Glossary

Peptide terms, defined

Plain-language definitions for the dosing, regulatory, pharmacology, and sourcing vocabulary used throughout the peptide library, protocols, and calculator.

503ARegulatory Status: The FDA section governing traditional compounding pharmacies, which prepare patient-specific prescriptions. Compounds on the 503A bulks list can legally be compounded this way.
503BRegulatory Status: The FDA section governing outsourcing facilities, which compound in larger batches under stricter manufacturing standards than 503A, often without a patient-specific prescription in hand first.
AgonistPharmacology: A compound that binds a receptor and activates it, triggering the same response the body's own signaling molecule would. GLP-1 agonists activate the GLP-1 receptor, for example.
Amino AcidPharmacology: The building block peptides are made of. Short chains (2–50 amino acids) are peptides; longer chains are proteins.
AntagonistPharmacology: A compound that binds a receptor and blocks it, without activating it, preventing the body's own signaling molecule from having its usual effect.
Bacteriostatic WaterDosing & Administration: Sterile water with a small amount of benzyl alcohol added, which inhibits bacterial growth. Used to reconstitute lyophilized peptides so they can be drawn into a syringe.
BioavailabilityPharmacology: The fraction of a dose that reaches systemic circulation intact. Injectable routes (SubQ, IM) generally have higher bioavailability than oral for peptides, since peptides are broken down by digestion.
COA (Certificate of Analysis)Quality & Sourcing: A lab report verifying a compound's identity, purity, and contaminant levels (heavy metals, endotoxin, microbial) for a specific batch. Independent third-party testing carries more weight than a supplier's own in-house COA.
CompoundableRegulatory Status: A compound listed on the FDA's 503A or 503B bulks list, meaning a licensed compounding pharmacy can legally prepare it for a specific patient under a valid prescription.
Cycle LengthDosing & Administration: The duration a protocol runs before a planned break. Some peptides are dosed continuously; others are cycled to preserve receptor sensitivity or limit exposure.
EndotoxinQuality & Sourcing: A bacterial cell-wall byproduct that can trigger fever, inflammation, or a severe immune reaction if injected. Endotoxin testing (measured in EU/mL) is one of the panels a thorough COA covers.
ExcipientQuality & Sourcing: Any inactive ingredient in a vial besides the peptide itself, such as a bulking agent or stabilizer used during lyophilization.
FDA-ApprovedRegulatory Status: The compound has cleared FDA review for at least one specific indication and is available by prescription in its approved form. Off-label or research use of an FDA-approved compound still falls outside that approval.
GLP-1Pharmacology: Glucagon-like peptide-1, a gut hormone that stimulates insulin release, slows gastric emptying, and reduces appetite. GLP-1 agonists (semaglutide, tirzepatide) mimic this hormone.
Gray AreaRegulatory Status: Not on the FDA's approved or compoundable lists, but not explicitly scheduled or banned either. Legal status is unsettled or jurisdiction-dependent; treat with more caution than a compoundable listing.
Half-LifePharmacology: The time it takes for half of an administered dose to clear the body. Short half-lives need more frequent dosing to maintain stable levels; long half-lives allow less frequent injections.
IM (Intramuscular)Dosing & Administration: Injection into muscle tissue, deeper than subcutaneous. Generally faster absorption than SubQ, used when a protocol calls for it.
IntranasalDosing & Administration: Administered as a nasal spray. Bypasses digestion and, for some compounds, offers more direct access to the central nervous system than injection.
IV (Intravenous)Dosing & Administration: Injection directly into a vein. Fastest onset of any route, but requires sterile technique and is normally a clinical-setting procedure, not self-administered.
Loading DoseDosing & Administration: A higher initial dose (or shorter interval) used to reach steady-state levels faster, before dropping to a lower maintenance dose.
LyophilizedDosing & Administration: Freeze-dried into a stable powder for storage and shipping. Lyophilized peptides must be reconstituted with bacteriostatic or sterile water before they can be dosed.
Maintenance DoseDosing & Administration: The steady, ongoing dose used after any loading phase, sized to sustain effect rather than to rapidly build up levels.
Mechanism of ActionPharmacology: The specific biological pathway a compound acts through, e.g. receptor binding, enzyme inhibition, or gene expression change. Distinct from its observed effects, which can have more than one plausible mechanism behind them.
PeptidePharmacology: A short chain of amino acids (roughly 2–50), linked by peptide bonds. Peptides sit between individual amino acids and full proteins in size, and often act as signaling molecules in the body.
PurityQuality & Sourcing: The percentage of a vial's mass that is the intended peptide, versus synthesis byproducts or degradation products, as measured by HPLC on a COA. Below roughly 98% purity is generally considered substandard for research use.
ReceptorPharmacology: A protein, usually on a cell's surface, that a compound binds to in order to trigger (agonist) or block (antagonist) a biological response.
ReconstitutionDosing & Administration: Adding bacteriostatic or sterile water to a lyophilized peptide vial to dissolve it into an injectable solution. The ratio of water to peptide sets the resulting concentration.
RestrictedRegulatory Status: On the FDA's do-not-compound list or otherwise explicitly prohibited from compounding pharmacy preparation. Restricted compounds are not legally compoundable regardless of prescription.
RUO (Research Use Only)Regulatory Status: Labeled for laboratory research use, not for human consumption. RUO compounds have not gone through FDA review for human safety or efficacy at any dose.
StackDosing & Administration: Two or more peptides run concurrently as a combined protocol. Synergy between stacked peptides can be additive, neutral, or in some pairings, contraindicated.
SubQ (Subcutaneous)Dosing & Administration: Injection into the fatty tissue just under the skin. The most common self-administered route for research peptides, using a short, fine-gauge needle.
TitrationDosing & Administration: Gradually increasing a dose over time, typically to assess tolerance or reach an effective dose while minimizing side effects.
TopicalDosing & Administration: Applied directly to the skin. Used for peptides intended for local, surface-level effects rather than systemic circulation.
U-100 SyringeDosing & Administration: An insulin syringe calibrated so its full barrel holds 100 units, equal to 1.0 mL. Also available in 30-unit (0.3 mL) and 50-unit (0.5 mL) sizes for smaller, more precise draws.
VialQuality & Sourcing: The sealed container a peptide ships in, typically holding a specific milligram amount of lyophilized powder. Vial size is one input into the reconstitution math on the dosage calculator.