A Plain-English Peptide Glossary

Talking about peptides comes with a lot of jargon. Sellers often toss these words around loosely, or just plain wrong. So here are the terms you are most likely to bump into, explained in everyday language and listed A to Z. None of this is medical advice. And just because a word shows up here does not mean we are suggesting you use any of these compounds.

Basic biology and chemistry

Agonist — Think of a cell receptor as a lock. An agonist is a molecule that fits the lock and switches it on, which makes the cell do something. A lot of peptides work this way: they copy a natural signal in the body and flip its switch. The opposite is an antagonist, which blocks the lock instead of turning it on.

Amino acid — The basic building block of peptides and proteins. There are 20 common ones, and the order you string them together is what decides what a peptide actually does.

Peptide — A short chain of amino acids linked together (the links are called peptide bonds). “Peptide” and “protein” are really the same idea at different lengths, and peptides are the shorter chains. Many of them act as signals in the body.

Secretagogue — Anything that tells a cell to release (secrete) something. So a “growth hormone secretagogue” is something that nudges the pituitary gland to put out growth hormone.

GHRH / GHRP — Two separate families that often get treated as the same thing. GHRH stands for growth-hormone-releasing hormone, and GHRH analogs act on the GHRH receptor. GHRPs (growth-hormone-releasing peptides) work on a different receptor, the ghrelin receptor (GHS-R1a). They are built differently and take different routes to do their job.

How a substance behaves in the body

Pharmacokinetics (PK) — The study of what your body does to a substance over time. It is usually summed up as ADME: how it is absorbed, where it spreads (distribution), how the body breaks it down (metabolism), and how it leaves (excretion).

Bioavailability — The share of a dose that actually makes it into your bloodstream. An IV (intravenous, straight into a vein) dose counts as 100% by definition. Other ways of taking something come in lower, because some of it gets lost along the way to absorption or gets broken down first.

Half-life — How long it takes for the amount in your blood to drop by half. It is a rough sense of how long something sticks around, not a hard cutoff or expiration timer.

Subcutaneous (SubQ/SC) — Into the fatty layer just under the skin, rather than into a vein (IV) or a muscle (IM). It is simply a way of getting something in, nothing more than that.

Form, handling, and units

Lyophilized — A fancy word for freeze-dried into a stable powder. Most research peptides ship this way because the dry form holds up better than a liquid during shipping and storage.

Reconstitution — Mixing that freeze-dried powder back into a liquid, using a sterile diluent (a clean liquid made for this), before use. The math is simple: a 5 mg vial dissolved in 2 mL of liquid gives you 2.5 mg per mL. (That is just figuring out the concentration. It is not a suggestion about how much to take.)

Bacteriostatic water — Sterile water with benzyl alcohol added (usually 0.9%). The benzyl alcohol is a preservative that holds back bacteria, which lets you use the same vial more than once over about 28 days. Plain “sterile water for injection” has no preservative and is meant to be used only once. One important note: the label for benzyl alcohol warns that it should not be used in newborns, where it has been tied to a fatal “gasping syndrome.”

IU vs mg vs mcg — These measure two different things. A milligram (mg) and a microgram (mcg, which is one-thousandth of a mg) measure mass, meaning actual physical weight. An International Unit (IU) measures biological activity compared against a reference standard. There is no one-size-fits-all way to convert IU into mg. It is different for every substance, so you cannot swap them.

Quality and labeling

HPLC — Short for high-performance liquid chromatography. It is the standard lab method for estimating how pure a peptide is, by splitting it into its parts and measuring the main one. Results can shift depending on the equipment and the conditions, so numbers from two different labs might not match exactly. And HPLC checks purity, but on its own it does not prove what the compound actually is.

COA (Certificate of Analysis) — A lab document that reports the identity and purity of one specific batch, usually using HPLC plus mass spectrometry. A COA worth trusting names the lab that did the testing, the lot number, and the methods used. Keep in mind a COA only confirms what is in the vial. It says nothing about whether a substance is safe or legal to use.

RUO (“Research Use Only”) — A label that means a product is sold for lab research, not for people. It is just information. The FDA does not treat “RUO” or “not for human consumption” as a green light for human use. It judges what something is really meant for by looking at all the evidence, not at what the label says.

Bottom line

These words describe chemistry, biology, and paperwork. They do not describe safety or legality. “RUO,” a clean COA, or a familiar route like subcutaneous tells you nothing about whether a compound is safe or approved for a person. Most of these substances are not FDA-approved for human use. Treat this vocabulary as a tool for reading critically, not as reassurance.

Sources

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