The most important words – simply explained

When it comes to peptides, the same technical words keep popping up. Here are the five most important ones – in everyday language, with pictures from real life.

Peptide

A peptide is a short chain of amino acids – like a little string of beads, where each bead is a building block. Such chains are found all over the body, for example as messengers that pass signals along. But many peptides are broken down again very quickly inside the body.

Receptor & agonist

A receptor is like a power outlet on a cell, and a peptide is the matching plug. An “agonist” is a substance that fits into that outlet and switches the device on – so it triggers an effect. The opposite, an “antagonist,” blocks the outlet so that nothing happens.

Half-life

The half-life is the time in which half of a substance has disappeared from the body. Picture a cup of hot coffee: after a while it is only half as warm as at the start. It is purely an informational figure – and explicitly not a dosing aid here.

“Research chemical” / not approved

Many peptides are only “research chemicals”: they have not been tested as a medicine and are not approved. That is like a kit carrying the note “Not cleared for everyday use.” Without approval, no authority has confirmed the purity, safety or benefit for humans.

Gray market

Gray market is the name for sales in a legal gray zone – often labelled “for research purposes only.” It is like a meal with no ingredient list and no kitchen inspection: you can never be sure what is really in it. Authorities such as the EMA and WHO have repeatedly documented counterfeit or contaminated products here.

Why these words matter

Anyone who understands these five terms can judge advertising promises better and tell claims apart from established facts. This page only explains – it is not a usage guide and does not replace a conversation with a doctor.

Glossary

The key terms — explained briefly and clearly.

5

503B / Outsourcing Facility: A US-registered manufacturing pharmacy that may also produce medicines in larger quantities without an individual prescription — with higher quality requirements than the classic 503A pharmacy. For several GLP-1 drugs the compounding permissions expired in 2025.

A

Accelerated Approval: An expedited US approval pathway by the FDA based on promising but not yet definitive endpoints. The approval can be tied to confirming the benefit in further studies.
Agonist: A substance that activates a receptor and thereby triggers a biological response — as opposed to an antagonist, which blocks the receptor.
Angiogenesis: The formation of new blood vessels. In peptide research often mentioned in connection with healing and repair processes.

B

Bioavailability: The fraction of an active substance that reaches the bloodstream unchanged and can take effect. Depends strongly on the route of uptake.

C

Compounding (503A): The individual preparation of medicines by pharmacies. In the US the FDA reviews which bulk substances are permissible for this; some peptides were assessed with safety concerns.
Contraindication: A circumstance (e.g. illness, pregnancy) in which an active substance should not be used because the risk outweighs the benefit.

F

Falsified medicine: A counterfeit product that imitates a real medicine but may contain the wrong, missing or contaminated active ingredient. Documented repeatedly for GLP-1 products by EMA, WHO and BfArM — in part with dangerous active-ingredient mix-ups.

G

GIP: Glucose-dependent insulinotropic polypeptide — an incretin hormone of the gut. A target receptor of modern metabolic drugs (e.g. tirzepatide).
GLP-1: Glucagon-like peptide-1 — a gut hormone that promotes insulin secretion in a glucose-dependent way, curbs appetite and slows gastric emptying.
Glucagon: A hormone that raises blood sugar and influences energy expenditure. Triple agonists also target the glucagon receptor.

H

Half-life: The time in which the concentration of a substance in the body falls by half. A pharmacological parameter — here purely informative, not a dosing aid.

I

Ileus: Intestinal paralysis or obstruction in which the passage through the gut comes to a standstill — a medical emergency. As a rare, serious signal of slowed gastrointestinal motility it was added to the US product information for semaglutide.
In vitro / In vivo: “In vitro” = in the test tube/cell culture, “in vivo” = in the living organism. Preclinical findings often come from cell or animal experiments.
Incretin: Collective term for gut hormones (e.g. GLP-1, GIP) that promote insulin secretion after eating. Incretin mimetics imitate this effect.
Investigational: An investigational substance that is in clinical trials and does not (yet) have market approval.

L

Lipopolysaccharide (LPS): A component of the outer membrane of certain bacteria, also called an “endotoxin”. If LPS enters the body through gray-market product made under non-sterile or contaminated conditions, it can trigger fever, inflammatory reactions and, in the worst case, life-threatening responses — a central purity risk of unapproved peptides.

M

MASH / NASH: Metabolic (formerly: non-alcoholic) steatohepatitis — a fatty liver disease with inflammation, considered a research field for several metabolic drugs.

N

NAION: Non-arteritic anterior ischemic optic neuropathy — a rare circulatory disorder of the optic nerve that appears as sudden, often painless vision loss in one eye and can be permanent. Classified in 2025 by the EMA safety committee as a very rare side effect of semaglutide.

O

Off-label: The use of an approved medicine outside the authorised indication — a medical decision carrying its own responsibility and counselling.

P

Peptide: A short chain of amino acids. Peptides act in the body as hormones and signaling molecules, among other things; many are broken down enzymatically very quickly.
Pharmacokinetics: What the body does with an active substance: uptake, distribution, metabolism and excretion.
Preclinical: Research before clinical trials in humans — typically cell and animal experiments. Promising preclinical data are no proof of benefit in humans.

R

Reconstitution: Dissolving a powdered active substance in a liquid. Deliberately not included as a guide in this app — it is a knowledge resource, not a usage resource.

S

Secretagogue: A substance that prompts a gland or cell to release (secrete) one of the body’s own hormones. Growth-hormone secretagogues (e.g. certain peptides) are meant to nudge the body’s own GH release rather than supplying the hormone directly.
Subcutaneous: “Under the skin” — a route of administration into the subcutaneous fat tissue. Explained here only as a pharmacological term, not as a guide.

W

WADA: World Anti-Doping Agency. Maintains the prohibited list, which includes several research peptides (e.g. BPC-157, TB-500).