Peptide Glossary: Key Research, Purity and Legal Terms

A peptide is a short chain of amino acids linked by peptide bonds. The line between a peptide and a protein is a question of size, and for regulatory purposes the United States draws it precisely: in a rule codified in the Federal Register (83 FR 63817, December 2018), the FDA defines a peptide as any alpha amino acid polymer of 40 or fewer amino acids, with anything larger treated as a protein. The IUPAC-IUB nomenclature convention uses a looser range of 2 to 50 residues. By the FDA standard, the compounds most often discussed in this space sit firmly in peptide territory: BPC-157 is 15 amino acids, MOTS-c is 16, and thymosin beta-4 is 43, while insulin, first administered clinically in 1922, is a 51-amino-acid two-chain hormone that falls on the protein side of the cutoff.

Term	Meaning
Peptide	A chain of amino acids; the FDA defines it as 40 or fewer amino acids in length.
Amino acid	The individual building block of a peptide or protein.
Residue	A single amino acid unit as it sits within a chain.
Sequence	The specific order of amino acids that defines a given peptide.
Analogue	A modified version of a natural peptide, altered to change stability or activity.
Agonist	A molecule that binds and activates a receptor.
GLP-1 receptor agonist	A compound that activates the GLP-1 receptor; the class semaglutide and tirzepatide belong to.
Secretagogue	A substance that prompts the body to secrete something, such as a growth-hormone secretagogue.
GHRH / GHRP	Growth-hormone-releasing hormone and growth-hormone-releasing peptide; two classes that act on the pituitary.
Half-life	The time it takes for a compound’s concentration in the body to fall by half.
Bioavailability	The proportion of an administered compound that actually reaches circulation.

The bottom line: a peptide is an amino acid chain, and in the United States the regulatory dividing line from a protein is a hard 40-amino-acid cutoff.

Research peptides are almost always supplied as a lyophilised (freeze-dried) powder that has to be dissolved before use. The terms below cover the physical form, the units, and the diluents involved. PeptideGuider does not publish reconstitution or injection protocols; these entries define the vocabulary only.

Term	Meaning
Lyophilised	Freeze-dried; the powdered state in which most research peptides ship.
Reconstitution	Dissolving a lyophilised powder in a sterile diluent to produce a solution.
Bacteriostatic water	Sterile water with 0.9% benzyl alcohol, the standard multi-use diluent. See our full explainer on bacteriostatic water.
Sterile water for injection	Preservative-free water; single-use because it has no antimicrobial agent.
Benzyl alcohol	The preservative that makes bacteriostatic water reusable and drives its safety warnings.
Subcutaneous / intramuscular	Administration into the layer under the skin, or into muscle, respectively.
mcg vs mg	Microgram and milligram; 1 mg equals 1,000 mcg, a frequent source of error.
Multiple-dose vial	A container designed for repeated withdrawals, which requires a preservative.
Aliquot	A measured portion divided off from a larger quantity, often stored separately.

The bottom line: peptides ship lyophilised and are dissolved in a diluent, and the choice between a preservative-containing and a preservative-free diluent decides whether a vial is single or multiple use.

The most consequential terms in research-peptide sourcing are the ones about what is actually in the vial. Two of them get confused constantly: chromatographic purity (how clean the peptide is) and net peptide content (how much peptide there is by weight). They are different measurements, and a product can score high on one and low on the other.

A large part of that gap comes down to counterions, a term most consumer glossaries skip entirely. Every synthetic peptide made by standard Fmoc solid-phase synthesis carries charged counterion molecules that pair with basic sites on the chain. The default is trifluoroacetate (TFA), a byproduct of the synthesis and purification steps, which weighs 114 daltons per counterion; an acetate counterion weighs only 59 daltons. Because TFA is heavier, a TFA-salt vial contains less actual peptide by weight than an acetate-salt vial of the same labelled mass, and for a peptide with several basic sites the counterion can account for 15 to 25% of the total powder weight. How that translates into measured net peptide content, and how a lab reports it, is covered in our walkthrough of what a certificate of analysis proves.

Term	Meaning
Certificate of Analysis (COA)	The lab document reporting a batch’s identity, chromatographic purity, and contaminant testing.
HPLC	High-performance liquid chromatography, the standard purity test. See how the testing methods compare.
Mass spectrometry	A method that confirms a peptide’s molecular weight and identity.
Chromatographic purity	The percentage of the sample that is the target peptide rather than impurities.
Net peptide content	The actual mass of peptide in a vial after counterions, water, and salts are removed.
Counterion / salt form	The charged molecule paired with the peptide; TFA (114 Da) and acetate (59 Da) are the common forms.
Endotoxin	A bacterial contaminant tested for separately from purity; reported on a thorough COA.
Nonpyrogenic	Free of fever-inducing substances such as endotoxin.
USP	United States Pharmacopeia, which sets official quality standards and monographs.
cGMP	Current Good Manufacturing Practice, the FDA quality-system standard for manufacturing.
Third-party testing	Independent-lab testing rather than a seller’s own; the leading trust signal in vendor vetting.
Lot / batch number	The identifier tying a vial to a specific production run and its test data.
Pyrogen	A fever-inducing substance; “nonpyrogenic” means a preparation is free of them.

The bottom line: purity and net peptide content measure different things, and the counterion salt form is the hidden variable that drives much of the difference.

The regulatory vocabulary is where the most consequential misunderstandings happen, because phrases like “research use only” read like permissions when they are really just sales-channel labels. The entries below are short by design; each topic has a dedicated guide that covers it in full.

Term	Meaning
Research use only (RUO)	A sales-channel designation; it carries no approval for human use. Unpacked in the US legal framework.
FDA Category 1 vs Category 2	The compounding bulk-substance categories. See what the categories mean.
503A / 503B	The two compounding pathways: patient-specific pharmacies (503A) and outsourcing facilities (503B).
Compounding	Preparing a tailored medication outside standard mass manufacturing.
Bulk drug substance	An active ingredient a compounder may use; the category lists govern which peptides qualify.
NDA	New Drug Application, the FDA approval route a prescription drug must clear.
DSHEA	The 1994 supplement law; most research peptides do not qualify as dietary supplements under it.
MHRA / TGA	The UK and Australian medicines regulators, covered in the respective country legality guides.

The bottom line: regulatory terms describe how a product may be sold and compounded, not whether it is approved for use, and that distinction is the one most often missed.