Peptide glossary — every term, plainly defined
What reconstitution, bacteriostatic water, insulin units, titration, half-life, compounded, research-use-only and 62 other terms actually mean — each with the misreading that causes real harm. Free, no account, no doses.
Lyophilised
Water removed under vacuum, leaving a dry cake or a scatter of white powder in the bottom of the vial. Peptides are shipped this way because dry material is far more stable than liquid.
Reconstitution
Adding liquid to a dry vial to dissolve the powder. The amount of liquid decides the concentration of the finished solution, and nothing else about the vial changes.
Bacteriostatic water
Sterile water containing a small amount of benzyl alcohol, which stops bacteria multiplying. That preservative is why a vial mixed with it can be entered more than once.
Sterile water for injection
Water that has been sterilised and contains nothing else — no preservative, no salt. Used where a preservative is unwanted.
Vial
The small glass bottle with a rubber stopper and a metal crimp. Single-dose and multi-dose vials are different products: only the multi-dose kind is designed to be entered repeatedly.
Septum
The rubber disc a needle passes through. It reseals itself after each entry, which is what makes a multi-dose vial possible.
Coring
A fragment of the rubber stopper punched out by the needle and dropped into the solution, usually from entering at an angle or reusing a blunted needle.
Cold chain
The unbroken run of temperature-controlled handling between manufacture and use. Approved products state their storage range on the label and their maker validated it.
Temperature excursion
A period spent outside the labelled storage range. Approved labels define how much is tolerable; unapproved material has no such definition to fall outside of.
Concentration
How much substance sits in a given volume of liquid — usually written as milligrams per millilitre. It is a property of the mixed vial, decided at the moment it is reconstituted.
Insulin units
The markings on an insulin syringe. On a standard U-100 syringe, 100 units is one millilitre, so each unit is one hundredth of a millilitre. They are volume markings being used as a measuring scale.
Milligram and microgram
A milligram is a thousandth of a gram. A microgram is a thousandth of a milligram — a thousand micrograms make one milligram.
International unit
A measure of biological activity rather than mass, used for substances where the mass does not map cleanly onto the effect. Each substance has its own conversion, and there is no general formula.
Dead space
The liquid left in the needle and hub after the plunger is fully depressed. On the small syringes used here it is a meaningful fraction of a small volume.
Half-life
The time taken for the amount in circulation to fall by half. It governs how often something is given and how long it lingers after the last one.
Steady state
The point where the amount going in each interval matches the amount clearing, so the level stops climbing. It takes roughly four to five half-lives to arrive.
Titration
Starting low and increasing in planned steps. Approved incretin labels are written this way to let gastrointestinal effects settle at each step before the next.
Supply arithmetic
How long a supply lasts depends on the total in each container and how it is used as prescribed. For a prescribed product, the pharmacist or prescriber can say how long a fill should last.
Peptide
A short chain of amino acids — the same building blocks as proteins, in a shorter string. The boundary with 'protein' is a matter of convention around a few dozen amino acids rather than a hard rule.
Agonist
A molecule that binds a receptor and switches it on, mimicking whatever the body's own signal does there. An antagonist binds the same receptor and blocks it instead.
Analogue
A molecule deliberately modified from a natural one — usually to survive longer in the body — while keeping the part that fits the receptor.
Fragment
A piece of a larger natural molecule, synthesised on its own.
GLP-1
Glucagon-like peptide-1, a gut hormone released after eating. It prompts insulin, suppresses glucagon, slows gastric emptying and reduces appetite through the brain. The pathway the whole modern weight-loss class works on.
GIP
Glucose-dependent insulinotropic polypeptide, the other main incretin hormone. Adding GIP to GLP-1 is the mechanistic difference between tirzepatide and semaglutide.
Glucagon
The hormone that raises blood glucose — insulin's counterweight. Engaging its receptor deliberately, as retatrutide does, targets energy expenditure and liver fat rather than blood sugar.
Amylin
A hormone released alongside insulin that signals fullness through a route separate from GLP-1. Cagrilintide is an amylin analogue, which is why CagriSema is described as engaging two different satiety pathways.
Incretin
The family name for gut hormones — GLP-1 and GIP — that trigger insulin release in response to food. 'Incretin drug' is the accurate collective term for what people call GLP-1s.
Secretagogue
A substance that makes a gland secrete something. Growth hormone secretagogues act on the pituitary to release growth hormone the body already had, rather than supplying it from outside.
GHRH
Growth hormone-releasing hormone — the hypothalamic signal telling the pituitary to release growth hormone. Sermorelin, CJC-1295 and tesamorelin are all analogues of it.
Ghrelin receptor
A separate switch on the pituitary, reached by ghrelin — the hunger hormone. Ipamorelin and the other 'morelins' act here, which is why they pair with GHRH analogues rather than compete with them.
IGF-1
Insulin-like growth factor 1, made largely in the liver in response to growth hormone. It is the blood marker used to judge whether a growth hormone intervention did anything measurable.
Subcutaneous
Into the fat layer under the skin rather than into a muscle or a vein. Absorption is slower and steadier, which suits most of this class.
Bioavailability
The fraction of what you administer that actually reaches circulation intact. Peptides are usually injected because digestion destroys most of them, and oral formulations need specific engineering to survive the gut.
Selectivity
How narrowly a molecule hits its intended receptor and leaves others alone. Ipamorelin's design goal was selectivity — releasing growth hormone with little effect on cortisol or prolactin.
Phase 1, 2 and 3
Phase 1 asks whether it is tolerable, in a small group. Phase 2 looks for a signal and a workable range. Phase 3 is the large trial that decides whether the effect is real and durable enough to approve.
Topline
The headline result released by the sponsor before the full paper: the primary endpoint and little else. It is a press release, chosen by the company, not a peer-reviewed dataset.
Endpoint
The thing a trial was set up to measure, chosen before it began. A primary endpoint is what the trial succeeds or fails on; everything else is secondary or exploratory.
Cross-trial comparison
Placing results from two separate trials side by side as if they were measured against each other. Different populations, durations, baselines and analysis methods make the gap unreliable.
Head-to-head trial
One trial, one protocol, participants randomised between the two drugs and measured on the same endpoint at the same time. The only design that can rank two drugs.
Cardiovascular outcomes trial
A long, large trial counting actual events — heart attacks, strokes, cardiovascular deaths — rather than surrogate measures like weight or blood sugar. The most expensive and most persuasive evidence a metabolic drug can carry.
MACE
Major adverse cardiovascular events — the composite counted in an outcomes trial, typically cardiovascular death, non-fatal heart attack and non-fatal stroke.
Placebo-controlled
Compared against an inactive injection rather than against another drug. It measures the full size of an effect; an active-comparator trial measures only the difference between two treatments.
Mean versus median
The mean is the average; the median is the middle person. Weight-loss results are usually reported as means, and the distribution behind a mean is wide.
Preclinical
Cell and animal studies done before humans are involved. It establishes plausibility and shapes what a first human trial looks like.
Adverse event report
A report that something bad happened to somebody taking something. Reports are collected without any finding that the drug caused the event.
FDA-approved
A specific product, from a specific manufacturer, for specific indications, reviewed and cleared for sale. Approval attaches to the product and its label — never to a molecule in the abstract.
Indication
The condition and population a product is approved to treat, printed on the label. Everything the label does not name is outside the approval.
Off-label
A licensed prescriber using an approved product for something its label does not cover. Legal, common, and a clinical judgement they carry responsibility for.
Compounded
Mixed to order by a pharmacy for a specific patient. A legitimate part of medicine, governed by sections 503A and 503B of the Food, Drug and Cosmetic Act.
503A and 503B
The two compounding lanes: 503A is a pharmacy compounding for individual patients on prescription; 503B is a registered outsourcing facility that may compound in bulk under stricter manufacturing rules.
Bulk drug substances list
The FDA's evaluation of which raw substances may be used in compounding. Substances judged to raise significant safety concerns are excluded — which is what closed the compounding route for BPC-157.
Research use only
A label that positions a product outside medicines regulation. It is a legal posture adopted by the seller, not a statement about what the substance is or how pure it happens to be.
Warning letter
A formal FDA notice to a company that it appears to be breaking the law, with a deadline to respond. Aimed at sellers, not at the people who bought from them.
Import alert
An instruction letting FDA field staff detain shipments at the border without examining each one, unless the maker is on a cleared list.
Boxed warning
The most serious warning a US label can carry, printed inside a black border. Its presence signals a specific, documented, serious risk.
Filed / under review
The dossier is with the regulator and a decision has not been made. It is the start of the review, not a preview of its outcome.
Biologic
A regulatory category for larger, biologically-produced molecules, licensed on a different pathway from conventional drugs. Whether a given peptide counts can be contested — the classification of retatrutide is currently in dispute with the FDA.
WADA Prohibited List
The World Anti-Doping Agency's list of substances banned in sport, with a category prohibited at all times rather than only in competition. Most peptides discussed for recovery or growth hormone sit in it.
Certificate of analysis
A document reporting laboratory results for a batch — typically identity and purity. Meaningful only when the lab is independent, the batch number matches the vial in your hand, and the report is the lab's own.
HPLC
High-performance liquid chromatography — the standard purity test. It separates a sample into components and reports what proportion is the intended one.
Mass spectrometry
Measures molecular mass, which confirms identity — that the material is the molecule claimed. The complement to a purity assay rather than a substitute for it.
Endotoxin
Fragments of bacterial cell wall. They survive sterilisation, are not removed by filtering, and cause fever and inflammatory reactions when injected.
Third-party testing
Testing commissioned from a laboratory with no commercial interest in the result — ideally by the buyer, from their own vial, after it arrives.
Grey market
The trade in substances sold without approval, usually under a research-use label. Not a black market — the sellers are visible and take card payments — but outside medicines regulation entirely.
Stack
Community shorthand for taking more than one substance in the same period. It is a description of a practice, not a validated combination — combinations are almost never studied as combinations.
Blend
Two or more peptides pre-mixed in one vial and sold as a single product — 'CJC/Ipa' being the best-known.
Protocol
In clinical research, the pre-registered plan a trial follows. In community use, whatever somebody has decided to do — which is a personal account, not a protocol in the research sense.
Non-responder
Someone who sees little effect where others see a lot. Real in every trial in this class, and a reminder that a mean result contains a wide spread of individual ones.
Plateau
The point where a change that had been continuing levels off. Expected in weight trials, where curves flatten as the body's energy requirements fall with its mass.
Educational use only, never medical advice. No doses.