A Plain-English Peptide Glossary

Peptide discussions are dense with jargon, and a lot of it gets used loosely or inaccurately by sellers. Below are plain-English definitions of the terms you will run into most, in alphabetical order. None of this is medical advice, and the appearance of a term here is not an endorsement of using any compound.

Basic biology and chemistry

Agonist — A molecule that binds a cellular receptor and switches it on, producing a response. Many peptides are agonists: they mimic a natural signaling molecule and activate its receptor. (The opposite is an antagonist, which blocks the receptor.)

Amino acid — The building block of peptides and proteins. There are 20 common amino acids; the order in which they are strung together determines what a peptide does.

Peptide — A short chain of amino acids joined by peptide bonds. “Peptide” and “protein” sit on a spectrum of length; peptides are the shorter chains. Many are signaling molecules in the body.

Secretagogue — Anything that triggers a cell to secrete a substance. A “growth hormone secretagogue” prompts the pituitary to release growth hormone.

GHRH / GHRP — Two distinct families often lumped together. GHRH (growth-hormone-releasing hormone) analogs act on the GHRH receptor. GHRPs (growth-hormone-releasing peptides) instead activate the ghrelin receptor (GHS-R1a). They are structurally unrelated and work through different pathways.

How a substance behaves in the body

Pharmacokinetics (PK) — The study of what the body does to a substance over time, summarized as ADME: absorption, distribution, metabolism, excretion.

Bioavailability — The fraction of an administered dose that actually reaches the bloodstream. By definition an intravenous dose is 100%; other routes are lower because some is lost to absorption or breakdown.

Half-life — The time it takes for the concentration in plasma to fall by half. It is a rough guide to how long something lingers, not a hard expiration timer.

Subcutaneous (SubQ/SC) — Into the fatty layer just under the skin, as opposed to into a vein (IV) or muscle (IM). It is a route of administration, nothing more.

Form, handling, and units

Lyophilized — Freeze-dried into a stable powder. Most research peptides ship lyophilized because the dry form survives shipping and storage better than a solution.

Reconstitution — Dissolving a lyophilized powder back into liquid with a sterile diluent before use. The math is straightforward: a 5 mg vial dissolved in 2 mL of liquid yields 2.5 mg per mL. (That is a concentration calculation, not a dose recommendation.)

Bacteriostatic water — Sterile water containing benzyl alcohol (commonly 0.9%), a preservative that inhibits bacterial growth and allows a multi-dose vial to be used over roughly 28 days. Plain “sterile water for injection” has no preservative and is single-use. Notably, the labeling for benzyl alcohol carries a warning that it is not for use in newborns, where it has been linked to a fatal “gasping syndrome.”

IU vs mg vs mcg — Two different kinds of measurement. A milligram (mg) and microgram (mcg, one-thousandth of a mg) are units of mass — physical weight. An International Unit (IU) is a unit of biological activity defined against a reference standard. There is no universal IU-to-mg conversion; it differs for every substance, so the two are not interchangeable.

Quality and labeling

HPLC — High-performance liquid chromatography, the standard lab method for estimating a peptide’s purity by separating its components and measuring the main peak. Results vary with equipment and conditions, so numbers from different labs may differ slightly. HPLC measures purity but does not by itself confirm a compound’s identity.

COA (Certificate of Analysis) — A lab document reporting a specific batch’s identity and purity, typically via HPLC plus mass spectrometry. A meaningful COA names the testing lab, the lot number, and the methods. A COA confirms what is in the vial; it says nothing about whether a substance is safe or legal to use.

RUO (“Research Use Only”) — A label meaning a product is sold for laboratory research, not human use. It is informational only. The FDA does not treat “RUO” or “not for human consumption” as authorization for human use, and judges actual intended use from the totality of evidence, not the label text.

Bottom line

These words describe chemistry, biology, and paperwork — not safety or legality. “RUO,” a clean COA, or a familiar route like subcutaneous tells you nothing about whether a compound is safe or approved for a person. Most of these substances are not FDA-approved for human use; treat the vocabulary as a tool for reading critically, not as reassurance.

Sources

• Pharmacokinetics — StatPearls, NIH/NCBI Bookshelf
• Drug Bioavailability — StatPearls, NIH/NCBI Bookshelf
• Elimination Half-Life of Drugs — StatPearls, NIH/NCBI Bookshelf
• Drug Absorption — StatPearls, NIH/NCBI Bookshelf
• Growth hormone secretagogues: history, mechanism of action, and clinical development (Ishida et al., 2020), JCSM Rapid Communications
• Growth hormone-releasing peptides and their analogs (Camanni, Ghigo, Arvat, 1998) — PubMed
• Distribution of In Vitro Diagnostic Products Labeled for Research Use Only or Investigational Use Only — FDA Guidance
• Bacteriostatic Water for Injection, USP — DailyMed label (benzyl alcohol; not for use in neonates)
• Designing peptide receptor agonists and antagonists (Hruby, 2002) — Nature Reviews Drug Discovery