What a peptide sequence settles, and what it leaves open

A sequence is a strong identity statement

A peptide is a chain of amino acids. A sequence writes down which ones and in what order, and that is a far more specific statement than any name.

Two listings printing the same sequence are describing the same chain. That is worth something, because names in this market do not have that property. The same substance appears under a laboratory code on one page and a different code on another, and a reader has no way to tell from the names alone whether they refer to one thing.

So when a sequence is published, take it seriously. It is the closest a listing comes to saying what the substance is rather than what it is called.

What follows is not an argument that a sequence is unreliable. It is a list of the questions a sequence does not answer, all of which affect what arrives.

A regulator has said in its own words that names do not work

This is not an outsider's complaint about drug naming. It is the reason a federal agency built a separate identification system.

The agency describes the problem plainly on the page for that system. Consumers, clinicians and researchers do not always have a way to tell what ingredients are in commercial products, because certain ingredients are not included in the labeling and others are labeled ambiguously.

Its answer was to stop using names as the key. Instead of relying on names, which the agency notes vary across regulatory domains, countries and regions, the system defines substances by standardized scientific descriptions. Those descriptions classify a substance as a chemical, a protein, a nucleic acid, a polymer, structurally diverse, or a mixture, following an international standard.

The system generates unique ingredient identifiers, which are used in electronic drug listing and throughout a product's regulatory life. The agency says it built the system beginning in 2006 because no other code system met its regulatory needs.

One line on that page is the whole lesson of this article. Those identifiers can be generated at any time in the regulatory life cycle for any substance, from an atom to an organism. An identifier is an index entry for something that has been described. It is not a test, an approval, or a statement about a container.

A registry number is an index of what has been written down

The other identifier that turns up on peptide paperwork is a chemical registry number, and it works the same way.

The organization that publishes the best known registry describes it as a database of disclosed chemical substances, curated from scientific literature and other sources. It says compounds are described in many ways, including molecular formulas, structures, and generic, systematic, common and trade names, and that the resulting lack of clarity can cause safety concerns.

A number is assigned so that one substance can be referred to unambiguously. It is assigned because a substance was described somewhere, not because anyone examined a batch of it.

That is why a registry number on a certificate is useful for one thing only: checking that the document is about the substance the label names. Whether the container holds that substance is a different question, answered by testing, and covered separately here.

The vocabulary the law uses for two things being the same

Federal regulation has a precise set of words for degrees of sameness, and reading them makes clear how much a shared identity claim leaves open.

The narrowest term is the active moiety. The regulation defines it as the molecule or ion responsible for the physiological or pharmacological action. From that it excludes the appended portions that make the drug an ester, or a salt including a salt with hydrogen or coordination bonds. It also excludes another noncovalent derivative such as a complex, chelate or clathrate.

Read the exclusion list rather than the definition. It names, one by one, the things a chemical name carries at its tail: an ester, a salt, a metal complex. The definition treats all of them as appended to the moiety rather than part of it.

Two products can therefore share an active moiety and still be different articles. That is exactly what the next two terms are for.

Alternative, equivalent, and equivalent in effect are three different things

The regulation sets out three tiers, and each one adds a requirement to the tier below it.

Pharmaceutical alternatives are products that contain the identical therapeutic moiety, or its precursor, but not necessarily in the same amount or dosage form, and not necessarily as the same salt or ester. Each product meets its own applicable standard of identity, strength, quality and purity.

Pharmaceutical equivalents are stricter. They are products in identical dosage forms and routes of administration, containing identical amounts of the identical active drug ingredient. The regulation glosses that as the same salt or ester of the same therapeutic moiety. They need not contain the same inactive ingredients, and they must meet the identical applicable standard of identity, strength, quality and purity.

Therapeutic equivalents are stricter still. They are approved products that are pharmaceutical equivalents for which bioequivalence has been demonstrated, and that can be expected to have the same clinical effect and safety profile under the conditions specified in the labeling.

Notice what moves between the tiers. Sharing a moiety is the first tier. Sharing the salt, the amount, the dosage form and the route is the second. Having a study behind the claim of the same effect is the third.

Whose rule that is, and whose it is not

This vocabulary has a stated scope, and it needs saying before anyone uses it as a standard.

The regulation introduces its definitions with the words that they apply to that part and to one other part of the chapter. Those two parts govern applications for approval of a new drug, and the bioavailability and bioequivalence work that supports them.

Nothing sold as a compounded peptide preparation has such an application. So none of these terms is a duty anyone here owes, and no product in this market is described by any of them.

What the vocabulary is good for is measuring a sentence. When a page says two things are the same, the three tiers show you which claim is being made, and whether anything was required to establish it.

A claim of a shared molecule is a first-tier claim, and it is the easiest to make. A claim of the same effect is a third-tier claim, and in the approval system it takes a study.

The four things a sequence does not settle

The salt or complex form. A sequence describes the chain, and the counter-ion or metal bound to it sits outside that description. Two vials of one sequence in two forms are different active ingredients, and how to check which one a document describes is set out in the article on reading a certificate.

The amount. A sequence says nothing about how much is in the container, and the figure that answers that has its own difficulties, covered in the article on what a strength actually states.

What else is in the container. A sequence describes a substance. The regulation quoted above distinguishes a drug substance, which is the active ingredient, from a drug product, which is the finished dosage form containing it, generally along with other ingredients. You buy the second one. What else is in it has its own article here.

Whether the container holds it at all. This is the largest gap and the plainest one. A sequence printed on a page is a claim about identity, and the only things that speak to whether the claim is true are testing and the records behind it.

What to do with an identifier on a product page

Treat a sequence as the strongest name on the page and still not as a description of the product.

Where a sequence, a formula, a registry number and a molecular weight are all printed, check that they agree with each other and with the name. Documents that disagree with themselves are the useful finding.

Where only a name is printed, ask for the sequence and the form. A seller that can supply both is describing a substance; one that can supply neither is describing a label.

Do not read an identifier as a credential. A registry number means a substance was described in the literature, and a federal ingredient identifier can be generated for anything from an atom to an organism.

Keep the three tiers of sameness in mind whenever a page says its product is the same as something better documented. That is usually a first-tier claim wearing third-tier clothes.

Sources

Each document below is named as it names itself, with the date printed on that document rather than the day it was read.

1. Title 21 Code of Federal Regulations Section 314.3, Definitions, read in full for active moiety, drug substance, drug product, pharmaceutical alternatives, pharmaceutical equivalents and therapeutic equivalents, including the scope sentence in paragraph (b) — Electronic Code of Federal Regulations, Office of the Federal Register, October 2016
2. Title 21 Code of Federal Regulations Section 320.1, Definitions, which as in force consists of a single sentence applying the definitions in section 314.3 to that part — Electronic Code of Federal Regulations, Office of the Federal Register, October 2016
3. FDA's Global Substance Registration System, home of the Unique Ingredient Identifier, including the statement that the system defines substances by standardized scientific descriptions instead of relying on names — U.S. Food and Drug Administration, March 2024
4. United States Code Title 21, Section 352(e)(1)(A), which requires a label to bear the established name to the exclusion of any other nonproprietary name, excepting the applicable systematic chemical name or the chemical formula — Office of the Law Revision Counsel, U.S. House of Representatives, September 2026