Peptide glossary

Amino acid

The basic building block of peptides and proteins. There are 20 standard amino acids; their order in a chain determines what a peptide does.

Peptide bond

The chemical bond that links amino acids together into a chain.

Peptide vs. protein

Both are amino-acid chains. Peptides are short (roughly 2–50 amino acids); proteins are long, folded chains of many more.

Analog

A modified version of a natural peptide, engineered to change its strength, duration, or selectivity — for example, a longer-acting version of a natural hormone.

Receptor agonist

A molecule that activates a receptor, switching on its signal (e.g. a GLP-1 receptor agonist mimics the hormone GLP-1).

Receptor antagonist

A molecule that blocks a receptor, preventing its normal signal (e.g. a GnRH antagonist).

GLP-1

Glucagon-like peptide-1 — a gut hormone that curbs appetite and improves insulin response. The target of weight-loss/diabetes drugs like semaglutide.

GIP

Glucose-dependent insulinotropic polypeptide — a second incretin hormone, combined with GLP-1 in dual agonists like tirzepatide.

Incretin

A gut hormone (such as GLP-1 or GIP) released after eating that boosts insulin and reduces appetite. 'Incretin mimetics' copy them.

Secretagogue

A substance that prompts the body to secrete something — e.g. a growth-hormone secretagogue triggers release of the body's own growth hormone.

GHRP

Growth-hormone-releasing peptide — acts on the ghrelin receptor to trigger a growth-hormone pulse (e.g. ipamorelin).

GHRH analog

A peptide modeled on growth-hormone-releasing hormone that stimulates the pituitary (e.g. sermorelin, CJC-1295).

IGF-1

Insulin-like growth factor 1 — a hormone downstream of growth hormone that drives tissue and muscle growth.

Myostatin

A protein that limits muscle growth; some experimental peptides aim to inhibit it to promote muscle.

Melanocortin system

A brain/skin signaling system involved in pigmentation and sexual desire — the target of peptides like PT-141 and melanotan.

GnRH

Gonadotropin-releasing hormone — the master switch of the reproductive hormone cascade. Agonists and antagonists are used in fertility and cancer care.

Nootropic

A substance studied for cognitive benefits (memory, focus). Several neuropeptides such as Semax are described this way.

Bioregulator

A term for ultra-short synthetic peptides (often of Russian origin) proposed to influence specific tissues; evidence is largely preclinical.

Subcutaneous

Injection into the fat layer just under the skin — the most common route for peptide injections.

Bioavailability

The fraction of a dose that reaches the bloodstream intact. Most peptides have very low oral bioavailability because they're digested.

Half-life

How long it takes for half of a substance to clear from the body — a key driver of how often a peptide must be dosed.

Research chemical

A compound sold 'for research use only' that is not approved for human use and often lacks human safety or efficacy data.

FDA

The U.S. Food and Drug Administration — the regulator that approves medicines and polices unapproved drug marketing.

WADA

The World Anti-Doping Agency, whose code prohibits many peptides in sport at all times.

Amino acid sequence

The specific order of amino acids in a peptide, which determines its shape and therefore which receptor it fits. Changing even one residue can change the effect entirely — this is why analogs behave differently from the natural peptide.

Cyclic peptide

A peptide whose chain is joined into a ring rather than left as a loose strand. Cyclisation makes a peptide much harder for enzymes to break down, which is why several long-acting drugs — octreotide among them — are cyclic.

Prodrug

An inactive compound that the body converts into the active drug after it is administered. Used to control the release rate: terlipressin and palopegteriparatide are both prodrugs designed to deliver a steady supply rather than a spike.

PEGylation

Attaching polyethylene glycol chains to a peptide to slow its clearance and extend its duration. One of several standard tricks — alongside cyclisation, fatty-acid attachment and amino-acid substitution — for turning a short-lived natural peptide into a viable drug.

Depot / long-acting release

A formulation that releases a drug slowly from an injection site over weeks, so one injection replaces many. Lanreotide's gel and octreotide's LAR microspheres are depots; the trade-off is that a depot cannot be withdrawn quickly if it is poorly tolerated.

Intramuscular (IM)

Injection into muscle rather than the fat layer beneath the skin. Used mainly for depot formulations; most peptide injections are subcutaneous instead.

Endogenous vs. exogenous

Endogenous means produced by the body itself; exogenous means introduced from outside. Vasopressin is endogenous as a hormone and exogenous as an injected medicine — the same molecule in two roles.

Receptor selectivity

How narrowly a compound binds one receptor subtype rather than several. Selectivity is often the whole difference between two drugs in the same family: octreotide targets mainly SSTR2, while pasireotide also reaches SSTR5, which is why they treat different conditions.

Tachyphylaxis

A rapid loss of response after repeated doses, usually because the drug depletes a store or the receptor stops responding. Desmopressin's effect on clotting factors shows this — it releases existing reserves, so repeat doses give progressively less.

Pharmacokinetics

What the body does to a drug: absorption, distribution, metabolism and elimination. Peptide drug design is mostly a pharmacokinetics problem, since the molecules themselves usually work — the difficulty is making them last.

Dose escalation (titration)

Starting at a low dose and increasing gradually to limit side effects. Standard practice with GLP-1 medicines, and the reason the full effect of an approved peptide drug may take a couple of months to appear. We do not publish dosing schedules.

Lyophilized

Freeze-dried into a stable powder, because most peptides degrade in solution. It is the form research-chemical vials are sold in, and a common source of dosing error once someone tries to work out concentrations themselves.

Amylin

A hormone released alongside insulin that signals fullness through the brainstem — a pathway separate from GLP-1. Amylin analogs (cagrilintide, petrelintide, eloralintide) are the most active frontier in obesity medicine after the incretin drugs.

Somatostatin

The body's broad inhibitory hormone, which switches off the release of growth hormone, insulin, glucagon and more. Natural somatostatin lasts about two minutes, so the clinically useful versions are engineered analogs.

Vasopressin (ADH)

Antidiuretic hormone — the peptide that controls how much water the kidneys retain, and separately constricts blood vessels. Its analogs split those two jobs: desmopressin keeps the water effect, terlipressin the vascular one.

Parathyroid hormone (PTH)

The peptide hormone that regulates blood calcium. It has an unusual property: given in brief daily pulses it builds bone (teriparatide, for osteoporosis), while given continuously it restores normal calcium handling (palopegteriparatide, for hypoparathyroidism).

Thymic peptide

A peptide derived from the thymus, the gland that trains T cells and shrinks with age. Thymosin alpha-1 is the most studied; the family is the basis of most "immune peptide" marketing.

Antimicrobial peptide

A short peptide of the innate immune system that kills microbes directly, usually by disrupting their membranes. LL-37 is the best-known human example. Also called host-defence peptides.

Matrikine

A peptide fragment released when the skin's matrix proteins break down, which signals cells to produce more collagen. Cosmetic "signal peptides" such as Matrixyl are synthetic matrikines.

SNARE proteins

The machinery nerve endings use to release neurotransmitters. Cosmetic peptides such as Argireline and SNAP-8 aim to interfere with it topically to soften expression lines — the same general target as injectable neuromodulators, with a far smaller effect.

Randomized controlled trial (RCT)

A study that randomly assigns participants to the treatment or a comparison group, so the groups differ only by chance. Randomisation is what makes a result attributable to the drug rather than to who chose to take it.

Placebo-controlled

A trial in which the comparison group receives an inactive treatment. Essential because people improve for many reasons — expectation, time, other changes — and only a placebo group separates those from a real drug effect.

Phase 1 / 2 / 3

The stages of human drug testing. Phase 1 checks safety and dosing in a small group; Phase 2 looks for an effect in a few hundred patients; Phase 3 confirms it in large trials that support approval. Most "promising" peptide results are Phase 2 or earlier.

Primary endpoint

The single main outcome a trial is designed to measure, chosen before it starts. Trials are judged on it, which is why a drug can produce impressive-looking numbers and still be reported as having failed.

Non-inferiority

A trial design that asks whether a new treatment is not meaningfully worse than an existing one, rather than better. Missing a non-inferiority endpoint — as CagriSema did against tirzepatide — means the drug failed to prove it matched the comparator, not that it did nothing.

Meta-analysis

A study that statistically combines the results of multiple trials to produce a more reliable overall estimate. Along with systematic reviews, it sits at the top of the evidence hierarchy.

Off-label use

Prescribing an approved medicine for a condition, population or route not covered by its approval. Legal and common in medicine, but it means the specific use has not been reviewed by a regulator.

Compounded

Made up by a compounding pharmacy for an individual patient, rather than manufactured as an approved product. Compounded versions of peptide drugs are not FDA-approved products and are not reviewed for safety, effectiveness or quality in the way the branded drug is.

Biosimilar

A near-copy of an approved biologic medicine, authorised after being shown to be highly similar with no clinically meaningful differences. Not the same as a generic, because large biological molecules cannot be reproduced exactly.

Frequently asked questions