/ Glossary

Every term, defined plainly.

55 terms from the chemistry, the pharmacology, the preparation vocabulary, the regulatory language, and the research words people quote without checking. Where a term is routinely misused in this category, the definition says so, because that correction is the part nobody else bothers with.

Chemistry

7 terms

Amino acid: The building block of peptides and proteins, made of an amino group, a carboxyl group, and a side chain that gives each one its distinct character. Twenty standard amino acids are encoded directly by the genetic code, and humans incorporate a twenty-first, selenocysteine, through a recoding mechanism. Synthetic peptides often contain modified or non-natural amino acids as well.
Amino acid sequence: The order in which amino acids appear in a peptide, read from the N-terminus to the C-terminus and written in single-letter or three-letter code. Sequence determines identity: the same amino acids in a different order form a completely different molecule.
Analogue: A molecule intentionally modified from a parent structure, usually to change how long it survives in the body, how tightly it binds a receptor, or how it can be manufactured. An analogue is a distinct substance from the compound it was derived from, so its properties, its regulatory status, and its evidence base all have to be established separately.
Counterion (acetate, trifluoroacetate): Synthetic peptides are normally isolated as a salt, so the peptide is paired with a small ion such as acetate or trifluoroacetate (TFA) left behind by synthesis and purification. The counterion contributes to the total weight of powder in a container, which is why net peptide content can be lower than the labeled weight of the contents.
Peptide: A chain of amino acids joined end to end, shorter than a protein. There is no universal cutoff, but chains of roughly 50 amino acids or fewer are usually called peptides. The word describes a class of molecule and nothing more, so calling something a peptide says nothing about what it does, whether it is legal, or whether it has been studied in people.
Peptide blend: A preparation containing more than one peptide in a single container. A blend makes interpretation harder, because any change or reaction that follows cannot be attributed to one component rather than another.
Peptide purity: The share of a tested batch that is the intended peptide, usually measured by high performance liquid chromatography (HPLC) and reported as a percentage. Purity is the most misread figure in this space: it says nothing about identity, sterility, endotoxin content, or what the remaining percentage actually consists of.

Pharmacology

10 terms

Bioavailability: The fraction of an administered amount that reaches the bloodstream in active form. Most peptides have extremely low oral bioavailability because digestive enzymes break them apart, which is why the route of administration dominates the pharmacology of this class.
Growth hormone secretagogue: A class of compounds that prompt the pituitary to release the body's own growth hormone rather than supplying growth hormone itself. They act at one of two receptors: the growth hormone releasing hormone receptor, or the ghrelin receptor, also written GHS-R1a. A secretagogue and growth hormone itself are pharmacologically and legally distinct, and the two are constantly conflated. Approval status has to be checked compound by compound. A small number have narrow approvals for specific uses in specific countries, and most of the compounds discussed under this heading do not.
Half-life: The time taken for the concentration of a substance in the blood to fall by half. It is an average drawn from measured data and varies between individuals. It describes how long a molecule persists, not how long any effect lasts, and those two are routinely treated as the same thing when they are not.
Pharmacodynamics: What a substance does to the body, meaning the relationship between its concentration and the effects that can be measured. Pharmacokinetics and pharmacodynamics are complementary and are commonly mistaken for each other.
Pharmacokinetics: What the body does to a substance: absorption, distribution, metabolism, and excretion, often abbreviated ADME. Half-life, peak concentration, and time to peak are all pharmacokinetic measures.
Receptor: A protein, usually sitting on a cell surface, that a signaling molecule binds to and thereby changes what the cell does. Peptides are generally active because they fit a specific receptor, which is why a small structural difference between two similar molecules can change the response completely.
Receptor agonist: A molecule that binds a receptor and switches it on, producing the same type of signal the body's own messenger would. Agonist describes a mechanism only. It carries no implication of benefit, strength, or approval.
Receptor antagonist: A molecule that binds a receptor and blocks it, preventing the usual signal without generating one itself. Agonist and antagonist are opposites in mechanism, and the two words are frequently swapped in casual writing.
Subcutaneous: Into the layer of fatty tissue between the skin and the muscle beneath it, abbreviated SC or subQ. The term describes anatomy and a route of administration, and says nothing about technique or amount.
Titration: The clinical practice of adjusting an amount gradually while monitoring response and tolerability. It is a prescriber-directed process built on assessment and follow-up, and the word is widely borrowed online to describe unsupervised self-experimentation, which is not what it means.

Preparation and handling

9 terms

Bacteriostatic water: Sterile water containing a preservative, most often benzyl alcohol, that inhibits bacterial growth so a container can be entered more than once. It is not a sterilizing agent and cannot rescue a container that has already been contaminated, and benzyl alcohol is not appropriate for every product or every person.
Beyond-use date: The date after which a prepared, repackaged, or reconstituted product should no longer be used, assigned by the pharmacy or facility that prepared it. It is not the same as a manufacturer's expiration date, which applies to the sealed product as made, and the two are frequently confused.
Cold chain: The unbroken, temperature-controlled handling of a temperature-sensitive product from manufacture through storage to the point of use. As a general principle, a temperature excursion can degrade a product with no visible change, and specific storage conditions come from the product's own labeling.
Excipient: Any ingredient in a finished preparation that is not the active substance, such as a buffer, stabilizer, bulking agent, or preservative. Excipients are a common source of reactions that then get attributed to the active ingredient.
Insulin syringe units (U-100): The markings on a standard insulin syringe are insulin units on a U-100 scale, where 100 units correspond to one milliliter of U-100 insulin. They are volume graduations calibrated for insulin alone, so a unit is not a measure of any other substance. Treating the two as interchangeable is among the most common and most serious errors made with this vocabulary.
Lyophilized: Freeze-dried: the material is frozen and its water removed under vacuum, leaving a dry cake or powder that stores far more stably than a solution. Also spelled lyophilised. The word describes a physical state and implies nothing about purity, sterility, or regulatory status.
Reconstitution: Returning a freeze-dried solid to liquid form by adding a suitable sterile diluent. Which diluent is appropriate, and what concentration results, are properties stated on a given product's own labeling and are specific to that product.
Sterile water for injection: Water that has been sterilized and contains no preservative and no other additive. Because there is no preservative, labeling for it is normally single use, which is the practical difference between it and bacteriostatic water.
Vial: A small glass or plastic container sealed with a rubber stopper and a crimped metal cap, used to hold injectable products. Labeling designates a vial as single-dose or multiple-dose, and that designation reflects whether the contents include a preservative.

GLP-1 and metabolic

10 terms

DPP-4 (dipeptidyl peptidase-4): An enzyme that rapidly inactivates GLP-1 and GIP after they are released. Resistance to DPP-4 is a central design goal behind long-acting incretin analogues, while inhibiting the enzyme itself is the mechanism of a separate, older class of medicines.
Dual agonist (co-agonist): A single molecule engineered to activate two receptors, for example GLP-1 and GIP. Activating two pathways with one molecule is not the same as combining two products, and the evidence for any given combination applies only to that specific molecule.
Gastric emptying: The rate at which stomach contents pass into the small intestine. Slowed gastric emptying is one route by which incretin signaling influences appetite and post-meal glucose, and it also accounts for a large share of the gastrointestinal effects reported with this class.
GIP (glucose-dependent insulinotropic polypeptide): The other major incretin hormone released from the gut after eating. It also stimulates insulin release in response to glucose, and its effects on fat tissue differ from those of GLP-1.
GLP-1 (glucagon-like peptide-1): A hormone released by the gut after eating that increases insulin release when blood glucose is high, suppresses glucagon, slows gastric emptying, and signals to appetite centers in the brain. The natural hormone is broken down within minutes, which is the problem that long-acting analogues were designed around.
GLP-1 receptor agonist: A molecule that activates the GLP-1 receptor, reproducing the hormone's signal while resisting the enzyme that degrades the natural version. The phrase names a mechanism and a class. Individual products within that class differ in structure, approved indications, and the evidence behind them, so they are not one interchangeable thing.
HbA1c: Glycated hemoglobin, a blood measure reflecting average glucose exposure over roughly the preceding two to three months. It is a laboratory result interpreted by a clinician alongside other findings, and it is not a score to be self-graded or acted on alone.
Incretin effect: The observation that glucose taken by mouth produces a larger insulin response than an intravenous glucose infusion that holds blood glucose at the same levels. The difference is attributed to hormones released by the gut, which amplify the insulin signal. Matching blood glucose across the two routes is what makes the comparison meaningful, because the point is to isolate the contribution of the gut hormones rather than the glucose itself. This effect is the physiological basis of the entire incretin drug class.
Insulin resistance: A state in which cells respond less to insulin, so more of it is needed to move the same amount of glucose out of the blood. It is identified through clinical assessment and laboratory testing, not from symptoms, appearance, or a body measurement.
Satiety: The suppression of hunger in the period after eating, which governs how long it is before someone eats again. It is distinct from satiation, which is what brings a single meal to an end, and the two words are used interchangeably far more often than they should be.

Regulatory

10 terms

503A pharmacy and 503B outsourcing facility: Two categories under US federal law: a 503A pharmacy compounds for an identified individual patient against a prescription, while a 503B outsourcing facility may compound larger batches under stricter manufacturing and reporting requirements. Neither status makes a compounded preparation FDA approved.
Category 2 bulk drug substance: A designation under FDA's interim policy on bulk drug substances nominated for use in compounding. Placing a substance in Category 2 records that the agency has identified significant safety risks with it, and FDA has stated it does not intend to exercise enforcement discretion for compounding with substances in that category. Several peptides discussed widely online sit here. The classification is a regulatory finding about the substance itself and is not a comment on any individual's experience.
Compounded drug: A medication prepared by a licensed pharmacy or outsourcing facility to meet a need a manufactured product does not, such as an allergy to an excipient or a form a patient cannot take. Compounded preparations are not FDA approved and are not reviewed for safety, effectiveness, or manufacturing quality before they are dispensed.
Dietary supplement: A US regulatory category created by DSHEA for products intended to supplement the diet, which are not reviewed or approved by the FDA before they go on sale. The FDA has taken the position that certain peptides are excluded from this category, so a peptide marketed as a supplement is not thereby lawful, reviewed, or verified.
FDA approval: A decision by the US Food and Drug Administration that a specific product, from a specific manufacturer, for a specific use, has shown that its benefits outweigh its risks on the evidence submitted. Approval attaches to a product and an indication, not to a molecule in general, so the same substance can be approved in one form and entirely unapproved in another.
FDA drug shortage list: The FDA's published record of products currently in shortage. It matters beyond supply, because certain compounding activity is permitted while a product is listed and restricted once the shortage is resolved, which is why the list is watched so closely in the GLP-1 space.
INN (International Nonproprietary Name): The generic name assigned to an active substance by the World Health Organization so it can be identified across borders. It names the substance itself, while a brand name identifies one company's product containing that substance. The system aims for a single global name and mostly achieves it, though a few countries retain different national names for the same substance. Shared name stems, such as the ending common to many peptide analogues, signal a shared class rather than similar effects or interchangeability.
Investigational: A substance being studied in humans under a regulatory authorization such as an Investigational New Drug (IND) application, and not approved for general use. Investigational describes a stage of study and nothing more. It does not indicate that approval is coming, and it does not apply to material held under no such authorization, even when the word appears on a label or in marketing.
Off-label use: Use of an approved product for an indication, population, age group, or route not covered by its approved labeling. A licensed prescriber may lawfully decide to prescribe off-label. The phrase describes a clinical prescribing decision, and it is not a status that makes an unapproved substance available to anyone.
Research use only (RUO): A labeling designation stating that a material is intended for laboratory research and is not for diagnostic or human use. It is a statement of intended use, and it carries no assurance of identity, purity, sterility, or safety in a person.

Research

9 terms

Clinical trial phases: Phase 1 studies look primarily at safety and how the body handles a substance in a small group, Phase 2 explores whether it produces a measurable effect and at what exposure, and Phase 3 compares it against a control in a large population. Regulatory decisions normally rest on Phase 3 evidence, so a Phase 1 or Phase 2 result is not a finding of effectiveness.
Double-blind: A design in which neither the participants nor the researchers assessing them know who received which treatment, which reduces bias in what gets reported and in what gets measured. Single-blind means only the participants are unaware, and the two are often quoted as if they were equivalent.
In vitro: Literally in glass: experiments performed on cells, tissue, or isolated proteins outside a living organism. In vitro work can establish a mechanism at best, and findings there routinely fail to reproduce in a living body.
In vivo: In a living organism. In vivo very often means an animal study rather than a human one, so the phrase on its own is not evidence of anything in people.
Peer review: Evaluation of a manuscript by independent specialists in the field before a journal publishes it. Peer review filters obvious errors and overreach. It does not verify the underlying data and does not certify that a finding is correct.
Preclinical: The stage of research before any testing in humans, covering laboratory and animal work. The large majority of preclinical findings never translate into human results, which is why claims sourced from this stage mislead when they are repeated as established fact.
Preprint: A manuscript posted publicly before it has been peer reviewed. Preprints circulate results quickly and can be revised substantially or withdrawn entirely, so they carry less weight than a published, reviewed paper despite often being quoted as though they carried more.
Randomized controlled trial: A study in which participants are assigned by chance to the intervention or to a comparator, so differences between the groups are unlikely to reflect who chose what. It is the strongest routine design for showing that an intervention caused an outcome rather than merely accompanying it.
Statistical significance (p-value): A p-value estimates how likely results at least this extreme would be if there were no real effect, with 0.05 the conventional threshold. Significant does not mean large, lasting, or clinically meaningful, and that conflation is the most common misuse of research language in this field.

Definitions are educational reference, not medical advice. None of them recommends a dose, a schedule, or a compound, and a term being defined here is not a statement that anything is safe, legal where you are, or worth using. See how we source, or keep reading in the guides and the compound library.