Peptide Glossary

Terms are grouped by what they describe rather than alphabetically, because the distinctions only make sense next to the thing they are usually confused with.

Regulatory status

FDA-approved. The FDA has reviewed evidence that a specific product works for a specific use, in a specific population, at a specific dose, and that its benefits outweigh its risks — and has inspected how it is manufactured. Approval attaches to a product for an indication, never to a molecule in the abstract. The same active ingredient can be approved in one form and not another.

Off-label use. Prescribing an approved drug for a use it was not approved for. Legal, common, and a normal part of medical practice. It means a clinician has judged it appropriate — not that the FDA reviewed evidence for that use.

Compounded. Prepared by a pharmacy for an individual patient, rather than manufactured and approved as a finished product. Compounded drugs are not FDA-approved: the agency does not verify their safety, effectiveness or quality before they reach a patient. Compounding is legal and genuinely necessary for some patients. It is a statement about who prepared something, not about what has been checked.

503A and 503B. The two sections of US law that compounding pharmacies operate under. 503A covers traditional pharmacy compounding for an individual prescription. 503B covers registered outsourcing facilities, which may compound in larger batches and are held to stricter manufacturing standards. Neither produces an FDA-approved drug.

Drug shortage list. An FDA-maintained list of drugs in short supply. Its significance for peptides is indirect but large: shortage status changes what compounders may legally prepare. When a drug leaves the list, compounding allowances tied to that shortage end — which is how a compounded product can become unlawful without the compound itself changing at all. Always check the date on any claim about shortage status.

Research use only (RUO). A labelling term for material sold for laboratory work. It is not a regulatory category that permits human use, and it is not a safety assessment. In practice it frequently functions as a disclaimer attached to compounds sold to people who intend to take them.

IND — Investigational New Drug. The application a sponsor files to give an unapproved drug to humans in a study. An active IND means a compound is being formally investigated. It does not mean the compound works, and it does not permit sale to the public.

Evidence

Preclinical. Studies in cells or animals. Genuinely informative about mechanism and a poor predictor of human outcomes — most compounds that look promising preclinically do not work out in people. A compound with striking rat data and no human trials is not "backed by science" in the way most readers hear that phrase.

Clinical trial. A study in humans. The size, design and endpoints decide how much it can tell you. A trial is not automatically strong evidence.

Phase 1 / 2 / 3. Roughly: is it safe and how is it handled by the body (1), does it appear to do anything (2), does it work better than the existing option in a large population (3). Most compounds fail. A Phase 1 result is not a finding about effectiveness.

Randomised controlled trial (RCT). Participants assigned to treatment or control by chance, which is what allows a difference between the groups to be attributed to the treatment.

Placebo-controlled / double-blind. The control group receives an inactive preparation, and neither participants nor investigators know who is in which group until the study ends. Both features exist to stop expectation from producing the result.

Endpoint. The specific outcome a study measured. It is the most commonly skipped detail in health coverage, and often the most important — a trial can succeed on a surrogate marker while telling you nothing about whether anyone felt better or lived longer.

Peer-reviewed. Assessed by other researchers before publication. A quality filter, not a guarantee, and it says nothing about study size or design.

Systematic review / meta-analysis. A structured synthesis of the existing studies. Usually the strongest available evidence, and only as good as the studies it pools.

Sport and anti-doping

WADA Prohibited List. The list of substances and methods banned in sport, maintained by the World Anti-Doping Agency and updated annually.

FDA status is not WADA status. These are separate systems answering separate questions. An FDA-approved prescription medicine can be prohibited in competition, and a compound no regulator has approved can still be banned. Never infer one from the other.

S0 — non-approved substances. A Prohibited List category covering substances not currently approved by any governmental regulatory health authority for human therapeutic use. Several compounds discussed in peptide communities sit here.

Therapeutic Use Exemption (TUE). Formal permission for an athlete to use an otherwise prohibited substance for a documented medical condition.

Chemistry

Peptide. A short chain of amino acids. Convention puts the boundary with proteins somewhere around 50. It describes structure — not safety, legality or effect.

Amino acid. The individual units peptides and proteins are built from.

Sequence. The order of the amino acids. It determines the shape the chain folds into, and the shape determines what it can bind to. Two peptides of the same length can do entirely unrelated things.

Bioavailability. How much of a dose actually reaches circulation. Most peptides have poor oral bioavailability because digestion breaks them down — which is why they are usually injected, and why oral versions are an engineering achievement rather than a formulation choice.

Half-life. How long it takes for half of a dose to be cleared. It drives dosing frequency, and it is why some compounds are taken weekly and others daily.

Analogue. A modified version of a natural molecule, usually altered to last longer or bind more selectively. Most peptide medicines are analogues rather than the natural peptide itself.

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Definitions are written for a general reader and are not a substitute for regulatory or medical advice. Where a term has a precise legal meaning, the citation above points to the authority that sets it.