Peptide Glossary

The terms you'll run into on peptide pages, vendor listings, and research papers, in plain language.

Last updated September 23, 2026 · 55 terms

A

Acetylationalso: N-acetyl: Adding an acetyl group, usually to the N-terminus (start) of a peptide. It can slow breakdown by enzymes, and it can also change how the peptide binds its targets, so an acetylated version is a chemically distinct compound rather than the same peptide with a label.See:N-Acetyl Semax
Agonist: A compound that binds a receptor and activates it, producing a response similar to the body's own signal. A partial agonist activates the receptor less than fully.See:Semaglutide
Amidationalso: Amidate: Replacing the carboxyl group at a peptide's C-terminus (end) with an amide. Many natural peptide hormones are amidated, and the change often improves stability. "Amidate" in a product name refers to this modification.
Amino acid: The building block of peptides and proteins. Twenty standard amino acids are encoded by human genes, and the order they are linked in (the sequence) determines what a peptide does.
Amylin: A hormone released by the pancreas alongside insulin. It slows stomach emptying and signals fullness to the brain. Long-acting amylin analogs are studied for weight loss, alone or paired with GLP-1 drugs.See:Cagrilintide
Antagonist: A compound that binds a receptor without activating it, blocking the natural signal. The opposite of an agonist.
Aromatase inhibitor (AI): A drug that blocks aromatase, the enzyme that converts testosterone to estradiol. Used in breast cancer treatment and discussed in TRT contexts for managing estrogen.See:Anastrozole

B

Bacteriostatic wateralso: BAC water: Sterile water containing 0.9% benzyl alcohol, a preservative that inhibits bacterial growth. That preservative is what allows a vial to be drawn from more than once, unlike plain sterile water, which is intended for single use.See:Reconstitution calculator
Bioavailability: The fraction of a dose that reaches the bloodstream in active form. Most peptides have very low oral bioavailability because digestive enzymes break them down, which is why many are injected.
Bioregulatoralso: Khavinson peptide: A term from Russian research for very short peptides (often 2 to 4 amino acids) proposed to act on specific tissues. Most of the published work comes from a single research group, so independent replication is limited.See:Epitalon Pinealon
Blend: A vendor product containing two or more peptides in one vial. Compare with a stack, where compounds are used together but kept in separate vials. A blend fixes the ratio between its components.See:Stacks

C

Certificate of analysis (COA): A document reporting test results for one production lot, typically identity and purity. It is only meaningful if it comes from an independent lab and its lot number matches the vial in hand.See:COA red flags
Clinical trial phasesalso: Phase 1, Phase 2, Phase 3: Phase 1 tests safety and dosing in a small group; Phase 2 looks for efficacy and side effects in a larger group; Phase 3 compares against standard treatment or placebo in hundreds to thousands of people and usually underpins regulatory approval.
Compounding pharmacyalso: 503A, 503B: A pharmacy that prepares medications itself rather than dispensing a manufacturer's product. In the US, 503A pharmacies compound for individual prescriptions under state oversight; 503B outsourcing facilities register with the FDA, may produce batches without patient-specific prescriptions, and must follow current good manufacturing practice.
Contraindication: A condition or circumstance in which a substance should not be used because the risk outweighs any benefit, such as pregnancy or a history of a specific cancer.
Cyclealso: Washout: A planned period of use followed by a break (the washout). Cycling is common in community protocols, often to limit desensitization, though for most research peptides the optimal on/off pattern has not been studied.

D

DAC (Drug Affinity Complex): A chemical linker that lets a peptide bind to albumin in the blood, greatly extending its half-life. CJC-1295 with DAC lasts for days, while the version without DAC (also called Mod GRF 1-29) is cleared within about half an hour.See:CJC-1295
Desensitizationalso: Tachyphylaxis: A weakening response to a compound with repeated use, often because receptors are reduced or become less responsive. It is one reason dosing breaks are discussed for some peptides.

E

Endotoxin: A toxic component of certain bacteria's outer membrane that can contaminate products made without sterile controls. Even small amounts injected can cause fever and inflammation; pharmaceutical products are tested for it (the LAL test).See:COA red flags
Ester: A fatty-acid chain attached to a hormone such as testosterone to slow its release from an injection site. Cypionate and enanthate are esters; a longer ester generally means a slower, longer-lasting release.See:Testosterone Cypionate Testosterone Enanthate

G

GHRH analog: A synthetic version of growth hormone-releasing hormone, the brain's signal telling the pituitary to release growth hormone. It preserves the body's normal pulsed release pattern.See:Sermorelin Tesamorelin CJC-1295
GHRP (ghrelin mimetic): Growth hormone-releasing peptides activate the ghrelin receptor, triggering growth hormone release. Some, like GHRP-6, also strongly increase hunger; ipamorelin is described as more selective.See:GHRP-6 Ipamorelin
GIP: Glucose-dependent insulinotropic polypeptide, an incretin hormone released from the gut after eating. Tirzepatide activates both the GIP and GLP-1 receptors.See:Tirzepatide
GLP-1: Glucagon-like peptide-1, a gut hormone that increases insulin release when blood sugar is high, slows stomach emptying, and reduces appetite. GLP-1 receptor agonists are approved drugs for type 2 diabetes and obesity.See:Semaglutide Tirzepatide
Growth hormone secretagogue: Any compound that prompts the pituitary gland to release growth hormone. The two main classes are GHRH analogs and ghrelin mimetics (GHRPs), which act on different receptors and are often combined.See:Ipamorelin CJC-1295

H

Half-life: The time it takes for the amount of a compound in the blood to fall by half. It largely sets how often a compound must be dosed; after about five half-lives, most of a dose has been cleared.
HPLC: High-performance liquid chromatography, the standard test for peptide purity. It separates a sample into its components and reports the main peak as a percentage. It shows how pure a sample is, not what it is; identity is confirmed by mass spectrometry.See:COA red flags

I

IGF-1: Insulin-like growth factor 1, a hormone made mainly by the liver in response to growth hormone. It carries out many of growth hormone's effects and is the usual blood marker used to gauge growth hormone activity.See:IGF-1 LR3
In vitro / in vivo: In vitro means in cells or tissue outside a living organism (a dish or test tube); in vivo means in a living animal or person. Many peptide claims rest on in vitro or animal data that has not been confirmed in humans.
Incretin: A gut hormone released after eating that boosts insulin secretion. GLP-1 and GIP are the two main incretins and the targets of modern weight-loss drugs.See:Semaglutide Tirzepatide
Insulin syringe (U-100): The small syringe commonly used for subcutaneous peptide injections. U-100 means 100 units per mL, so one unit is 0.01 mL of liquid. The unit is a volume, not a dose: how much peptide a unit holds depends on how the vial was reconstituted.See:Reconstitution calculator
International unit (IU): A unit based on biological activity rather than mass, defined separately for each substance. IU cannot be converted to mg without that substance's own factor; for example, the standard for somatropin (growth hormone) is about 3 IU per mg.See:HCG (Human Chorionic Gonadotropin)
Intramuscular (IM): Injected into a muscle. Typically used for oil-based preparations like testosterone esters; absorption is generally faster than from fat but depends on the preparation.
Intranasal: Given as a nasal spray and absorbed through the nasal lining. Some neuropeptides, such as Semax and Selank, are studied this way because it avoids digestion and may reach the brain more directly.See:N-Acetyl Semax

L

Lyophilizedalso: Freeze-dried: Freeze-dried into a powder, the form most research peptides ship in. The powder is more stable for storage and shipping than a solution, and must be reconstituted with liquid before use.See:Reconstitution calculator

M

Mass spectrometry (MS): A test that measures a molecule's mass, used to confirm a peptide is actually the compound on the label. Paired with HPLC purity, it is what a credible COA should include.See:COA red flags
mcg vs mg: 1 milligram (mg) = 1,000 micrograms (mcg). Vials are usually labeled in mg while doses are often given in mcg, and confusing the two is a thousand-fold error.See:Reconstitution calculator
Melanocortin receptors: A family of receptors involved in skin pigmentation, appetite, and sexual function. Melanotan II and PT-141 act on these receptors, which is why tanning, nausea, and libido effects overlap between them.See:Melanotan II PT-141
Meta-analysis: A study that statistically pools the results of several earlier studies to get a more precise overall estimate. Its quality depends on the studies it includes.
Mitochondrial-derived peptide: A peptide encoded in mitochondrial DNA rather than the cell nucleus. MOTS-c and humanin are the best-known examples; both are studied for metabolic and aging-related effects, mostly in animals.See:MOTS-c Humanin

O

Off-label use: Prescribing an approved drug for a condition, dose, or population other than those it was approved for. It is legal for physicians in the US, but it is not the same as a research chemical being approved.

P

Peptide: A short chain of amino acids joined by peptide bonds. The cut-off is not strict, but chains of up to roughly 50 amino acids are usually called peptides and longer ones proteins.
Pharmacodynamics: What a compound does to the body: the receptors it acts on and the effects that follow, including how the effect changes with dose.
Pharmacokinetics: What the body does to a compound: how it is absorbed, distributed, broken down, and eliminated over time. Half-life and bioavailability are pharmacokinetic measures.
Post-cycle therapy (PCT): Protocols intended to restart the body's own testosterone production after it has been suppressed by external hormones. They commonly involve SERMs and sometimes HCG.See:Clomiphene Enclomiphene HCG (Human Chorionic Gonadotropin)
Preclinical: Research done in cells or animals before any human testing. Promising preclinical results often fail to carry over to people, so a preclinical label means effects in humans are unknown.
PubMed / PMID: PubMed is the US National Library of Medicine's free index of biomedical literature. Every indexed paper has a PMID (PubMed ID); each study cited on this site links to its PubMed record so it can be checked.

R

Randomized controlled trial (RCT): A study in which participants are randomly assigned to the treatment or a control (often placebo). Randomization is what lets an RCT show cause and effect rather than just association.
Reconstitution: Adding liquid, usually bacteriostatic water, to a lyophilized peptide to dissolve it. The amount of liquid added sets the concentration, and therefore how many syringe units a given dose takes.See:Reconstitution calculator
Research chemicalalso: Research use only, RUO: A compound sold for laboratory research and not approved for human use. It is not made under pharmaceutical manufacturing standards, so identity, purity, and sterility are not guaranteed.See:COA red flags

S

SERM: Selective estrogen receptor modulator: a drug that blocks estrogen in some tissues and mimics it in others. Clomiphene and enclomiphene are used to raise the body's own testosterone production; tamoxifen is used in breast cancer.See:Clomiphene Enclomiphene Tamoxifen
Stack: Two or more compounds used together, usually for complementary mechanisms. Stacks are rarely tested as combinations, so evidence for each component does not automatically transfer to the stack.See:Stacks
Subcutaneous (SubQ): Injected into the fat layer just under the skin, typically on the abdomen or thigh, with a short insulin needle. The most common route for injectable peptides.See:Reconstitution calculator

T

Titration: Starting at a low dose and increasing gradually. GLP-1 drugs are titrated over weeks because stepping up slowly reduces nausea and other GI side effects.See:Semaglutide Tirzepatide
Triple agonist: A single molecule that activates three receptors, in the case of GLP-3 R the GLP-1, GIP, and glucagon receptors. The glucagon component is thought to add energy expenditure on top of the appetite effects.See:GLP-3 R

For educational and research purposes only; not medical advice.