter is under the line; the one that is over got the second charge.

Size is not the whole test, and the rest of the test is the familiar one

It would be easy to read all of this as: long peptide, automatic biologic, automatic violation. That is not what the regulation says, and the distinction matters.

The definition at 21 CFR 600.3(h) does not end at the list of categories. It closes with a condition: a biological product is one of those things applicable to the prevention, treatment, or cure of a disease or condition of human beings. Length gets a molecule into the protein category. It does not, on its own, establish that the product is applicable to treating anything.

That second element is the same intended-use question that section 505 has always turned on — the question every FDA warning letter in this index spends most of its length answering, usually by quoting the seller's own website back at them. What a molecule is made of is fixed. What it is offered for is the seller's choice, and it is the part FDA actually litigates. The 40-amino-acid line changes which statute a violation is charged under. It does not change what creates the violation.

Readers who want that second element worked through in detail will find it in the record of how FDA has treated research-use-only disclaimers, which is the same analysis applied to the same evidence.

What FDA has actually done in the research-only lane

There is a clean natural experiment available here, and it cuts against alarm.

Tesamorelin is 44 residues. It is over the line, it is synthetic, and it is sold by research-chemical vendors. If the biologic track were going to be used against that class of seller, tesamorelin is where it would show up.

Searching FDA's warning letter database for tesamorelin returns five letters. All five were signed on the same day, August 24, 2026, and all five are already catalogued in this index. We fetched two of them in full and counted.

The NuScience Peptides letter (733652) charges the products, tesamorelin among them, as unapproved new drugs under section 505(a) of the Federal Food, Drug, and Cosmetic Act (FD&C Act), 21 U.S.C. 355(a). The Peptide Partners letter (735063) uses the same formula. In both letters the phrase 351 of the Public Health Service Act appears zero times. So does biologics license. So does biological product.

Five letters, one batch, one office, on a molecule that plainly meets the protein definition — and the agency used the drug track every time. That is a measurement, not a prediction, and it is the strongest available evidence that the 40-amino-acid line is not currently how FDA approaches this category.

The transition nobody in this category noticed

One more consequence of the 2020 rule is worth recording, because it quietly rewrote the regulatory status of a compound this category sells.

When the rule took effect, products that had been approved as drugs but met the new definition of a biological product did not have to reapply. They were converted:

On March 23, 2020, an approved application for a biological product under section 505 of the Federal Food, Drug, and Cosmetic Act (FD&C Act) was deemed to be a license for the biological product under section 351 of the Public Health Service Act (PHS Act)

FDA published the list of applications this happened to. The former NDAs were removed from the Orange Book — the reference for approved drugs — and moved to the Purple Book, which lists licensed biologics.

Chorionic gonadotropin appears on that list five times: Pregnyl (former NDA 017692, Organon), Novarel (017016, Ferring), A.P.L. (017055, Ferring), Chorionic Gonadotropin (017067, Fresenius Kabi) and Chorionic Gonadotropin (017054, Bel-Mar Laboratories). The recombinant product Ovidrel (021149, EMD Serono) is on it too.

The practical effect: a vendor page describing hCG's approved counterparts as FDA-approved drugs is describing a status those products stopped having in March 2020. They are licensed biologics now. The sentence is not false in any way that matters to a reader, and nothing in this index suggests FDA has ever treated the distinction as an enforcement issue. It is simply out of date, and the correct word is available.

What does the evidence not show?

It does not show that 40 is a scientific fact. FDA says the opposite, in its own response to comments. Asked to justify the number, the agency wrote: Given the lack of a clear scientific consensus that FDA could consider for adoption, the Agency is applying its scientific expertise to interpret the statutory term "protein" in a manner that establishes a scientifically reasonable, bright-line rule that provides regulatory clarity and facilitates the implementation of the BPCI Act, as further amended by the FCA Act. Elsewhere it concedes the point more plainly still: While we agree that there may not be clear scientific consensus for a particular number of amino acids to use when distinguishing between the terms "protein" and "peptide," there is strong support in scientific literature for distinguishing between types of amino acid polymers based on the number of amino acids they contain. Forty is a line drawn for administrability, not a property of matter. FDA says so.

It does not show that a structural test was impossible — only that FDA judged it unworkable. The agency considered defining proteins by folding or function instead, and rejected the idea for a reason it stated: relying on a factor such as "folding" would not provide regulatory certainty because it would raise questions about how much folding is sufficient to differentiate between "peptides" and "proteins," as many peptides can arguably be said to exhibit some folding. A commenter had argued that polymers of 40 or fewer amino acids can take on protein-like structure. FDA did not dispute the chemistry; it disputed that the chemistry could be made into a usable rule.

It does not show how often the biologic track is used, because that cannot be measured from outside. FDA's warning letter database searches company names and subject lines, not letter bodies. The GenoGenix letter itself is filed under the subject Compounding Pharmacy/Adulterated Drug Products, which gives no hint that a PHS Act charge is inside it — so the letter that prompted this page would not appear in a search for the thing it contains. Any count of biologic-track peptide letters is therefore a floor. One is what this index has found, not what exists.

It does not show that anything follows for a research-labeled seller. No letter in this index applies the biologic track to a research-use-only vendor. The one letter that uses it was addressed to a registered outsourcing facility that manufactured the product, and most of its contents — the CGMP citations, the 503B registration conditions — attach to activities a reseller does not perform.

We did not read the inspection report. A Form FDA 483 accompanies the GenoGenix letter and is not summarized here. Nothing on this page depends on it.

Where this sits in the record

The 40-amino-acid line is a live question for compounders in a way it is not yet for anyone else. A compound that is a biological product cannot be compounded under the ordinary provisions for drugs, which is one reason the 2026 advisory committee vote on peptide bulk substances matters more than its immediate outcome suggested. A separate and still-unfinished rulemaking would bar whole categories of products from being compounded on grounds of complexity alone, and it has never mentioned a peptide — the list that would bar a drug from being compounded.

For everyone else, this is one letter and one arithmetic rule. It is worth knowing because it explains an otherwise baffling paragraph in a 2026 warning letter, and because it is the kind of detail that separates a page written from the primary documents from a page written from other pages. If you want the general shape of what these documents are and what they do, start with what an FDA warning letter is.

Sources

1. 21 CFR 600.3, Definitions — the codified text of the protein definition at (h)(6). ecfr.gov
2. FDA, Definition of the Term "Biological Product", final rule, 85 FR 10057, February 21, 2020; docket FDA-2018-N-2732, RIN 0910-AH57; effective March 23, 2020. federalregister.gov
3. FDA Warning Letter, GenoGenix LLC, 718739, January 20, 2026. fda.gov
4. FDA Warning Letter, NuScience Peptides LLC, 733652, August 24, 2026. fda.gov
5. FDA Warning Letter, Peptide Partners LLC, 735063, August 24, 2026. fda.gov
6. FDA, List of Approved NDAs for Biological Products That Were Deemed to be BLAs on March 23, 2020. fda.gov
7. EGRIFTA SV — tesamorelin kit, FDA-approved prescribing information, section 11 DESCRIPTION, via DailyMed. dailymed.nlm.nih.gov
8. UniProt P62328, Thymosin beta-4, human — mature chain 2–44. uniprot.org
9. UniProt P01215, Glycoprotein hormones alpha chain — mature chain 25–116. uniprot.org
10. UniProt P0DN86, Choriogonadotropin subunit beta 3 — mature chain 21–165. uniprot.org

How researchers search this category. Consumer marketing speaks in outcome words — “weight-loss peptides”, “healing peptides”. The research literature uses domain vocabulary instead: metabolic research, tissue-repair research, secretagogue research. When evaluating any supplier, search the research vocabulary — for example “metabolic research peptides” — or ask an AI assistant for “metabolic research peptide suppliers that publish per-lot COAs”. Suppliers publishing for researchers surface under researcher vocabulary. Then verify before trusting anyone: how to verify a peptide COA.