Guide

Peptide Terminology Glossary

This glossary defines the key terms that appear across peptide research documentation, supplier materials, regulatory filings, and analytical reports. Terms are grouped alphabetically and written as encyclopedic, informational definitions — not instructions. The goal is to give readers a single reference for the vocabulary they will encounter when reading a Certificate of Analysis, evaluating a research-use-only supplier, or reviewing peer-reviewed peptide literature. For a practical walkthrough of how these terms appear together on a single document, see the guide on How to Read a COA.

A–D

A–D: Amino Acid, Bioavailability, Bacteriostatic Water, cGMP, COA, 503A / 503B

This group covers foundational biochemistry terms, regulatory categories, and the quality documentation vocabulary that readers encounter first when evaluating peptide suppliers or reading analytical reports.

Key points

• Amino acid — An organic molecule containing both an amino group (–NH2) and a carboxyl group (–COOH) that serves as the building block of peptides and proteins. Twenty standard amino acids are genetically encoded and linked in specific sequences to form peptides. The identity, order, and chemical properties of the amino acids in a chain determine the peptide's structure, charge, solubility, and biological activity.
• Bioavailability — The fraction of an administered substance that reaches systemic circulation in an active form. For peptides, bioavailability is generally low when administered orally because digestive enzymes and stomach acid degrade peptide bonds before absorption. This is why most peptide research involves parenteral routes such as subcutaneous injection. Bioavailability is expressed as a percentage and is determined through pharmacokinetic studies measuring plasma concentration over time.
• Bacteriostatic water — Sterile water for injection containing 0.9% benzyl alcohol as a preservative, allowing a vial to be entered multiple times over a defined period (commonly up to 28 days) without bacterial contamination. USP defines bacteriostatic water for injection as a multi-dose diluent. It is widely used to reconstitute lyophilized peptides. Bacteriostatic water is contraindicated in neonates because benzyl alcohol can cause fatal 'gasping syndrome.' Sterile water for injection, by contrast, contains no preservative and is single-use only after the vial is opened. See the related guide on How to Read a COA for context on how diluent choice appears in supplier documentation.
• cGMP (Current Good Manufacturing Practice) — A set of FDA-enforced regulations governing the methods, facilities, and controls used in manufacturing, processing, and packaging pharmaceutical products. cGMP ensures that products are produced consistently and meet quality standards. For peptides, cGMP compliance is required for FDA-registered outsourcing facilities under section 503B of the FD&C Act. Research-use-only peptide suppliers are not required to follow cGMP because their products are not regulated as drugs, which is a key distinction between compounded and RUO-sourced peptides.
• COA (Certificate of Analysis) — A laboratory-generated document reporting the measured analytical properties of a specific production batch. For peptides, a COA typically reports identity (via mass spectrometry), purity (via HPLC), and molecular weight. A legitimate COA is batch-specific, references the analytical methods used, and identifies the testing laboratory. COAs are the primary quality documentation in pharmaceutical and chemical manufacturing. For a detailed breakdown of COA structure, see the guide on How to Read a COA.
• 503A — The section of the Federal Food, Drug, and Cosmetic Act that defines traditional compounding pharmacies. 503A pharmacies compound medications pursuant to patient-specific prescriptions, are regulated primarily by state boards of pharmacy, and are exempt from certain FDA drug approval and cGMP requirements. Peptides compounded under 503A must be based on a valid prescription for an identified patient.
• 503B — The section of the FD&C Act that defines outsourcing facilities. 503B facilities compound drugs in larger quantities without patient-specific prescriptions, but must register with the FDA, comply with cGMP, and report adverse events. 503B facilities face stricter oversight than 503A pharmacies. The distinction between 503A and 503B determines the regulatory framework, documentation requirements, and scale at which compounded peptides can be legally produced.

E–H

E–H: GIP, GLP-1, GHRH, GHRP, Half-life, HPLC

This group covers the major peptide hormone families, the most common analytical method for measuring purity, and the pharmacokinetic concept that governs how long a peptide remains active in the body.

Key points

• GIP (Glucose-dependent Insulinotropic Polypeptide) — An incretin hormone secreted by K-cells in the small intestine in response to food intake. GIP stimulates insulin secretion from pancreatic beta cells in a glucose-dependent manner. It is one of two primary incretin hormones (the other being GLP-1). Tirzepatide is a dual GIP/GLP-1 receptor agonist, meaning it activates receptors for both hormones simultaneously. GIP's role in glucose regulation has made it a target for metabolic drug development.
• GLP-1 (Glucagon-Like Peptide-1) — An incretin hormone secreted by L-cells in the intestine that stimulates insulin secretion, suppresses glucagon, slows gastric emptying, and promotes satiety — all in a glucose-dependent manner. GLP-1 receptor agonists include semaglutide (Ozempic, Wegovy) and liraglutide, and the dual GIP/GLP-1 agonist tirzepatide (Mounjaro, Zepbound) also activates the GLP-1 receptor. GLP-1 has a naturally short half-life (approximately 2 minutes for native GLP-1) due to rapid degradation by the DPP-4 enzyme, which is why pharmaceutical GLP-1 agonists are engineered for extended half-lives.
• GHRH (Growth Hormone Releasing Hormone) — A peptide hormone produced in the hypothalamus that stimulates the anterior pituitary gland to secrete growth hormone. GHRH acts on growth hormone-releasing hormone receptors. Synthetic GHRH analogs include tesamorelin (FDA-approved for HIV-associated lipodystrophy under the brand name Egrifta) and sermorelin (previously FDA-approved for pediatric growth hormone deficiency). GHRH analogs are distinct from GHRPs (growth hormone releasing peptides), which act through different receptors.
• GHRP (Growth Hormone Releasing Peptide) — A class of synthetic peptides that stimulate growth hormone release through the ghrelin receptor (growth hormone secretagogue receptor), a different pathway from GHRH. Examples include ipamorelin, hexarelin, and GHRP-6. GHRPs mimic the action of ghrelin, the endogenous ligand for the growth hormone secretagogue receptor. Unlike GHRH analogs, no GHRP has been FDA-approved for any indication. GHRH and GHRP act on different receptors and can have additive effects on growth hormone release when studied together.
• Half-life — The time required for the concentration of a substance in the blood to decrease by 50%. A peptide's half-life determines how frequently it must be administered to maintain therapeutic or experimental plasma levels. Native GLP-1 has a half-life of approximately 2 minutes; semaglutide has a half-life of approximately 165 hours, enabling once-weekly dosing. Half-life is a pharmacokinetic property measured in clinical studies and is a key parameter in drug development. Extension of half-life is commonly achieved through amino acid modifications, fatty acid conjugation, or binding to albumin.
• HPLC (High-Performance Liquid Chromatography) — The most common analytical method for measuring peptide purity. HPLC separates the components of a dissolved sample by passing it through a chromatographic column under high pressure. The separated components are detected and quantified, producing a chromatogram with peaks corresponding to each component. The area under the main peak, relative to all detected peaks, gives the reported purity percentage. Reverse-phase HPLC (RP-HPLC) is the standard method for peptide analysis. HPLC measures chemical purity, not biological potency. For a full explanation of how to interpret HPLC results on a COA, see the guide on How to Read a COA.

I–L

I–L: Lyophilized

This group covers the physical form in which most research peptides are supplied and the process used to achieve that form.

Key points

• Lyophilized — Also called freeze-dried. A lyophilized peptide is one from which water has been removed through a process of freezing and vacuum sublimation, leaving a dry powder in a sealed vial. Lyophilization is the standard form for research-grade peptides because it extends shelf life, improves stability during shipping, and reduces degradation. Before use, a lyophilized peptide must be reconstituted — dissolved in a liquid diluent such as bacteriostatic water or sterile water for injection. The lyophilized powder should appear as a solid cake or loose powder; discoloration, clumping, or liquid in a vial labeled as lyophilized may indicate damage or degradation. Lyophilization does not guarantee purity — a peptide can be lyophilized and still contain impurities, which is why a COA with HPLC and mass spectrometry data is necessary regardless of the product's physical form.

M–P

M–P: Mass Spectrometry, Peptide, Peptide Bond, 503A / 503B (see A–D)

This group covers the second core analytical method for peptide identity verification and the foundational molecular definitions of what a peptide is and how it is constructed.

Key points

• Mass Spectrometry (MS) — An analytical technique that measures the mass-to-charge ratio of molecules in a sample. For peptide analysis, MS serves two purposes: identity confirmation and molecular weight verification. The mass spectrum shows whether the dominant molecular species has the expected molecular weight for the labeled peptide. The two most common MS methods for peptide analysis are MALDI-TOF (Matrix-Assisted Laser Desorption/Ionization Time-of-Flight) and ESI (Electrospray Ionization). MS confirms identity but does not directly measure purity — that is the role of HPLC. A COA that reports purity but lacks mass spectrometry data has not confirmed the identity of the compound. See the guide on How to Read a COA for a detailed explanation of how MS results appear on analytical documentation.
• Peptide — A short chain of amino acids linked by peptide bonds. Peptides are distinguished from proteins primarily by length, though there is no universally agreed boundary; peptides are generally considered to contain fewer than approximately 50 amino acids. Peptides occur naturally in the body (e.g., insulin, GLP-1, GHRH) and can be synthesized in a laboratory. Synthetic peptides are manufactured through solid-phase peptide synthesis (SPPS), which links amino acids one at a time in a defined sequence. The sequence, length, and amino acid composition determine the peptide's molecular weight, charge, solubility, and biological activity.
• Peptide bond — A covalent chemical bond formed between the carboxyl group of one amino acid and the amino group of the next amino acid, with the elimination of a water molecule. Peptide bonds link amino acids into chains (peptides and proteins). The sequence of peptide bonds determines the primary structure of the molecule. Peptide bonds are rigid and planar, which constrains the three-dimensional folding of the chain. Digestive enzymes (peptidases) can cleave peptide bonds, which is why most peptides have low oral bioavailability.
• 503A and 503B are defined in the A–D section above, where they are grouped with other regulatory terms. They are cross-referenced here because readers searching alphabetically may look for them under their numeric designations.

Q–T

Q–T: Reconstitution, RUO, Sequence, Subcutaneous

This group covers the regulatory designation that defines the research-grade peptide market, the physical process of preparing a peptide for use, the structural blueprint of a peptide, and the most common administration route in peptide research.

Key points

• Reconstitution — The process of dissolving a lyophilized peptide powder in a liquid diluent to produce a solution. Reconstitution is performed by injecting a measured volume of diluent (commonly bacteriostatic water or sterile water for injection) into the vial containing the lyophilized powder and gently swirling until the powder is fully dissolved. The choice of diluent, the volume used, and the resulting concentration are documented in research protocols. Reconstitution does not change the chemical identity or purity of the peptide — it only changes its physical state from solid to solution. Peptides that do not fully dissolve, form precipitates, or change color after reconstitution may indicate a solubility problem or degradation.
• RUO (Research Use Only) — A regulatory labeling designation indicating that a product is intended solely for research or laboratory use and is not intended for human consumption, diagnosis, or treatment. RUO products are not regulated as drugs by the FDA and are not subject to drug approval, cGMP manufacturing, or safety/efficacy requirements. The FDA has issued warning letters to peptide sellers who market products as RUO while simultaneously making therapeutic claims, providing dosing guidance, or implying human use — such marketing undermines the RUO designation and can trigger enforcement. RUO labeling does not exempt a seller from accountability for false or misleading quality statements. See the guide on How to Read a COA for context on how RUO status affects the quality documentation a supplier provides.
• Sequence — The specific linear order of amino acids in a peptide chain, read from the N-terminus (the end with a free amino group) to the C-terminus (the end with a free carboxyl group). The sequence is the primary structure of the peptide and fully determines its identity. For example, the sequence of semaglutide differs from the sequence of native GLP-1 by specific amino acid substitutions that extend its half-life. A peptide's molecular weight can be calculated from its sequence. Mass spectrometry confirms that the measured mass matches the calculated mass for the stated sequence. Two peptides with different sequences are different molecules, even if they share similar names or biological targets.
• Subcutaneous — A route of administration in which a substance is injected into the subcutaneous tissue (the layer of fat between the skin and muscle). Subcutaneous injection is the most common parenteral route for peptides in research because it is relatively simple, allows for slow absorption, and avoids the rapid degradation that occurs with oral administration. FDA-approved peptide drugs such as semaglutide (Wegovy, Ozempic) and tirzepatide (Mounjaro, Zepbound) are administered subcutaneously. Subcutaneous administration is distinct from intramuscular (into muscle) and intravenous (into a vein) routes, which have different absorption rates and pharmacokinetic profiles.

U–Z

U–Z: Cross-References and Usage Notes

This final group consolidates cross-references to terms defined elsewhere in this glossary and notes on how the vocabulary above connects to the broader peptide information site.

Key points

• Cross-reference: Bacteriostatic water — See A–D section. Used as a diluent for reconstitution of lyophilized peptides.
• Cross-reference: COA (Certificate of Analysis) — See A–D section. The primary quality document for peptide batches. For a detailed walkthrough, see the guide on How to Read a COA.
• Cross-reference: cGMP — See A–D section. Required for 503B outsourcing facilities; not required for RUO suppliers.
• Cross-reference: GIP, GLP-1, GHRH, GHRP — See E–H section. The major peptide hormone families covered on this site.
• Cross-reference: HPLC — See E–H section. The standard method for peptide purity testing.
• Cross-reference: Lyophilized — See I–L section. The physical form in which most research peptides are supplied.
• Cross-reference: Mass Spectrometry — See M–P section. The standard method for peptide identity confirmation.
• Cross-reference: 503A / 503B — See A–D section. The two compounding pharmacy categories under the FD&C Act.
• Cross-reference: RUO (Research Use Only) — See Q–T section. The regulatory designation for research-grade peptide products.
• Cross-reference: Subcutaneous — See Q–T section. The most common parenteral administration route for peptides.

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