Peptide Glossary: 50 Essential Terms Explained
A reference for the most-used vocabulary in peptide research, from agonist to zwitterion
Peptide research has accumulated a vocabulary that's part biochemistry, part pharmacology, part regulatory law, and part informal community jargon. This is the working glossary for the most-used 50 terms.
A
Agonist: A molecule that binds to a receptor and activates it. Most therapeutic peptides are agonists. Example: semaglutide is a GLP-1 receptor agonist.
Amino acid: The building block of peptides and proteins. The 20 standard amino acids form essentially all human peptides.
Analog: A modified version of a natural compound. Semaglutide is a GLP-1 analog with a fatty acid attached for stability.
Antagonist: A molecule that binds to a receptor and blocks its activation. Less common in peptide therapeutics than agonists.
Aseptic processing: Manufacturing under sterile conditions. Required for injectable pharmaceuticals.
B
Bioavailability: The fraction of a dose that reaches systemic circulation. Most peptides have low oral bioavailability (digestive degradation), high subcutaneous bioavailability.
BPC-157: 15-amino-acid peptide derived from gastric juice, used in healing research.
Bulk drug substance: The active pharmaceutical ingredient in unformulated form.
C
Category 2: FDA classification for compounding bulk substances with insufficient safety data. Several peptides moved to Category 2 in September 2023.
cGMP: current Good Manufacturing Practice. Pharmaceutical manufacturing standard.
Certificate of Analysis (COA): Document accompanying a peptide batch describing identity, purity, and other quality parameters.
CJC-1295: GHRH analog used in growth hormone protocols. DAC variant has extended half-life.
Compounding: Pharmacy preparation of customized medications. 503A is patient-specific; 503B is bulk facility.
Counter-ion: Charged molecule paired with peptide. TFA from synthesis, acetate preferred for human use.
D
DAC: Drug Affinity Complex. A modification (often albumin-binding) that extends a peptide's half-life.
Deamidation: Common peptide degradation pathway affecting Asn and Gln residues.
Degradation: Loss of peptide integrity over time through oxidation, hydrolysis, or aggregation.
E
Endogenous: Produced by the body. Insulin and oxytocin are endogenous peptides.
Endotoxin: Bacterial cell wall fragment that causes fever and inflammation when injected. Tested in EU/mg.
Epitalon: Tetrapeptide developed in Russia for telomerase research.
F
FDA-approved: Cleared for marketing by the US Food and Drug Administration. Stricter regulatory category than compounded.
Free base: Peptide form without counter-ion. Less common in commercial supply.
G
GHK-Cu: Tripeptide-copper complex used in cosmetic and wound healing applications.
GHRP: Growth Hormone Releasing Peptide class (GHRP-2, GHRP-6, hexarelin, ipamorelin).
GIP: Glucose-dependent Insulinotropic Polypeptide. Activated by tirzepatide alongside GLP-1.
GLP-1: Glucagon-Like Peptide 1. Activated by semaglutide and others; central to incretin therapy.
H
Half-life: Time for peptide concentration to fall to half. Determines dosing frequency.
HPLC: High-Performance Liquid Chromatography. Standard purity measurement.
I
IGF-1: Insulin-like Growth Factor 1. Downstream marker of GH activity.
IND: Investigational New Drug application. Required for FDA-supervised human research.
Incretin: Gut hormone family that triggers insulin release in response to food. Includes GLP-1 and GIP.
Ipamorelin: Selective ghrelin receptor agonist used in growth hormone protocols.
L
Lyophilized: Freeze-dried. Most peptide vials ship as lyophilized powder for stability.
M
Mass spectrometry (MS): Analytical method confirming peptide identity by molecular weight.
MOTS-c: Mitochondrial-derived peptide with metabolic regulation function.
Mounjaro: Brand name for tirzepatide approved for type 2 diabetes.
N
NAD+: Nicotinamide Adenine Dinucleotide. Cofactor for sirtuin and PARP activity. Not technically a peptide.
NMN / NR: Nicotinamide Mononucleotide / Riboside. NAD+ precursors.
O
Off-label use: Use of an FDA-approved drug for an indication or population not approved.
Oral bioavailability: Fraction of an oral dose reaching circulation. Generally low for peptides.
Ozempic: Brand name for semaglutide for type 2 diabetes.
P
Peptide bond: Covalent bond between amino acid residues forming the peptide backbone.
Pharmacokinetics (PK): Study of how the body processes a drug (absorption, distribution, metabolism, excretion).
Phase I/II/III: Stages of human clinical trials. Phase III provides primary efficacy and safety evidence for FDA approval.
Purity: Percentage of total mass that is the target compound. Measured by HPLC.
R
Receptor: Cell surface or intracellular protein that binds ligands (including peptides) to trigger response.
Research-grade: Material intended for laboratory use, not human administration. Different quality standards from clinical-grade.
Retatrutide: Triple-receptor agonist (GLP-1/GIP/glucagon) in late-stage development.
S
Selank: Russian-developed anxiolytic peptide.
Semaglutide: GLP-1 receptor agonist; backbone of Ozempic and Wegovy.
Semax: Russian-developed neuroprotective peptide.
Sequence: Order of amino acids in a peptide chain.
Subcutaneous: Under-the-skin injection route. Standard for many peptide therapies.
SURPASS / SURMOUNT trials: Phase III trial programs for tirzepatide.
SUSTAIN / STEP / SELECT trials: Phase III trial programs for semaglutide.
T
TB-500: Synthetic 7-amino-acid fragment of Thymosin Beta-4.
TFA: Trifluoroacetic acid. Common counter-ion from peptide synthesis; less ideal for clinical use than acetate.
Thymosin Alpha-1: Immunomodulator approved in 35+ countries.
Tirzepatide: Dual GLP-1/GIP agonist; backbone of Mounjaro and Zepbound.
W
WADA: World Anti-Doping Agency. Maintains the Prohibited List that includes many peptides.
Wegovy: Brand name for semaglutide approved for obesity.
Z
Zepbound: Brand name for tirzepatide approved for obesity.
Zwitterion: Molecule with both positive and negative charges. Most amino acids are zwitterionic at physiological pH.