The Vocabulary: What the Terms Actually Mean

A peptide is a chain of amino acids joined by peptide bonds. A protein is the same chemistry at greater length. The line between them is arbitrary in nature but not in regulation: the US Food and Drug Administration draws it at 40 amino acids. More than 40 residues and it counts as a protein, and therefore a biologic. Forty or fewer and it is a peptide, regulated as a conventional drug. That single number decides which approval pathway a compound can travel.

Small molecules are a different class: compact synthetic structures, usually under about 900 daltons, stable enough to survive the stomach. Most familiar tablets are small molecules.

Peptides sit between the two. They are large enough to be broken apart by digestive proteases and cleared quickly by the kidney, and small enough to be made by chemical synthesis rather than grown in cell culture.

A receptor is a protein that changes shape when the right molecule binds to it. An agonist switches it on, an antagonist blocks it. GLP-1 is glucagon-like peptide-1, an incretin hormone released by L-cells in the intestinal wall after eating, acting on a class B G-protein-coupled receptor. A GLP-1 receptor agonist is any molecule that switches that receptor on.

Native GLP-1 is broken down by the enzyme DPP-4 within minutes. An analogue is a modified version of a natural molecule, and the modifications in this class mostly exist to defeat that clearance: substitutions at the cleavage site, and fatty acid chains that let the molecule ride on serum albumin. Dual agonists add activity at a second receptor such as GIP.

A secretagogue does not do the job itself: it prompts the body to secrete something. Growth hormone secretagogues signal the pituitary through the ghrelin receptor GHS-R1a or the GHRH receptor, rather than supplying growth hormone directly. Most compounds described this way are not approved medicines for human use anywhere.

Lyophilisation is freeze-drying. The solution is frozen and held under vacuum so the ice sublimes straight to vapour. Peptides degrade in water through hydrolysis, oxidation and aggregation, so removing the water buys shelf life. What remains is the cake, a dry plug that usually contains bulking agents and lyoprotectants such as mannitol, sucrose or trehalose.

Reconstitution is adding a liquid, the diluent, back to that powder. How that is done, and with what, belongs to a manufacturer's label and to a clinician.

Bacteriostatic water for injection is sterile water containing a preservative, usually benzyl alcohol, which inhibits bacterial growth. Sterile water for injection has no preservative and is labeled for single use. Labels for products containing benzyl alcohol warn against use in neonates, following the gasping syndrome cases of the early 1980s. Those are label facts, not advice. As a general principle the category is temperature sensitive, and a vial's history since manufacture is not visible by looking at it.

Half-life is the time taken for a compound's concentration in plasma to fall by half. It is a measured property, not an instruction. Half-lives here run from minutes to a week or more, and the difference is usually engineering: albumin binding, PEGylation, protease resistance. A half-life figure describes how quickly the body clears a molecule and nothing more. It is not a basis for planning use, and no such planning appears in this guide. Those decisions sit with a licensed clinician and with the approved product label.

Subcutaneous describes the tissue layer that sits beneath the skin and above the muscle. It is a standard anatomical term used across medicine and is not specific to this category. Blood flow through that layer is limited compared with muscle, which is why absorption from it is generally slower and steadier. Which route applies to any given product is set by that product's label and by the prescribing clinician.

Bioavailability is the fraction of what is given that reaches circulation intact, with intravenous defined as 100 percent by convention. An unmodified peptide swallowed as a tablet is destroyed by gut proteases, so oral bioavailability sits far below one percent without deliberate engineering such as the absorption enhancer SNAC.

Three different kinds of quantity get loosely called units. Milligrams and micrograms are units of mass, and one is a thousand times the other. The same numeral therefore means wildly different amounts depending on the suffix beside it, which is the most common source of serious confusion here.

An international unit, IU, is not a mass. It measures biological activity, defined separately for each substance against a reference preparation held by the World Health Organization's Expert Committee on Biological Standardization. Any conversion between IU and milligrams is specific to one substance and one assay. A conversion borrowed from another compound means nothing.

Units marked on a syringe barrel are a third thing: marks of volume. An insulin syringe is graduated for one standard concentration of insulin. Outside that context the marks describe how much liquid the barrel holds, not how much of any compound is in it, and the same mark corresponds to a different mass whenever the concentration differs. That is why no worked example appears here.

Research chemical is a legal and labeling status, not a statement about quality. Material sold for research use only has not been reviewed or cleared for human use. Many compounds in this category live here permanently: no completed human trials, no approved indication anywhere, and no regulator standing behind identity, purity or content.

An approved medicine has passed a regulator's review for a named indication and population. Approval is narrow and jurisdictional: approved in one country for one condition is not approved everywhere or for everything, and popularity is not approval.

Compounding pharmacies are separate. In the United States they work under sections 503A and 503B of the Federal Food, Drug, and Cosmetic Act: a 503A pharmacy compounds against an individual prescription, a 503B outsourcing facility produces larger batches under stricter oversight. Compounded preparations are not FDA approved products, and the room to compound a copy of a commercially available drug narrows once a shortage is declared resolved, as happened around the GLP-1 shortages across 2024 and 2025.

Generic and brand describe naming, not quality: the brand name is proprietary, and the generic, or international nonproprietary name, is the molecule. Biologics have no generics, only biosimilars on a separate pathway, while peptides at or under 40 amino acids are regulated as drugs and can face conventional generic competition.

Serious errors in this space usually begin as language errors: a mass unit read as an activity unit, a syringe mark read as an amount of compound, a research label read as a quality claim, a compounded preparation assumed to carry the oversight of an approved one.

OptimusPep is a tracking and education tool. It records what you and your clinician have already decided. It does not recommend compounds, amounts or schedules, and nothing here is medical advice. Decisions about starting, stopping or changing anything belong with a licensed clinician who knows your history.