Peptide Glossary: Key Terms Explained Simply
This peptide glossary explains 75 terms that come up when you read about peptides, GLP-1 drugs and lab reports, each in one or two plain sentences.
The terms sit in five groups, from peptide basics to research and regulation. We wrote every definition from scratch and checked it in November 2026 against named records: the NCI Dictionary of Cancer Terms and NCI Thesaurus entry for each term, NLM MeSH records, MedlinePlus, FDA pages and drug labels, US federal rules and PubMed reviews. Each group ends with links to the exact entries it used. Where we have a longer guide on a term, a More link follows its definition.
At a glance
These 11 terms are a quick start. They cover what a peptide is, how GLP-1 drugs act, how a vial is mixed and measured, what a lab report shows and what FDA status means. Each one-line meaning is a short form of the full definition further down this page.
| Term | What it means | Read more |
|---|---|---|
| Peptide | A chain of two or more amino acids, often shorter than a protein. | Full definition, Understanding peptides |
| GLP-1 | A gut hormone sent out after meals that helps steer blood sugar and appetite. | Full definition, What is GLP-1? |
| Agonist | A substance that binds a receptor and turns it on. | Full definition |
| Half-life | How long the amount of a drug in the body takes to fall by half. | Full definition |
| Reconstitution | Dissolving a freeze-dried powder by adding a liquid called a diluent. | Full definition, Reconstitution calculator |
| Bacteriostatic water | Sterile water with benzyl alcohol added, sold in vials made for repeat use. | Full definition, What is bacteriostatic water? |
| U-100 | A strength of 100 units per milliliter, the scale on most insulin syringes. | Full definition, Insulin syringe units chart |
| Certificate of analysis (COA) | The test results for one batch, set against the limits it had to meet. | Full definition, How to verify a peptide COA |
| Research use only | A label for lab research products, not for diagnosing or treating people. | Full definition |
| Compounded drug | A medicine mixed for one patient that has no FDA approval. | Full definition, Compounded tirzepatide |
| Phase 3 | A large human trial, often 300 to 3,000 people, that tests whether a drug works. | Full definition |
Sources: NCI Dictionary: peptide; Januvia FDA label; NCI Dictionary: agonist; Ozempic FDA label; Mounjaro FDA label; FDA guide: lyophilization of parenterals; Bacteriostatic Water for Injection label; Humulin R FDA label; FDA guidance: ICH Q7; FDA guidance: research use only labeling; FDA: human drug compounding; FDA: clinical research phases
What are peptides, receptors and half-life?
Most of these terms answer one of three questions: what a peptide is made of, how it acts on a cell, and what happens to it once it is in the body.
Peptide
A chain of two or more amino acids joined end to end by peptide bonds. Peptides are often shorter than proteins, and gut hormones such as GLP-1 and GIP are peptides.
More: Understanding peptides
Amino acid
One of the small building blocks that link together to form peptides and proteins. Proteins are built mainly from 20 common amino acids.
Peptide bond
The chemical link that joins one amino acid to the next in a chain. It forms between the carboxyl group of one amino acid and the amino group of its neighbor.
Polypeptide
A longer chain of amino acids, often described as about 13 or more. A very long polypeptide that folds into a working shape is called a protein.
Protein
A large molecule made of one or more long amino acid chains folded into a shape that does a job in the body. Under US FDA rules, a chain of more than 40 amino acids counts as a protein.
Analog
A substance designed to be similar, but not identical, to another one. Semaglutide is a GLP-1 analog: its label says 94 percent of its sequence matches human GLP-1, with changes that help it resist breakdown and last longer.
Fragment
A piece of a larger protein or peptide. It can be cut from the full molecule or made in a lab as a copy of just one part of the sequence.
Receptor
A molecule on or inside a cell that a specific substance fits, much like a key fits a lock. When the right substance binds, the cell responds in a set way.
Agonist
A substance that binds to a receptor and switches it on, copying the action of a natural signal in the body. Semaglutide, for example, is a GLP-1 receptor agonist.
Antagonist
A substance that blocks a receptor or stops another substance from having its effect. It works in the opposite direction to an agonist.
Half-life
The time it takes for the amount of a drug in the body to drop by half. Each drug has its own: the semaglutide label lists about one week and the tirzepatide label about five days.
Bioavailability
How much of a drug reaches the bloodstream and the site where it acts, and how fast it gets there. Peptides swallowed by mouth meet gut enzymes and poor absorption, which is why most peptide drugs are given as shots instead of pills.
Pharmacokinetics
What the body does to a drug over time: how it is absorbed, spread through the body, broken down and removed. It is often shortened to PK.
Pharmacodynamics
What a drug does to the body: the effects it causes and the way it causes them. It pairs with pharmacokinetics, which covers what the body does to the drug.
Sources: NCI Dictionary: peptide; NCI Dictionary: amino acid; NCI Dictionary: polypeptide; NCI Dictionary: protein; NCI Dictionary: analog; NCI Dictionary: receptor; NCI Dictionary: agonist; NCI Dictionary: antagonist; NCI Dictionary: pharmacokinetics; NCI Thesaurus: Peptide (C735); NCI Thesaurus: Peptide Bond Constituents (C118427); NCI Thesaurus: Half Life (C70916); NCI Thesaurus: Pharmacodynamics (C15720); MeSH: Peptides (D010455); MeSH: Peptide Fragments (D010446); 21 CFR 600.3; Januvia FDA label; Ozempic FDA label; Mounjaro FDA label; StatPearls: Elimination Half-Life of Drugs; 21 CFR 314.3; Drucker 2020, oral peptide therapeutics; StatPearls: Pharmacokinetics
What are GLP-1, incretins and growth hormone?
GLP-1 drugs copy hormones the gut releases after a meal, so these terms explain how those drugs act, while the growth hormone terms describe a separate system run by the brain and pituitary gland.
GLP-1
Glucagon-like peptide-1, a hormone the gut releases after a meal. It helps the pancreas release insulin when blood sugar runs high, lowers glucagon and helps regulate appetite.
More: What is GLP-1?
GIP
Glucose-dependent insulinotropic polypeptide, another gut hormone released after eating. Like GLP-1, it raises insulin release when blood sugar is normal or high.
Glucagon
A hormone made by the pancreas that raises blood sugar, mainly by signaling the liver to put more glucose into the blood. Its effect runs opposite to insulin.
Incretin
A gut hormone that prompts the pancreas to release insulin after a person eats. GLP-1 and GIP are incretins, and several diabetes and weight management drugs copy their action.
Dual agonist
A drug that switches on two different hormone receptors. Tirzepatide is one: its FDA label says it binds to and activates both the GIP and GLP-1 receptors.
Triple agonist
A drug that switches on three hormone receptors at once. Retatrutide, tested in phase 3 trials, acts on the GIP, GLP-1 and glucagon receptors.
More: Retatrutide calculator
DPP-4
Dipeptidyl peptidase-4, an enzyme that quickly breaks down GLP-1 and GIP in the body. DPP-4 inhibitor tablets for type 2 diabetes work by slowing that breakdown.
Insulin
A hormone made by the pancreas that moves sugar out of the blood and into cells, where it is used for energy. As cells take in that sugar, the blood sugar level falls.
Insulin resistance
A state in which muscle, fat and liver cells do not respond well to insulin, so sugar moves out of the blood less easily. Blood insulin levels often rise, and blood sugar can climb over time.
Satiety
The sense of fullness that keeps hunger away after a meal is over. Researchers separate it from satiation, the process that brings a meal to an end.
Gastric emptying
The passage of food from the stomach into the first part of the small intestine. FDA labels for GLP-1 drugs such as Ozempic state that they slow it down.
Food noise
An informal term for unwanted thoughts about food that keep coming back and feel hard to switch off. It is not a medical diagnosis. It spread online in talk about GLP-1 drugs, and researchers are still working out how to define and measure it.
More: Food noise
Growth hormone
A hormone made by the pituitary gland at the base of the brain. It drives growth in children and helps control how the body uses sugar and fat.
More: HGH peptides guide
GHRH
Growth hormone-releasing hormone, a peptide from the hypothalamus that signals the pituitary to make and release growth hormone. Tesamorelin is an FDA-approved analog of it.
More: HGH peptides guide
GHRP
Growth hormone-releasing peptide, the name for a family of peptides that make the pituitary release growth hormone in pulses. They belong to the wider class called growth hormone secretagogues.
More: HGH peptides guide
Secretagogue
Any substance that makes a gland or cell release another substance. An insulin secretagogue triggers insulin release, and a growth hormone secretagogue triggers growth hormone release.
More: HGH peptides guide
IGF-1
Insulin-like growth factor 1, a hormone that carries out many effects of growth hormone on bone and tissue. Its blood level stays steadier than the growth hormone level, so an IGF-1 test is a common way to track growth hormone activity.
Sources: Januvia FDA label; Wegovy FDA label; Ozempic FDA label; Mounjaro FDA label; TRIUMPH-1, NEJM 2026; NCI Dictionary: insulin; NCI Dictionary: glucagon; NCI Dictionary: growth hormone; NIDDK: insulin resistance; MeSH: Incretins (D054795); MeSH: Gastric Emptying (D005746); MeSH: Secretagogues (D000078604); Gibbons 2019, satiety and satiation; Hayashi 2023, food noise; Wylie 2026, food noise review; MedlinePlus: growth hormone test; Egrifta WR FDA label; Sigalos 2018, growth hormone secretagogues; MedlinePlus: IGF-1 test
What do reconstitution, U-100 and mg mean?
A vial and syringe bring together three different kinds of measure: weight in milligrams, volume in milliliters or syringe units, and biological activity in international units.
Lyophilized
Freeze-dried. The product is frozen and its water is pulled off under vacuum, leaving a dry powder that keeps better than a liquid.
More: Lyophilized peptides
Reconstitution
Adding a liquid, called a diluent, to a freeze-dried powder so it dissolves into a solution. Sterile water and bacteriostatic water are common diluents.
Bacteriostatic water
Sterile water that contains benzyl alcohol as a preservative to hold back the growth of bacteria. It is sold in multiple-dose vials that are made to be entered more than once.
Sterile water
Water for injection that is free of germs and has no preservative in it. It is sold only in single-dose containers.
Benzyl alcohol
A preservative added to bacteriostatic water and some other injectable products to slow the growth of bacteria. The bacteriostatic water label warns that it is not for use in newborn babies.
Vial
A small glass or plastic bottle closed with a stopper, used to hold an injectable liquid or powder. Single-dose vials are meant for one use, while multiple-dose vials have passed tests showing they resist germ growth between uses.
Concentration
How much of a substance is dissolved in a set amount of liquid. Labels often write it as milligrams per milliliter, shortened to mg/mL.
Insulin syringe
A small syringe marked in units instead of milliliters. Most are built for U-100 insulin, so a full 1 mL syringe holds 100 units.
More: Insulin syringe sizes
Units
The scale printed on an insulin syringe. For a U-100 syringe, 100 units equal one milliliter, so a unit is really a volume, and the amount of drug in it depends on the concentration in the vial.
U-100
A strength label meaning 100 units of insulin in each milliliter, the standard strength. Most insulin syringes are marked for U-100, and stronger insulins such as U-300 and U-500 also exist.
Milligram
One thousandth of a gram, written mg. A thousand micrograms make one milligram.
Microgram
One millionth of a gram, or one thousandth of a milligram. It is written mcg or with the Greek letter mu as µg.
International unit (IU)
A unit that measures a substance by its biological effect instead of its weight. The amount in one IU is set by international agreement and is different for each substance.
Subcutaneous
Under the skin, in the fatty layer that sits between the skin and the muscle. Insulin and injectable GLP-1 drugs are given this way.
More: SubQ vs intramuscular
Intramuscular
Into a muscle. An intramuscular injection goes deeper than a subcutaneous one, through the fat layer and into the muscle itself.
More: SubQ vs intramuscular
Titration
Changing a dose in small steps over time until a target effect or a set dose is reached. GLP-1 drug labels step the dose up on a fixed schedule to limit stomach side effects.
Loading dose
A larger first dose, or a few larger early doses, used to bring a drug up to working levels quickly. Smaller maintenance doses follow it.
Maintenance dose
The steady, ongoing dose used after any loading or step-up phase. On GLP-1 drug labels it is the dose reached at the end of the step-up schedule.
Sources: FDA guide: lyophilization of parenterals; NCI Dictionary: freeze-dried; NCI Dictionary: concentration; NCI Dictionary: microgram; NCI Dictionary: milligram; NCI Dictionary: International Unit; NCI Dictionary: subcutaneous; NCI Dictionary: intramuscular; NCI Thesaurus: Lyophilized Powder for Injectable Solution Dosage Form (C42957); NCI Thesaurus: Vial (C41275); NCI Thesaurus: Concentration (C41185); NCI Thesaurus: Titration Study (C90475); Bacteriostatic Water for Injection label; Sterile Water for Injection label; FDA guidance: package type terms; MedlinePlus: giving an insulin injection; Humulin R FDA label; NIST: metric SI prefixes; MedlinePlus: subcutaneous injections; Ozempic FDA label; Wegovy FDA label; Mounjaro FDA label; StatPearls: Loading Dose
What do COA, HPLC and endotoxin tests show?
Each test on a certificate of analysis answers its own question, such as whether a sample is the right molecule, how pure it is, or whether it is free of germs and bacterial toxins.
Certificate of analysis (COA)
A lab document for one batch of a product that lists each test run, the limits the batch had to meet and the actual results. Each certificate applies only to the batch it names.
HPLC
High-performance liquid chromatography, a lab method that pushes a dissolved sample through a column at high pressure so its parts separate and can be measured one by one. It is one of the standard methods for checking purity.
More: Peptide testing labs
Mass spectrometry
A lab method that measures charged molecules by their mass-to-charge ratio. Each peptide has a known molecular weight (the tirzepatide label lists 4,813.53 daltons), so the result can help confirm a sample is the expected molecule.
More: Peptide testing labs
Purity
How free a sample is of anything other than the intended substance, such as related impurities or leftover solvents. It is usually reported as a percentage.
Endotoxin
A toxic substance from the outer wall of a group of bacteria called Gram-negative bacteria. Injectable products are tested for it because in the bloodstream it can cause fever and other harmful reactions.
Sterility
Being free of living microorganisms such as bacteria and fungi. US drug rules require lab testing of each batch of a product sold as sterile.
Batch or lot
A set amount of product made in one production run and meant to be uniform throughout. A lot is a batch or a marked part of one, and its number ties the product to its test records.
Third-party testing
Testing done by an outside lab that does not make or sell the product. Studies of supplements describe it as an independent check, for example for banned substances.
More: Peptide testing labs
Sources: FDA guidance: ICH Q7; NCI Thesaurus: Clinical Trial Investigational Product Certificate of Analysis Documentation (C115473); NCI Thesaurus: High Performance Liquid Chromatography (C16434); NCI Thesaurus: Mass Spectrometry (C17156); NCI Thesaurus: Purity Assessment (C62352); NCI Thesaurus: Endotoxin (C50918); NCI Thesaurus: Sterility Test (C134278); NCI Thesaurus: Batch Number (C104504); Mounjaro FDA label; Janvier 2018, falsified peptide drugs; FDA guide: bacterial endotoxins and pyrogens; NCI Dictionary: sterile; 21 CFR 211.167; 21 CFR 210.3; Wardenaar 2025, third-party testing
What do phase 3, off-label and 503B mean?
Evidence about a drug builds in steps, from lab dish to animal studies to three phases of human trials, and the legal terms describe how drugs are approved, prescribed and made.
Research use only
A label stating that a product is meant only for laboratory research, not for diagnosing or treating people. For lab test kits and reagents, FDA guidance says this label means the product is not for use in diagnosing patients.
In vitro
In a lab dish or test tube, outside a living body. Studies on cells grown in a lab are in vitro.
In vivo
Inside a living body, such as an animal or a person. It is the opposite of in vitro.
Animal study
Research done in animals to learn whether a drug is likely to be useful or harmful. Animal studies come before human trials, which are still needed to show whether a drug works and is safe in people.
Randomized controlled trial
A study that splits people into groups by chance and compares a treatment with a placebo or another treatment. Chance tends to make the groups alike and lowers bias, though it does not guarantee they match, so trials also check how similar the groups were at the start.
Placebo
A dummy treatment that looks and is given exactly like the real one but holds no active drug. Comparing the two shows which effects come from the drug itself.
Phase 1
The first tests of a new drug in people, usually 20 to 100 healthy volunteers or patients. The main goals are safety and finding a workable dose range.
Phase 2
A trial in up to several hundred people who have the condition. It tests whether the drug works and what side effects it causes.
Phase 3
A large trial, often 300 to 3,000 people with the condition, that checks whether the drug works and watches for side effects. It is designed to show whether the drug gives a real benefit to a specific group.
Investigational
Describes a drug, dose or use that is still being tested in clinical trials. A drug can be approved for one condition and still be investigational for another.
FDA-approved
Means the US Food and Drug Administration reviewed the data and found that the benefits of a drug outweigh its known and possible risks for its intended use. The approval covers the uses listed on the label.
Prescription-only
A drug supplied only with a prescription from a doctor and meant for the one person it was prescribed for. In the US, the FDA decides whether a drug needs a prescription as part of approving it.
Off-label
Use of an approved drug outside what its label covers, such as for a different condition, by a different route or at a different dose. Doctors may legally prescribe off-label, but the FDA has not found that use safe and effective.
Compounded drug
A medicine mixed or changed by a pharmacist or doctor to fit the needs of one patient. Compounded drugs are not FDA-approved. The agency does not look at whether they are safe, whether they work or how well they are made before they are marketed.
More: Compounded tirzepatide
503A pharmacy
A state-licensed pharmacy that makes compounded drugs for individual patients under section 503A of US drug law. If they meet all the 503A conditions, these pharmacies are exempt from the federal CGMP manufacturing rules that 503B outsourcing facilities must follow.
503B outsourcing facility
A compounding site registered with the FDA under section 503B, which a 2013 law created. Outsourcing facilities must follow current good manufacturing practice (CGMP) rules and face FDA inspections on a risk-based schedule.
Generic
A drug made to match an existing brand-name drug in dose form, strength, route, quality, safety and intended use. The FDA reviews it before approval to make sure it works the same as the brand.
Biosimilar
A biologic medicine that is very close to one the FDA has already approved, with no differences that matter for safety or how well it works. Biologics are made in living cells, so a biosimilar is very close to the original but not an exact copy.
Sources: FDA guidance: research use only labeling; NCI Dictionary: in vitro; NCI Dictionary: in vivo; NCI Dictionary: animal model; NCI Dictionary: preclinical study; NCI Dictionary: randomized clinical trial; NCI Dictionary: placebo; NCI Dictionary: phase I clinical trial; NCI Dictionary: phase II clinical trial; NCI Dictionary: phase III clinical trial; NCI Dictionary: investigational; NCI Dictionary: off-label; NCI Dictionary: biosimilar drug; FDA: preclinical research; FDA: clinical research phases; FDA: what approval means; FDA: prescription and OTC drugs; FDA: off-label use; FDA: human drug compounding; FDA: compounding questions and answers; FDA: outsourcing facilities; FDA: generic drugs; FDA: biosimilar basics
Which glossary terms are easy to mix up?
These pairs look alike but mean different things, and mixing them up changes what a label or lab report is saying.
- Units and international units (IU): a unit on an insulin syringe marks a volume of liquid, while an IU measures how much biological effect a substance has.
- Bacteriostatic water and sterile water: both are germ-free water for injection, but only bacteriostatic water holds a preservative and comes in multiple-dose vials.
- Satiety and satiation: satiation is what ends a meal, while satiety is the fullness that lasts after it.
- Agonist and antagonist: an agonist switches a receptor on, while an antagonist blocks it or stops another substance from acting.
- In vitro and in vivo: in vitro work happens in a dish or test tube, while in vivo work happens in a living animal or person.
- Loading dose and maintenance dose: a loading dose is larger and comes first, while a maintenance dose is the steady amount that follows.
- Purity and identity: a purity figure says how much of a sample is a single substance, while an identity test such as mass spectrometry checks which substance it is.
- 503A and 503B: a 503A pharmacy compounds for individual patients and is exempt from federal CGMP rules only if it meets every 503A condition, while a 503B outsourcing facility registers with the FDA and must follow those rules.
- Generic and biosimilar: a generic matches a brand-name drug, while a biosimilar is a very close but not identical version of a biologic.
- Investigational and off-label: an investigational use is still being tested in trials, while off-label use means an approved drug is used outside its label.
Sources: MedlinePlus: giving an insulin injection; NCI Dictionary: International Unit; Bacteriostatic Water for Injection label; Sterile Water for Injection label; Gibbons 2019, satiety and satiation; NCI Dictionary: agonist; NCI Dictionary: antagonist; NCI Dictionary: in vitro; NCI Dictionary: in vivo; StatPearls: Loading Dose; NCI Thesaurus: Purity Assessment (C62352); NCI Thesaurus: Mass Spectrometry (C17156); NCI Dictionary: investigational; NCI Dictionary: off-label; NCI Dictionary: biosimilar drug; FDA: compounding questions and answers; FDA: outsourcing facilities; FDA: generic drugs; FDA: biosimilar basics; FDA: off-label use
Common questions about peptide terms
What is the difference between a peptide and a protein?+
Both are chains of amino acids. A peptide has two or more and is often shorter, while a protein is one or more long chains folded into a shape that does a job in the body. FDA rules draw the line at 40: a chain longer than 40 amino acids counts as a protein.
What does half-life mean for a drug?+
It is how long it takes for the amount of a drug in the body to fall by half. Each drug has its own figure, and the FDA labels list about one week for semaglutide and about five days for tirzepatide.
What does it mean to reconstitute a peptide?+
It means adding a liquid, called a diluent, to a freeze-dried powder so the powder dissolves. Sterile water and bacteriostatic water are common diluents, and only bacteriostatic water holds a preservative.
What does U-100 mean on a syringe label?+
U-100 means 100 units in each milliliter, so a full 1 mL U-100 syringe holds 100 units. On that scale a unit is a volume of liquid, so how much drug it carries depends on the concentration in the vial.
What does a certificate of analysis (COA) show?+
A COA lists the tests run on one batch of a product, the limit each result had to meet and the result the batch got. It covers only the batch it names.
What does research use only mean on a label?+
It marks a product as meant for laboratory research, not for diagnosing or treating people. For lab test kits and reagents, FDA guidance says the label means the product is not for use in diagnosing patients.
Sources: NCI Dictionary: peptide; NCI Dictionary: protein; 21 CFR 600.3; StatPearls: Elimination Half-Life of Drugs; Ozempic FDA label; Mounjaro FDA label; FDA guide: lyophilization of parenterals; Bacteriostatic Water for Injection label; Sterile Water for Injection label; MedlinePlus: giving an insulin injection; Humulin R FDA label; FDA guidance: ICH Q7; NCI Thesaurus: Clinical Trial Investigational Product Certificate of Analysis Documentation (C115473); FDA guidance: research use only labeling
This glossary is general education about words, not medical advice. A question about a specific medicine or condition is one for a licensed clinician.