This glossary defines the 50+ most important peptide terms a buyer encounters when researching compounds, reading Certificates of Analysis, evaluating vendor quality, or understanding protocol documentation — organized by category and written for practical buyer clarity, not academic abstraction.
Every definition answers two questions: what is this? and why does it matter when I'm buying or using peptides? Terms link to deeper Peptigrity articles where available. For compound-specific information, browse the 44 peptide guide pages. For the complete quality verification process, see the quality verification guide.
The glossary is organized into 6 categories: Peptide Basics, Quality & Testing, Handling & Administration, Compound Categories, Protocol & Cycling, and Regulatory & Market Terms.
Peptide Basics
Amino Acid
An amino acid is the molecular building block of peptides and proteins. There are 20 standard amino acids in human biology, each with a unique side chain that determines its properties. The specific amino acids in a peptide — and their order — define the peptide's identity and function. When reading a COA, the listed sequence (e.g., "Ac-SDAAVDTSSEITTKDLKEKKEVVEEAEN" for Thymosin Alpha-1) is the amino acid sequence. If mass spectrometry doesn't match the expected weight for that sequence, the product may not be what the label claims.
Bioavailability
Bioavailability is the fraction of an administered compound that reaches systemic circulation and is available for biological activity. Subcutaneous injection provides high bioavailability for most peptides. Oral bioavailability is typically very low for peptides longer than ~10 amino acids because gastrointestinal enzymes degrade them before absorption — this is why most research peptides are injectable, not oral. Semaglutide is a notable exception: its oral formulation (Rybelsus) uses an absorption enhancer to achieve sufficient bioavailability.
Dalton (Da) / Molecular Weight
A Dalton (Da) is the standard unit for measuring molecular weight of peptides and proteins. Each amino acid contributes approximately 110–130 Da to the total weight. BPC-157 (15 amino acids) has a molecular weight of ~1,419 Da. Thymosin Alpha-1 (28 amino acids) has a MW of ~3,108 Da. Molecular weight is the primary way mass spectrometry confirms peptide identity — if the observed MW doesn't match the expected value, the product may be a truncated fragment or a different compound.
Half-Life
Half-life is the time required for the concentration of a compound in the body to decrease by 50%. A peptide with a 30-minute half-life (like CJC-1295 without DAC) requires more frequent dosing than one with a 6–8 day half-life (like CJC-1295 with DAC). Half-life determines dosing frequency, injection timing, and cycling considerations.
Peptide
A peptide is a short chain of amino acids linked by peptide bonds, typically containing 2–50 amino acid residues. Peptides act as signaling molecules — they bind to specific receptors and trigger biological responses such as growth hormone release, tissue repair, immune modulation, or appetite regulation. Over 110 peptides have been approved globally as pharmaceutical drugs. Peptides are distinct from proteins (which are longer chains, typically >50 residues) and from small-molecule drugs like SARMs or steroids (which are not made of amino acids).
Peptide Bond
A peptide bond is the chemical bond that links one amino acid to the next in a peptide chain. It forms through a dehydration reaction between the carboxyl group of one amino acid and the amino group of the next. The number of peptide bonds in a chain is always one fewer than the number of amino acids — a 15-amino-acid peptide like BPC-157 has 14 peptide bonds.
Protein
A protein is a polypeptide chain typically exceeding 50 amino acids in length. The distinction between peptides and proteins is based on size — peptides are shorter chains. Insulin (51 amino acids) sits at the boundary. In the peptide market, all compounds sold as research peptides are below the protein threshold.
Receptor
A receptor is a protein on or inside a cell that binds to a specific molecule (called a ligand), triggering a biological response. Peptides work by binding to receptors — for example, Ipamorelin binds to the ghrelin receptor (GHS-R1a) to trigger growth hormone release. Understanding which receptor a peptide targets explains its mechanism and why different peptides produce different effects.
Receptor Agonist
A receptor agonist is a compound that binds to a receptor and activates it, triggering the receptor's biological response. Most therapeutic peptides are receptor agonists — they mimic the body's natural signaling molecules. For example, GLP-1 receptor agonists like semaglutide activate the GLP-1 receptor to regulate appetite and blood sugar.
Receptor Antagonist
A receptor antagonist binds to a receptor and blocks it, preventing the natural ligand from activating it. Antagonists are less common in the peptide space but exist in specific research applications.
Sequence
The amino acid sequence is the specific order of amino acids in a peptide chain — it defines the peptide's identity, structure, and biological function. Two peptides with the same amino acids in a different order are completely different compounds. When a COA lists the sequence, it should match the published sequence for the compound being sold. Sequence verification — typically through mass spectrometry confirming the expected molecular weight — is one of the most important quality checks.
Quality & Testing Terms
Batch / Lot Number
A batch or lot number is a unique identifier assigned to a specific production run of a peptide. Every COA should reference a specific batch number that matches the number on the vial. If the same COA appears across orders placed months apart, it is likely a generic template rather than a batch-specific test result. Batch-specific testing is the minimum standard for credible quality documentation. For more on documentation issues, see the COA red flags guide.
Certificate of Analysis (COA)
A COA is a document that reports the results of analytical testing performed on a specific batch of peptide. A credible COA includes: HPLC purity with chromatogram, mass spectrometry identity confirmation, batch/lot number, testing date, and the name of the testing laboratory. A COA that lacks a batch number, chromatogram, or testing method details is not a reliable quality document. Every vendor should provide a COA for every batch — if they don't, that is a red flag.
Chromatogram
A chromatogram is the graphical output of an HPLC test. It shows peaks corresponding to the compounds detected in the sample — the main peak represents the target peptide, and smaller peaks represent impurities. A purity percentage without the accompanying chromatogram cannot be independently verified. When reviewing a COA, look for a chromatogram showing a single dominant peak with minimal secondary peaks.
Endotoxin Testing / LAL Test
Endotoxins are lipopolysaccharides shed from gram-negative bacterial cell walls. The LAL (Limulus Amebocyte Lysate) test measures endotoxin levels in a sample, typically reported in EU/mg (Endotoxin Units per milligram). Endotoxin contamination in injectable peptides can cause fever, inflammation, and systemic immune reactions. This test is particularly important for immune-modulating peptides like Thymosin Alpha-1, where endotoxin contamination could trigger inflammatory responses independent of the peptide's mechanism.
GMP (Good Manufacturing Practice)
GMP refers to a set of manufacturing quality standards enforced by regulatory agencies (FDA in the US, EMA in Europe). GMP-compliant facilities follow standardized procedures for production, testing, and documentation. Pharmaceutical peptide drugs (semaglutide, tirzepatide) are manufactured under GMP. Research-grade peptides may or may not be produced in GMP-compliant facilities — this distinction affects quality consistency and cost.
HPLC (High-Performance Liquid Chromatography)
HPLC is the gold-standard analytical method for measuring peptide purity. It separates the target peptide from impurities in a sample and reports purity as a percentage of the main peak area. Research-grade peptides should achieve ≥98% purity by HPLC. HPLC tells you how clean the sample is, but not whether it is the correct compound — for identity confirmation, mass spectrometry is required. See the HPLC testing guide for the full methodology.
Mass Spectrometry (MS)
Mass spectrometry measures the molecular weight of a compound and confirms its identity. ESI-MS (Electrospray Ionization) and MALDI-TOF are the two most common methods for peptides. A sample can be 99% pure by HPLC but still be the wrong peptide — mass spectrometry is the check that catches wrong-compound substitution. The observed molecular weight must match the theoretical weight of the target peptide's amino acid sequence. See the mass spectrometry guide for details.
Purity (%)
Purity is the percentage of a sample that consists of the target peptide, as measured by HPLC. ≥98% is the standard for research-grade peptides. The remaining percentage (≤2% for a 98% pure sample) consists of impurities — truncation products, deletion sequences, residual salts, or degradation products from synthesis. Higher purity means a cleaner sample, but purity alone does not confirm identity. See the purity standards reference for context on what different levels mean.
Third-Party Testing
Third-party testing means analytical testing performed by a laboratory independent of the vendor selling the product. In-house vendor testing has an inherent conflict of interest — independent verification from an accredited lab provides unbiased quality data. Peptigrity maintains 378 independent lab tests across dozens of vendors and compounds, and shop reviews include trust scores based in part on testing verification.
USP (United States Pharmacopeia)
USP is an independent standards-setting organization that establishes quality, purity, identity, and strength standards for medications, including compounded peptides. USP-compliant compounding pharmacies follow these standards when preparing peptide formulations. USP compliance is a marker of pharmaceutical-grade quality — research-grade peptides sold under RUO labels may not adhere to USP standards.
Handling & Administration Terms
Bacteriostatic Water (BAC Water)
Bacteriostatic water is sterile water containing 0.9% benzyl alcohol as a preservative. The benzyl alcohol inhibits microbial growth, allowing reconstituted peptide solutions to be used over multiple days or weeks (typically up to 28–30 days refrigerated). BAC water is the standard diluent for reconstituting lyophilized peptides intended for multi-dose use.
Insulin Syringe / Units
Insulin syringes are designed for precise, small-volume injections and are the standard syringe type used for subcutaneous peptide administration. They are calibrated in "units" — 100 units = 1 mL on a standard U-100 syringe. When calculating peptide doses, convert the desired dose (in mcg or mg) to the corresponding volume (in mL) based on the reconstitution concentration, then draw that volume in units. For the full math, see the dose calculation guide.
Intramuscular Injection (IM)
Intramuscular injection delivers the peptide directly into muscle tissue, typically the deltoid or gluteus. IM injection provides faster absorption than subcutaneous but is less common for peptide protocols. Most research peptides are administered subcutaneously unless specifically indicated otherwise.
Intranasal Administration
Intranasal administration delivers peptides through the nasal mucosa, bypassing the digestive system. Used primarily for nootropic peptides like Semax and Selank, which target the central nervous system. Bioavailability varies by peptide and formulation. Not all peptides are suitable for intranasal delivery — larger peptides (>30 amino acids) generally have limited nasal absorption.
Lyophilized / Freeze-Dried
Lyophilized means freeze-dried — water has been removed from the peptide solution under vacuum to create a stable, dry powder. This is the standard form for research peptides because it extends shelf life from weeks (in solution) to months or years (as powder), preserves peptide structure, and enables quality testing on the dry material. Lyophilized peptides require reconstitution before use. See the storage guide for optimal handling.
mcg / mg / mL / IU
mcg (microgram) = one millionth of a gram. mg (milligram) = one thousandth of a gram. 1 mg = 1,000 mcg. Most peptide doses are measured in mcg (e.g., 250 mcg of Ipamorelin) or mg (e.g., 5 mg vial of BPC-157). mL (milliliter) measures liquid volume — used when calculating how much reconstituted solution to draw. IU (International Unit) is a standardized biological activity measurement used for some compounds (notably HGH). These are not interchangeable units — always verify the unit when reading a protocol.
Reconstitution
Reconstitution is the process of dissolving a lyophilized (freeze-dried) peptide powder in bacteriostatic water or sterile water to create an injectable solution. Technique matters: direct the water against the vial wall (not onto the powder), gently swirl until dissolved (never shake), and store refrigerated. The resulting concentration depends on how much water you add — more water = lower concentration per unit volume. See the reconstitution guide for step-by-step instructions.
Sterile Water
Sterile water is purified water with no preservatives. Unlike bacteriostatic water, it does not contain benzyl alcohol, so reconstituted solutions are not protected from microbial growth. Use sterile water only when the entire vial will be used immediately or when benzyl alcohol may interfere with specific research applications. For multi-dose use, bacteriostatic water is preferred.
Subcutaneous Injection (SubQ)
Subcutaneous injection delivers the peptide into the fat layer just beneath the skin — typically the abdominal area, thigh, or upper arm. SubQ is the most common administration route for research peptides because it provides consistent absorption, is technically simple, and causes minimal discomfort with fine-gauge insulin needles. The vast majority of clinical peptide trials use subcutaneous administration.
Vial
A vial is the small glass or plastic container holding lyophilized peptide powder. Standard research peptide vials contain 2–10 mg of peptide. The vial is sealed with a rubber stopper and aluminum crimp cap to maintain sterility. After reconstitution, peptides remain in the same vial for multi-dose withdrawal using an insulin syringe.
Compound & Category Terms
BPC (Body Protection Compound)
BPC is a family of peptides derived from human gastric juice protein. BPC-157, the most studied member, is a 15-amino-acid peptide researched for tissue repair, gut healing, and anti-inflammatory effects. The "Body Protection" name reflects the compound's origin in a protective gastric secretion.
DAC (Drug Affinity Complex)
DAC is a chemical modification added to certain peptides — most notably CJC-1295 — that binds to serum albumin in the bloodstream, significantly extending the peptide's half-life. CJC-1295 without DAC has a half-life of ~30 minutes; CJC-1295 with DAC has a half-life of 6–8 days. This modification changes dosing frequency and protocol design.
GH Secretagogue
A growth hormone secretagogue is any compound that stimulates the pituitary gland to release growth hormone. Peptide GH secretagogues include CJC-1295, Ipamorelin, GHRP-2, GHRP-6, Hexarelin, and Sermorelin. They work through the body's natural signaling — they do not introduce exogenous growth hormone. For the full category, see muscle growth peptides.
GHRH (Growth Hormone Releasing Hormone)
GHRH is a natural hormone produced in the hypothalamus that stimulates the pituitary to release growth hormone. GHRH analogs — synthetic peptides that mimic GHRH — include CJC-1295 and Sermorelin. They bind to the GHRH receptor on pituitary somatotroph cells.
GHRP (Growth Hormone Releasing Peptide)
GHRPs are synthetic peptides that stimulate growth hormone release by binding to the ghrelin receptor (GHS-R1a) — a different receptor than GHRH analogs target. Examples include Ipamorelin, GHRP-2, GHRP-6, and Hexarelin. GHRH analogs and GHRPs are often combined (e.g., CJC-1295 + Ipamorelin) because they converge on the same pituitary cells from two independent receptor systems, producing synergistic GH release.
GLP-1 Receptor Agonist
GLP-1 (Glucagon-Like Peptide-1) receptor agonists are peptide drugs that activate the GLP-1 receptor to regulate appetite, blood sugar, and metabolism. FDA-approved examples include semaglutide (Ozempic/Wegovy) and tirzepatide (Mounjaro/Zepbound). These are prescription pharmaceutical drugs with extensive Phase III clinical trial data — a different regulatory and evidence tier than most research peptides. For the full category, see metabolic peptides.
Immunomodulator
An immunomodulator is a compound that regulates — not simply "boosts" — the immune system. Immunomodulating peptides such as Thymosin Alpha-1 balance the immune response by enhancing T-cell maturation, dendritic cell activation, and cytokine regulation. The distinction from "immune booster" matters: modulation means the response is tuned up or down as needed, not uniformly amplified. For the full category, see immune peptides.
Melanocortin Peptide
Melanocortin peptides act on melanocortin receptors (MC1R through MC5R) to regulate pigmentation, sexual function, appetite, and inflammation. Examples include Melanotan II (pigmentation, MC1R/MC4R) and PT-141/Bremelanotide (sexual function, MC4R). PT-141 is the only melanocortin peptide with FDA approval (Vyleesi, for hypoactive sexual desire disorder).
Nootropic Peptide
Nootropic peptides are compounds that modulate cognitive function — focus, memory, verbal fluency, and neuroprotection. Examples include Semax (BDNF upregulation, monoamine modulation) and Selank (GABAergic anxiolytic). These are typically administered intranasally. For the full category, see cognitive peptides.
Thymosin
The thymosin family is a group of peptides originally isolated from the thymus gland. The two most commonly discussed members are Thymosin Alpha-1 (28 amino acids — immune modulation) and Thymosin Beta-4 (43 amino acids — tissue repair; the synthetic fragment TB-500 is derived from it). Despite sharing a family name, they have completely different mechanisms and applications.
Protocol & Cycling Terms
Cycling (On/Off Protocol)
Cycling is the practice of using peptides for a set period (the "on-cycle") followed by a rest period (the "off-cycle") to maintain receptor sensitivity. GH secretagogues typically follow 8–12 weeks on, 4–6 weeks off. Not all peptides require cycling — healing peptides like BPC-157 are used until the therapeutic goal is achieved, then discontinued. GLP-1 drugs are prescribed for continuous use. For the complete breakdown, see the peptide cycling guide.
Desensitization / Receptor Downregulation
Desensitization occurs when a receptor loses sensitivity due to continuous stimulation — the same dose produces a weaker effect over time. The body reduces the number or sensitivity of receptors as an adaptive response. This is the primary reason GH secretagogues require cycling. The clearest signal: needing to increase the dose to maintain the same effect.
Loading Phase
A loading phase is an initial period of higher-frequency or higher-dose administration intended to rapidly achieve effective tissue or blood levels. TB-500 protocols commonly use a loading phase (2–5 mg twice weekly for 4–6 weeks) followed by lower maintenance dosing. Not all peptides use loading phases.
Maintenance Phase
A maintenance phase follows a loading phase and uses a reduced dose or frequency to sustain the effects achieved during loading. The transition from loading to maintenance is typically based on symptom resolution or goal achievement.
PCT (Post-Cycle Therapy)
PCT is a protocol used to restart natural testosterone production after it has been suppressed by anabolic steroids or SARMs. Peptides do not require PCT — they do not suppress testosterone, LH, or FSH production. GH secretagogues operate on the growth hormone axis, not the hypothalamic-pituitary-testicular axis (HPTA). Healing peptides, nootropics, and GLP-1 drugs don't interact with the androgen system at all. For the full explanation, see Peptides vs SARMs vs Steroids.
Stacking
Stacking is the practice of combining multiple peptides in a single protocol to achieve complementary or synergistic effects. Common stacks include CJC-1295 + Ipamorelin (GH secretagogue synergy) and BPC-157 + TB-500 (complementary healing mechanisms). Peptides from different receptor classes can often be stacked without conflict. For combination protocols, see the peptide stacking guide.
Titration
Titration is the practice of gradually increasing a peptide dose over time rather than starting at the full target dose. This allows the body to adjust to the compound's effects and helps identify the minimum effective dose. Common in GH secretagogue protocols (start at 100 mcg, increase to 200–300 mcg over 2–4 weeks) and GLP-1 drug protocols (semaglutide starts at 0.25 mg, titrating to 2.4 mg).
Regulatory & Market Terms
503A Pharmacy / 503B Outsourcing Facility
503A pharmacies are traditional compounding pharmacies that prepare patient-specific medications under individual prescriptions. 503B outsourcing facilities can produce larger batches without patient-specific prescriptions but must register with the FDA and follow current good manufacturing practices. Both can compound peptides — the distinction affects scale, oversight, and pricing. For more, see compounding vs research peptide.
Category 1 / Category 2 (FDA Classification)
These FDA classifications determine whether a compound can be used by compounding pharmacies. Category 1 compounds are eligible for compounding under physician prescription. Category 2 compounds are restricted — compounding pharmacies cannot use them. In 2023, several peptides (BPC-157, CJC-1295, Ipamorelin, Thymosin Alpha-1, and others) were placed on the Category 2 list. As of 2026, approximately 14 of 19 Category 2 peptides are expected to be reclassified to Category 1. For the complete timeline, see the FDA regulatory timeline.
Compounding Pharmacy
A compounding pharmacy prepares customized medications — including peptides — based on physician prescriptions. Compounded peptides are produced under USP standards and are a legal route for peptide access when a physician determines medical necessity. They are typically more expensive than research-grade peptides but offer pharmaceutical-grade quality assurance.
INN (International Nonproprietary Name)
An INN is the official, non-proprietary name assigned to a pharmaceutical substance by the World Health Organization. For example, "thymalfasin" is the INN for synthetic Thymosin Alpha-1, and "semaglutide" is the INN for the GLP-1 agonist marketed as Ozempic and Wegovy. The INN is the compound's universal scientific name — brand names vary by region and manufacturer.
Investigational New Drug (IND)
An IND is an FDA designation that allows a compound to be studied in human clinical trials. Many research peptides have not filed IND applications. Having IND status means the compound is actively under clinical investigation but is NOT yet approved for commercial use.
Off-Label Use
Off-label use is the use of an FDA-approved drug for an indication, dose, or population not specified in its approved labeling. For example, tesamorelin is FDA-approved for HIV lipodystrophy, but some physicians prescribe it off-label for body composition goals. Off-label prescribing is legal when a physician determines it is medically appropriate.
RUO (Research Use Only)
RUO is a regulatory designation meaning the product is sold for laboratory research purposes only and is not approved, intended, or marketed for human therapeutic use. This is the standard label for research-grade peptides that have not completed the FDA approval process. Vendors selling RUO products cannot make therapeutic claims. For the full legal status by country, see the regulatory guide.
WADA Prohibited List
The WADA (World Anti-Doping Agency) Prohibited List specifies substances and methods banned in competitive sport. Most peptides discussed on Peptigrity — including GH secretagogues, BPC-157, TB-500, GLP-1 agonists, and Thymosin Alpha-1 — are prohibited both in-competition and out-of-competition. Athletes subject to anti-doping testing should assume all research peptides are banned.
Frequently Asked Questions
What's the difference between HPLC and mass spectrometry?
HPLC measures purity — what percentage of the sample is a single compound vs. impurities. Mass spectrometry measures identity — what the compound IS by confirming its molecular weight matches the expected peptide. A sample can be 99% pure by HPLC and still be the wrong peptide. You need both tests: HPLC confirms the sample is clean, mass spectrometry confirms it is the correct compound. When reviewing a COA, look for both results — purity percentage with chromatogram AND a mass spectrometry result matching the expected molecular weight.
Why do peptides come as powder instead of liquid?
Lyophilization (freeze-drying) removes water to create a stable powder that can be stored for months to years without significant degradation. Peptides in liquid solution begin degrading within days to weeks, even under refrigeration, through hydrolysis and oxidation. The powder form preserves the peptide's chemical structure until you reconstitute it immediately before use. This is also why lyophilized powder is the most verifiable form — COA testing is performed on the dry material.
What does ≥98% purity actually mean?
It means that at least 98% of the sample's mass (as measured by HPLC) is the target peptide. The remaining ≤2% consists of impurities — truncation products (peptides missing amino acids from the synthesis), deletion sequences, residual salts, or degradation products. For research-grade peptides, ≥98% is the standard benchmark. Pharmaceutical-grade peptides may exceed 99%. Purity below 95% suggests significant synthesis impurities and is generally considered below acceptable research-grade quality.
Do peptides require PCT (post-cycle therapy)?
No. Unlike anabolic steroids and SARMs, peptides do not suppress endogenous testosterone production. GH secretagogues operate on the growth hormone axis (GHRH receptor, ghrelin receptor), not the hypothalamic-pituitary-testicular axis (HPTA) that governs testosterone. Healing peptides, nootropic peptides, and GLP-1 drugs do not interact with the androgen system at all. No post-cycle therapy is needed after any peptide protocol. For the full comparison, see Peptides vs SARMs vs Steroids.
What does RUO mean on a peptide label?
RUO stands for "Research Use Only" — a regulatory designation meaning the product is sold for laboratory research purposes and is not approved, intended, or marketed for human therapeutic use. This is the standard label for research-grade peptides that have not completed the FDA approval process for any clinical indication. Vendors selling RUO products cannot legally make medical or therapeutic claims about their products.
This article is for educational and informational purposes only and does not constitute medical advice. Peptides discussed may be investigational compounds not approved by the FDA for human use. Always consult a qualified healthcare provider before using any peptide or research compound. Peptigrity is an independent review platform and does not sell, endorse, or recommend specific products or vendors.
