Home / Learn / Glossary
Reference

Peptide Glossary

Plain-English definitions of the 66 terms you’re most likely to meet reading about peptides — from the science to the syringe to the lab report.

The basics

↑ Back to top
Amino acid
The organic building block that chains together to form peptides and proteins. The human body uses 20 standard amino acids, sorted into three groups: essential (must come from food — the body can't make them), nonessential (the body makes on its own), and conditional (normally nonessential but needed in larger amounts during illness or stress). They also build tissue, hormones, neurotransmitters, and immune antibodies.
Analog
A deliberately modified copy of a natural peptide — typically with one or two amino acids swapped, added, or removed — usually so it lasts longer in the body or binds more strongly than the original. Most peptides you'll read about are analogs of something the body already makes.
Enzyme
A protein whose job is to speed up one specific chemical reaction. The enzymes that matter most for peptides are the chopping enzymes (peptidases) in blood and gut that break peptide chains apart — the reason many peptides don't survive being swallowed.
Fragment
A shorter piece snipped out of a longer peptide or protein's sequence. Researchers study fragments when one region of a molecule seems to carry most of its activity — the fragment alone is cheaper to make and easier to test.
Hormone
A chemical messenger released into the bloodstream to tell other organs what to do. Insulin is a hormone; so are growth hormone and GLP-1.
Peptide
A short chain of amino acids — typically 2 to around 50. Beyond that length the chains fold into what we call proteins, so peptides are the smaller cousins. A peptide's identity and effects come from which amino acids it contains and in what order.
Peptide bond
The chemical link that joins two amino acids into a chain. It forms when the carboxyl group of one amino acid reacts with the amino group of the next, releasing a molecule of water. Repeated peptide bonds are the backbone of every peptide and protein.
Protein
A long chain of amino acids — beyond roughly 50 — that folds into a specific three-dimensional shape to do its job. Peptides and proteins are the same chemistry at different lengths; the size difference changes how the body handles them.
Sequence
The exact order of amino acids in a peptide — its molecular identity. Change the order and you change the molecule: BPC-157 and TB-500 are different peptides precisely because their sequences differ, which is why they're studied for different things.

How peptides work in the body

↑ Back to top
Agonist
A compound that binds a receptor and switches it on, mimicking the body's own signal. Think of it as turning the key in a lock: a growth-hormone secretagogue is an agonist at the receptor that releases growth hormone.
Antagonist
The opposite of an agonist: it occupies the receptor without activating it, blocking the natural signal from getting through.
Binding affinity
How tightly a compound attaches to its receptor — the strength of the fit. Higher affinity generally means less compound is needed for the same effect.
Bioavailability
The fraction of a dose that actually reaches the bloodstream — or its target — still active. Injected peptides are typically high bioavailability; swallowed peptides usually aren't, because digestion breaks them down first.
Endogenous
Made inside your own body. Endogenous growth hormone is what your pituitary produces; many peptides copy, mimic, or amplify an endogenous signal.
Exogenous
Coming from outside the body — injected, inhaled, or otherwise supplied rather than made by you. The complement of endogenous.
Half-life
The time it takes for half of a dose to be cleared from the body. A peptide with a 2-hour half-life needs far more frequent dosing than one engineered to last a week — one of the biggest reasons analogs exist.
Receptor
A molecular docking station on or inside a cell. A compound that fits the receptor triggers it and the cell responds — most peptides work by binding receptors on cell surfaces, and each receptor type governs a different effect.
Signaling pathway
The chain reaction that runs inside a cell after a receptor is triggered — one messenger switches on the next until the instruction reaches the cell's machinery. Effects described as 'downstream' are the later links in that chain.
Transporter
A molecular machine that shuttles specific substances across barriers — from blood into brain, or gut into blood. Whether a peptide uses (or blocks) a transporter often decides where in the body it can go.

Use & administration

↑ Back to top
Bacteriostatic water
Sterile water containing a small amount of preservative (usually 0.9% benzyl alcohol). It's used to reconstitute lyophilized peptides for multi-dose vials because the preservative slows bacterial growth once the vial is punctured. Single-dose vials instead use plain sterile water.
Cycle
A defined period of use followed by a planned break — for example a 4-week cycle. 'Cycling' is community practice drawn from bodybuilding and anti-aging forums, not an established medical protocol, and the optimal lengths (if any) aren't well studied.
Insulin syringe
The standard measuring tool for injected peptides — marked in units on the same U-100 scale used for insulin. Adding more solvent when reconstituting lowers the concentration, so the same number of units delivers fewer micrograms; that conversion is what the reconstitution calculator does.
Intramuscular (IM)
An injection delivered into a muscle (common sites: deltoid, glute, thigh). Less common than SubQ for peptides; absorption can be faster but it tends to be more uncomfortable.
Nasal delivery
Absorbed through the lining of the nose. A middle ground between injection and swallowing: no needles, and some compounds partly bypass the barriers that stop them in the gut.
Protocol
Research shorthand for a reported dosing routine — the compound, the amount, how often, and for how long, as described in published sources or community reports. We call these routines on this site (a protocol can read like a prescription, and none of these are prescriptions); the word survives here because you will meet it everywhere in the literature.
Reconstitution
Mixing a lyophilized (freeze-dried) peptide powder with a sterile solvent — almost always bacteriostatic water — to turn it into a measurable injectable solution. The total volume of solvent determines the concentration per unit on an insulin syringe.
Route of administration
The way a compound gets into the body: injection under the skin or into muscle, nasal spray, by mouth, or on the skin. The route changes everything downstream — how fast it acts, how much survives, and how long it lasts.
Subcutaneous (SubQ)
An injection into the fatty tissue just beneath the skin (common sites: abdomen, thigh). It's the most common route for injectable peptides because it's easy to self-administer, low pain, and absorbs steadily.
Titration
Adjusting a dose step by step — often starting low — to find the smallest amount with the wanted effect. It's standard practice in prescription medicine; in community peptide use, reported titration schedules are just reports.
Topical
Applied to the skin surface. It works when a compound is small enough to penetrate — copper peptide in skincare creams is the common example.

Doses & amounts

↑ Back to top
Microgram (mcg)
One-millionth of a gram — the unit peptide doses are usually measured in. Typical research doses are tens to hundreds of micrograms, and 1 mg equals 1,000 mcg: mixing the two up is a thousand-fold error.
Milligram (mg)
One-thousandth of a gram. Peptide vials are labeled in milligrams of powder (a 5 mg vial), while the dose taken is usually measured in micrograms — a thousand times smaller.
Milliliter (mL)
A unit of volume — one-thousandth of a liter. Reconstitution decides how many milliliters of solution a vial holds, and dosing math converts the milliliters drawn into an insulin syringe into micrograms of peptide.

Categories you'll see here

↑ Back to top
Copper peptide (GHK-Cu)
A peptide complexed with a copper ion (GHK is the peptide sequence, Cu is the copper). Researched for skin, collagen, and tissue-repair effects and used topically in many skincare products as well as studied in research contexts.
GHRP / GHRH
Growth-hormone-releasing peptides (GHRP) and growth-hormone-releasing hormones (GHRH) — two classes that each stimulate the body's own growth-hormone pathway through different receptors (e.g. ipamorelin is a GHRP, CJC-1295 is a GHRH analog). They're often discussed, and sometimes combined, in research on GH support.
GLP-1
Glucagon-like peptide-1 — an incretin hormone pathway involved in insulin release and appetite. It's the target of approved medicines such as semaglutide (Ozempic/Wegovy) and tirzepatide (Mounjaro/Zepbound), used for diabetes and weight management.
Secretagogue
A substance that prompts the body to release something. A growth-hormone secretagogue, for instance, signals the body to release more of its own GH rather than supplying GH directly.

Blends & stacks

↑ Back to top
Blend
Multiple peptides premixed into a single product by the seller — a named blend vial — rather than combined by the user. Convenient, but you take the mix exactly as made and rely on the maker's ratios.
Stack
A combination of compounds taken together for a shared goal. Our stack pages describe reported combinations — what's in them, the claimed rationale, and what the evidence does and doesn't support.
Synergy
The idea that two compounds together produce more than the sum of their parts. Genuine synergy is hard to prove — it takes head-to-head testing of the combination against each compound alone, which most stack claims never had.

Quality & sourcing

↑ Back to top
Batch (lot) number
The identifier tying a specific vial to when and where it was made — printed on the label and matched to the COA. If a problem ever surfaces, the batch number is what makes a retest or recall possible.
COA (Certificate of Analysis)
A lab report confirming what's actually in a batch — typically the peptide's identity and purity, sometimes quantified by method (e.g. HPLC). Only a third-party (independent) COA is meaningful; a vendor's in-house lab has a built-in conflict of interest and isn't independent verification.
Counterion (salt form)
The mild acid or base attached to a peptide to keep it stable as a powder — usually acetate or TFA (trifluoroacetate). The same peptide can ship as either salt; it doesn't change the peptide itself, but it appears on the COA and slightly changes the powder's weight.
Endotoxin
Fragments of certain bacteria that survive sterilization and, if present in an injectable, can cause fever and strong immune reactions. A clean purity number says nothing about endotoxin — it's a separate test on a proper COA.
HPLC
High-performance liquid chromatography — the lab technique a COA usually relies on to separate what's in a sample and measure purity. When a vendor quotes '98% pure,' HPLC is typically how it was measured.
Lyophilized
Freeze-dried into a dry powder. It's the stable, shippable form most research peptides come in — dehydrated for shelf life, then reconstituted with bacteriostatic water before use.
Mass spectrometry
A lab technique that weighs molecules to confirm identity — the standard 'is this actually the peptide on the label' check, often reported alongside HPLC purity on a COA.
Purity
The fraction of the sample that is the stated peptide — e.g. '99%+ pure' — as measured on the COA (usually by HPLC). Purity is about how much of the powder is the right molecule, which is distinct from whether it's sterile, endotoxin-free, or accurately dosed.
Research chemical
The legal label under which most peptides are sold — 'for research use only, not for human consumption.' This is the category that allows them to be sold at all, and it's also exactly why buyers should be cautious: the label means no assurance of human-grade quality, sterility, or dosing accuracy.
Sterility
Freedom from living microbes. It's a separate property from purity — a vial can be 99% pure peptide and still be contaminated — and it's guaranteed by how and where the vial was filled, not by the peptide itself.

Reading the research

↑ Back to top
Anecdotal evidence
Individual reports of what happened to one person — forum posts, reviews, a friend's story. Useful for spotting questions worth asking; unreliable as proof, because there's no control group, no measurement, and memory keeps the highlights.
Clinical trial
A controlled study in humans, with defined doses and measured outcomes. It's the step that separates 'looks good in rats' from 'did what we expected in people.'
Double-blind
Neither the participants nor the researchers know who gets the real compound until the study ends — preventing expectation from coloring what's measured or reported.
Efficacy
How well something works under a study's conditions — the size of the measured effect, not just whether any effect appeared at all.
In vitro / in vivo
Latin shorthand you'll meet in study summaries: in vitro means outside a living body (cells or tissue in a dish); in vivo means inside a living organism. A result can hold in vitro and fail in vivo — the body is more complicated than a dish.
Mechanism of action
How a compound produces its effect — the specific biological steps, not just the outcome. 'Lowers appetite' is an effect; 'activates the GLP-1 receptor and slows stomach emptying' is a mechanism.
Peer-reviewed
Screened by independent experts before publication — a journal's basic quality gate. It catches many errors, but it's a floor, not a gold star: peer review approves a study for publication, not its conclusions for you.
Placebo
An inactive look-alike given to a control group so expectation alone can be ruled out. If a study has no placebo arm, 'people felt better' can simply mean 'people expected to.'
Preclinical
Research done before human testing — cells in a dish or animal studies. Most peptide claims start here; it's a promising signal, not proof of what happens in people.
Randomized controlled trial
The cleanest common study design: participants are randomly assigned to the compound or a control group, so differences in outcomes trace to the compound rather than to selection or expectation.
Trial phases
The staged sequence of human testing: Phase 1 checks safety in a small group, Phase 2 tests effect and dose, Phase 3 confirms the result against standard treatment in large groups. A compound can stall at any phase — most do.

Regulatory terms

↑ Back to top
Controlled substance
A drug scheduled under federal law (DEA schedules I–V) based on abuse potential — the category that restricts opioids and stimulants. Most peptides aren't controlled substances; their gray zone comes from being unapproved rather than scheduled.
Dietary supplement
A legal product category (vitamins, herbs, protein powders) regulated as food, with no approval needed before sale. Pure peptide products don't qualify — anything injectable sold as a 'supplement' is mislabeled.
FDA
The U.S. Food and Drug Administration — the agency that approves medicines and polices what may be sold for human use. Most research peptides are not FDA-approved; they occupy a gray zone sold 'for research use only.'
FDA-approved
Formally cleared by the FDA as safe and effective for a specific use in people — the bar prescription medicines clear. When a page here marks something approved, it means an approved medicine containing that ingredient (semaglutide, for example), not the research-grade market.
Investigational drug
A compound in the formal pipeline of human testing but not yet approved — it has an Investigational New Drug (IND) application on file with the FDA. That's a legal status, not a promise it will ever be approved.

Keep going

Now that the vocabulary’s clear, learn how to read a study or browse specific peptides.

Free: The Peptide Stack Cheat-Sheet

The most-used peptide combinations, what the research actually says, and where to buy — one page, one email.

No spam. Unsubscribe anytime.