Peptide Terminology: A Plain Glossary

Peptide listings and research summaries assume a vocabulary that nobody explains. These are the terms that come up most, in plain language, with links to where each one actually matters.

What the molecules are

• Peptide. A short chain of amino acids. Longer chains become proteins; the boundary is conventional rather than precise, usually around fifty residues.

• Analogue. A modified version of a natural molecule, altered to change stability, duration, or receptor selectivity. Semaglutide is a GLP-1 analogue.

• Fragment. A portion of a larger molecule, isolated in the hope of keeping one activity and dropping others. AOD-9604 and TB-500 are both fragments.

• Blend. Multiple peptides in one vial at a fixed ratio, such as GLOW and KLOW.

What they do

• Agonist. Binds a receptor and activates it. A dual agonist activates two receptor types — tirzepatide at GIP and GLP-1.

• Antagonist. Binds a receptor and blocks it rather than activating it.

• Secretagogue. Prompts the body to release something it already makes rather than supplying it. Ipamorelin is a growth hormone secretagogue — the distinction matters because the ceiling stays the body’s.

• Selective. Acts on one target without triggering related ones. Ipamorelin’s selectivity is its defining claim.

• Desensitisation. Receptors becoming less responsive to continued stimulation. It can reverse an effect entirely — see gonadorelin.

Handling and formulation

• Lyophilised. Freeze-dried to a stable powder. The form nearly all research peptides ship in.

• Reconstitution. Adding liquid to dissolve that powder. The volume sets concentration, not the amount of peptide — see the bacteriostatic water guide.

• Bacteriostatic water. Sterile water with 0.9% benzyl alcohol, allowing a vial to be re-entered for up to 28 days.

• Beyond-use date. When a vial should be discarded after first puncture — distinct from the printed expiry.

• Subcutaneous. Into the fat layer beneath the skin. See injection sites.

Measurement

• mg/mL. Concentration — milligrams of peptide per millilitre of solution. The number that turns a dose into a syringe reading.

• Unit (syringe). A volume mark. On a U-100 syringe, 100 units is 1 mL. It says nothing about drug content — see the syringe guide.

• IU. International unit — a measure of biological activity, not mass, and not interchangeable with syringe units. Relevant for oxytocin.

• mcg. Microgram, one thousandth of a milligram. Most research peptides are discussed at this scale.

Quality and testing

• COA. Certificate of analysis — a lab report for one production batch. See how to read one.

• HPLC purity. The proportion of peptide material that is the target compound. Typically 95–99%.

• Net peptide content. The proportion of the vial’s mass that is actually peptide rather than counter-ions and water. Usually 70–85%, and usually not reported.

• Mass spectrometry. Confirms molecular weight — answers whether it is the right molecule, which purity alone does not.

• Research use only. A statement about how a product is marketed, not a quality certification. See are peptides legal.

Evidence terms

• Preclinical. Before human trials — cell culture and animal work. Most research peptides never leave this stage.

• Phase 1 / 2 / 3. Human trial stages: safety and dosing, then efficacy signal, then confirmation at scale.

• Endpoint. What a trial set out to measure. A missed primary endpoint means the study did not show what it was designed to show.

• Dose conversion. Translating an animal dose to a human one, done by body surface area rather than weight. Skipping that step inflates the result several-fold — see BPC-157.