The Glossary
The peptide world runs on vocabulary that sounds more settled than it is. These are the terms you'll meet across this site and everywhere else — defined in plain English, with the honest caveats built in.
503A pharmacy
A compounding pharmacy operating under section 503A of the federal Food, Drug and Cosmetic Act: state-licensed, preparing customized medications for individual patients against specific prescriptions. Which bulk substances 503A pharmacies may use is governed by FDA's evolving lists — the mechanism behind most 'is this peptide compoundable?' questions.
503B outsourcing facility
An FDA-registered 'outsourcing facility' that compounds medications in bulk under current Good Manufacturing Practice requirements — a stricter tier than 503A. Clinics sourcing compounded medications at scale typically buy from 503B facilities.
Amino acid
The building-block molecules of peptides and proteins. Twenty standard amino acids, arranged in different sequences, produce essentially all of the body's peptide and protein diversity — sequence order determines what the resulting molecule does.
Beyond-use date
The date after which a compounded preparation should not be used, assigned by the compounding pharmacy based on stability standards — the compounded world's equivalent of an expiration date, and typically much shorter than a manufactured drug's shelf life.
Category 2 (bulk substances)
A tier of FDA's interim rule-making about which bulk substances compounding pharmacies may use. Category 2 designates substances FDA has flagged for significant safety risk — effectively removing them from lawful compounding while so designated. Several popular peptides have moved on and off this list since 2023, which is why 'is it compoundable?' has no permanent answer.
Compounded drug
A medication prepared by a licensed pharmacy rather than a pharmaceutical manufacturer — lawful when done by licensed 503A/503B operations using permitted ingredients under a valid prescription. Compounded drugs are not FDA-approved products: legitimacy comes from the pharmacy's license and the prescriber's oversight, not from an approval.
Evidence tier
The grading this site applies to each compound's published human evidence: 'human clinical trials' (controlled studies in people), 'limited human data' (small or early studies), or 'mostly animal/lab studies' (no meaningful human research). Popularity online has no relationship to tier — that mismatch is the library's central finding.
GLP-1
Glucagon-like peptide-1 — a gut hormone released after eating that slows stomach emptying, signals fullness and prompts insulin release. Engineered GLP-1 receptor agonists (semaglutide, liraglutide) and multi-receptor relatives (tirzepatide) are the most evidence-backed peptide drugs in current medicine.
Gray market
The sales ecosystem operating outside regulated pharmacy channels: 'research chemical' websites, imports and resellers. Products may or may not contain what their labels claim — published analyses have repeatedly found mislabeled contents — and no oversight body stands behind any of it.
Half-life
The time it takes for half of a substance to be eliminated from the body. Molecular engineering of half-life is much of modern peptide pharmacology: natural GLP-1 survives minutes, liraglutide about a day, semaglutide about a week — same receptor, different engineering.
Peptide
A short chain of amino acids — conventionally 2 to 50 — linked by peptide bonds. Peptides sit between single amino acids and full proteins, and many function as signaling molecules: insulin, GLP-1 and oxytocin are all peptides. The word describes size and structure, not safety, legality or effectiveness.
Peptide bond
The chemical bond that joins the carboxyl group of one amino acid to the amino group of the next, releasing a water molecule. Repeating this link builds the chains we call peptides and, at greater lengths, proteins.
Peptide therapy
The wellness industry's umbrella term for programs built around prescribed peptides — properly: a licensed prescriber, baseline labs, a licensed compounding pharmacy, and follow-up. The phrase itself is marketing shorthand; the prescriber-and-pharmacy chassis underneath is what separates a medical program from a website with vials.
Polypeptide
A longer amino-acid chain — conventions vary, but roughly 20 to 50 units. Past about 50, once the chain folds into a stable three-dimensional working shape, it is usually called a protein. The boundaries are conventions, not laws of chemistry.
Pulsatile release
Hormone secretion in discrete bursts rather than a constant stream. Growth hormone is naturally pulsatile, which is why secretagogue marketing emphasizes preserving pulses versus injecting the hormone directly — a real physiological distinction whose clinical significance for wellness uses remains untested.
Receptor agonist
A molecule that binds to a cell-surface receptor and activates it — the key that turns the lock. Most peptide drugs are receptor agonists: they imitate a natural signaling molecule precisely enough to trigger its receptor.
Research use only (RUO)
A label used by chemical suppliers to state that a product is not intended for human or veterinary use. On gray-market peptide sites it functions as legal insulation while marketing implies otherwise. RUO products carry no required purity testing, no verified dose and no regulatory oversight — the label is a posture, not a standard.
Secretagogue
A substance that prompts a gland to release a hormone. Growth-hormone secretagogues like ipamorelin and the GHRP family don't contain growth hormone — they signal the pituitary to release its own. 'Raises a hormone' is a mechanism, not an outcome; what that release does to muscle, fat or aging is a separate, usually untested question.