Peptide glossary

Plain-English definitions of the terms you'll meet on vial labels, prescription labels, study summaries and this site's calculators. Each entry lists the sources it was checked against.

Definitions describe what a word means. They aren't advice about any product, dose or schedule. For how to take a prescribed medicine, follow its label and your prescriber. Several compounds named here are investigational or not approved by the FDA for any use.

A

Acetate salt

A salt form in which a peptide is paired with acetate ions. The peptide and its acetate salt have different molecular weights, so a drug label may state both the peptide amount and the equivalent salt amount.

In practice: When checking calculator math against a label, confirm whether the stated milligrams refer to the peptide itself or to its salt form.

Related terms: Synthetic peptide, Peptide, Peptide salt form, Counter-ion, Free base

Sources: Sikora et al. (2020) (opens in a new tab); DailyMed: Vyleesi label (opens in a new tab)

Adverse event

An adverse event is any unwanted medical event that happens while someone is using a drug, whether or not the drug caused it. MedWatch is FDA's safety reporting program, where patients, consumers, and health professionals can voluntarily report serious reactions, product quality problems, and product use errors.

In practice: Prescribing labels list FDA's phone number, 1-800-FDA-1088, and www.fda.gov/medwatch for reporting suspected adverse reactions.

Related terms: Boxed warning, Contraindication, Prescribing information, Adverse reaction, Side effect, MedWatch reporting program

Sources: 21 CFR 312.32 (opens in a new tab); FDA: MedWatch reporting (opens in a new tab)

Agonist

A drug or other substance that binds to a receptor and activates it, causing the same kind of action as the natural substance that normally binds there.

In practice: Labels often name a drug's class this way; the Vyleesi label, for example, calls bremelanotide a melanocortin receptor agonist.

Related terms: Antagonist, Receptor, Melanocortin receptor

Sources: NCI: agonist (opens in a new tab); DailyMed: Vyleesi label (opens in a new tab)

Amino acid

A small molecule that serves as a building block of peptides and proteins. There are 20 common amino acids found in proteins, and linking them in different combinations and orders produces different peptides and proteins.

In practice: Peptide labels and studies often describe a compound by how many amino acids it contains and which ones they are.

Related terms: Peptide, Amino acid sequence, Peptide bond

Sources: NHGRI: Amino Acids (opens in a new tab); NCI: amino acid (opens in a new tab)

Amino acid sequence (primary structure, peptide sequence)

The order in which amino acids are linked along a peptide or protein chain, also called its primary structure. The sequence is a key factor in the molecule's three-dimensional shape and therefore how it works.

In practice: A sequence identifies a molecule’s structure; it does not tell you how many milligrams are in a vial or what volume to draw.

Related terms: Amino acid, Peptide, Polypeptide

Sources: MeSH: Amino Acid Sequence (opens in a new tab)

Analog (drug analog, analogue)

In chemistry, a substance that is similar to another but not identical. Many peptide drugs are analogs of natural hormones: the FDA label for setmelanotide (Imcivree), for example, describes it as a cyclic peptide analog of the body's own alpha-MSH.

In practice: An analog is a different compound from the natural molecule it resembles, with its own name, label, and properties.

Related terms: Melanocortin, Synthetic peptide

Sources: NCI: analog (opens in a new tab); DailyMed: Imcivree label (opens in a new tab)

Antagonist (blocker)

A substance that stops or blocks the action or effect of another substance. Many antagonist drugs act at a receptor, preventing a natural messenger or another drug from producing its usual effect there.

In practice: An antagonist blocks a receptor’s action; an agonist activates it.

Related terms: Agonist, Receptor

Sources: NCI: antagonist (opens in a new tab)

Aseptic technique (asepsis)

Practices used when handling and preparing medicines, syringes and needles to prevent contamination by germs. CDC recommends aseptic technique whenever injections are prepared and given.

In practice: Contamination during mixing or drawing up can cause infection, regardless of whether the dose math is correct.

Related terms: Reconstitution, Multiple-dose vial

Sources: WHO: aseptic technique guidelines (opens in a new tab); CDC: injection safety (opens in a new tab)

B

Bacteriostatic water for injection (BWFI, bacteriostatic water)

Sterile water that contains benzyl alcohol as a preservative, commonly 0.9% (one FDA-labeled presentation contains 1.1%). It comes in multiple-dose vials and is labeled only for diluting or dissolving drugs. Its label warns it is not for use in neonates because benzyl alcohol has been linked to toxicity in newborns.

In practice: The preservative is what allows repeated withdrawals from one vial, but it also limits who the diluent is suitable for.

Sources: DailyMed: Bacteriostatic Water label (opens in a new tab); DailyMed: Bacteriostatic Water label (opens in a new tab)

Beyond-use date (BUD)

The date, or date and hour, after which a compounded or opened preparation must not be used, counted from when it was prepared or first entered. An expiration date is different: the manufacturer assigns it to the unopened product. A beyond-use date should never be later than the original expiration date.

In practice: A mixed or opened vial can reach its beyond-use date long before the expiration date printed on the box.

Related terms: Multiple-dose vial, Reconstitution

Sources: USP: beyond-use dates (opens in a new tab); CDC: injection safety (opens in a new tab)

Bioavailability

Bioavailability is how much of a dose reaches the bloodstream, and how fast, in a form the body can use. It depends partly on the route. The Ozempic label lists the absolute bioavailability of semaglutide under the skin as 89%.

In practice: Two routes with the same dose on paper can deliver different amounts of drug to the blood.

Related terms: Pharmacokinetics, Route of administration

Sources: Merck Manual: Drug Bioavailability (opens in a new tab); DailyMed: Ozempic label (opens in a new tab)

Biomarker (biological marker, molecular marker)

A measurable substance in blood, other body fluids, or tissue that signals a normal process, an abnormal process, or a disease. Biomarkers can also show how the body is responding to a treatment.

In practice: Studies and lab reports track biomarkers such as HbA1c, hs-CRP, and IGF-1 to measure change over time.

Sources: NCI: biomarker (opens in a new tab)

Body composition

The makeup of the body in terms of fat mass and fat-free mass, such as muscle, bone, and water. Body weight and BMI alone cannot show how much of a person's weight is fat versus lean tissue.

In practice: Some labels report body composition, not just weight; the Zepbound label, for example, states it lowers fat mass more than lean mass.

Related terms: Visceral fat, Biomarker, Fat mass, Lean mass, Fat-free mass

Sources: Holmes et al. (2021) (opens in a new tab); DailyMed: Zepbound label (opens in a new tab)

Boxed warning (black box warning)

A boxed warning is a warning printed inside a box at the top of a prescribing label. FDA may require it for serious risks, especially ones that can lead to death or serious injury. Ozempic and Zepbound labels both carry a boxed warning about the risk of thyroid C-cell tumors.

In practice: It is the most prominent safety warning on a U.S. prescription drug label.

Sources: 21 CFR 201.57 (opens in a new tab); DailyMed: Zepbound label (opens in a new tab)

C

Clinical trial phases

Drugs are tested in people in phases. Phase 1 studies safety and dosage in about 20 to 100 people. Phase 2 looks at effectiveness and side effects in up to several hundred people. Phase 3 checks effectiveness and watches for adverse reactions in 300 to 3,000 people.

In practice: A Phase 1 result is early safety data, not proof that a drug works.

Related terms: Investigational drug, Placebo, Washout period, Phase 1, Phase 2, Phase 3

Sources: FDA: clinical trial phases (opens in a new tab)

Compounded drug

A compounded drug is a medicine mixed or changed by a pharmacy or outsourcing facility, often for a patient who cannot use an FDA-approved product. Compounded drugs are not FDA-approved, and FDA does not check their safety, effectiveness, or quality before they are sold.

In practice: FDA has reported dosing errors with compounded GLP-1 drugs when patients measured and gave themselves the wrong amount.

Sources: FDA: compounding questions and answers (opens in a new tab); FDA: unapproved GLP-1 drugs (opens in a new tab)

Compounding pharmacy (503A) (503A compounder, traditional compounding pharmacy)

Under section 503A of federal drug law, a licensed pharmacist in a state-licensed pharmacy or federal facility, or a physician, may compound drugs based on valid prescriptions for individual patients. These compounders are exempt from FDA approval and federal manufacturing-quality (CGMP) rules.

In practice: A drug from a 503A pharmacy is made for a named patient and is not an FDA-approved product.

Sources: FDA: compounding laws (opens in a new tab); FDA: compounding questions and answers (opens in a new tab)

Concentration (strength, mg/mL)

The amount of drug dissolved in each unit of liquid, often written as milligrams per milliliter (mg/mL). For example, an FDA label may say that mixing a vial with a stated volume of diluent gives a solution of 8 mg per mL.

In practice: Concentration links a dose by weight to a volume to draw up; it changes whenever the amount of diluent changes.

Sources: DailyMed: Egrifta WR label (opens in a new tab)

Contraindication

A contraindication is a situation in which a drug should not be used because the risk clearly outweighs any possible benefit. For example, the Ozempic label says it is contraindicated in people with a personal or family history of medullary thyroid carcinoma (MTC) or with MEN 2.

In practice: Contraindications appear in their own section of the prescribing label.

Sources: 21 CFR 201.57 (opens in a new tab); DailyMed: Ozempic label (opens in a new tab)

D

DAC (Drug Affinity Complex) (Drug Affinity Complex)

A chemical add-on that lets a peptide attach to albumin, a protein in the blood, so it stays in circulation much longer. It is best known from CJC-1295, a long-acting GHRH analog that is not FDA-approved.

In practice: "With DAC" and "without DAC" describe different molecules with very different durations in the body, not two strengths of one product.

Related terms: GHRH analog, GHRH (growth hormone-releasing hormone), CJC-1295 with DAC

Sources: Jetté et al. (2005) (opens in a new tab); Dominikowski et al. (2026) (opens in a new tab)

Dead space (syringe dead space, residual volume)

The small amount of liquid left inside the syringe hub and needle after the plunger is pushed all the way down. Syringes with a permanently attached needle usually have less dead space than syringes with detachable needles.

In practice: Dead space is not delivered, and researchers note it can make very small injection volumes less precise.

Related terms: Insulin syringe

Sources: Trickey et al. (2022) (opens in a new tab); Krisdiyanto et al. (2022) (opens in a new tab)

Diluent (solvent, mixing liquid)

The sterile liquid used to dissolve or dilute a drug before injection. Common diluents include sterile water for injection and bacteriostatic water for injection. A product's label names which diluent may be used.

In practice: Using a different diluent than the label names can change the product's stability or how long it may be kept after mixing.

Sources: DailyMed: Bacteriostatic Water label (opens in a new tab); DailyMed: Egrifta WR label (opens in a new tab)

Displacement volume (powder volume, displacement value)

The extra volume a powder adds once it dissolves in a diluent. Because of it, the final solution volume can be larger than the diluent volume added. In one published example, 10 mL of water added to a 2 g ceftriaxone powder gives 11.37 mL.

In practice: Ignoring displacement can make the true concentration lower than a simple drug-amount-divided-by-diluent calculation suggests.

Sources: Kumwenda et al. (2024) (opens in a new tab)

Dose

A dose is one measured amount of a medicine that is taken or given at a single time.

In practice: A prescription or study states the dose by weight, but a syringe measures volume, so a calculator converts one into the other.

Related terms: Titration, Prescribing information

Sources: NCI: dose (opens in a new tab); EUPATI: Dose (opens in a new tab)

Downstream effect (signaling cascade, downstream signaling)

A result produced at a later step of a signaling pathway. Once a receptor is activated, the first molecule in the pathway activates another, and that relay repeats step by step, like a cascade, until the cell carries out its final response. Steps and results further along the chain are called downstream.

In practice: Papers may report a downstream marker changing in cells or animals; that is a lab finding, not evidence that a compound works in people.

Related terms: Signaling pathway, Receptor

Sources: NCI: signaling pathway (opens in a new tab)

Dual agonist (dual GIP/GLP-1 receptor agonist, twincretin)

A single medicine that activates two different hormone receptors. Tirzepatide, sold as Mounjaro and Zepbound, is FDA-approved and activates both the GIP and GLP-1 receptors.

In practice: Labels and studies compare single, dual, and triple agonists; the term tells you how many hormone pathways a drug is designed to act on.

Sources: DailyMed: Zepbound label (opens in a new tab); DailyMed: Mounjaro label (opens in a new tab)

E

Expiration date (expiry date)

The date a manufacturer prints on an unopened product, after which it should not be used. FDA calls it a key part of deciding whether a product is safe and will work as intended. Once a product is opened or mixed, a shorter beyond-use date may apply.

In practice: The printed expiration date applies to the product as packaged, not after mixing or first puncture.

Related terms: Beyond-use date

Sources: FDA: medicine expiration dates (opens in a new tab); CDC: injection safety (opens in a new tab)

F

FDA-approved

An FDA-approved drug has been reviewed by FDA, which found that its benefits outweigh its potential risks for a specific use. Approval does not mean the drug has no side effects. In the U.S., a new drug must be tested and reviewed before it can be sold.

In practice: Compounded drugs and products labeled for research are not FDA-approved, even if they contain the same active ingredient as an approved drug.

Sources: FDA: off-label use (opens in a new tab); FDA: drug approval process (opens in a new tab)

Final volume (final solution volume, total volume)

The total volume of liquid in a vial after a powder is fully dissolved. It equals the diluent added plus any displacement volume from the powder, and it is the number used to work out the final concentration.

In practice: A calculator that treats the diluent volume as the final volume is ignoring displacement, which can matter when the powder takes up noticeable space.

Sources: Kumwenda et al. (2024) (opens in a new tab)

G

GHRH (growth hormone-releasing hormone) (GHRH, GRF, GHRF, growth hormone-releasing factor)

A peptide hormone made in the hypothalamus, a brain region, that signals the pituitary gland to make and release growth hormone in pulses.

In practice: Lab-made versions of GHRH, called GHRH analogs, include the FDA-approved drug tesamorelin; knowing the parent hormone explains what they are designed to do.

Related terms: GHRH analog, Growth hormone (GH)

Sources: DailyMed: Egrifta WR label (opens in a new tab)

GHRH analog (GHRH analogue, GRF analog)

A lab-made peptide modeled on GHRH that binds the same pituitary receptor and prompts the body to release its own growth hormone. Tesamorelin (Egrifta) is FDA-approved to reduce excess abdominal fat in adults with HIV who have lipodystrophy.

In practice: The Egrifta label notes it is not indicated for weight loss; a label's indication section tells you exactly what a drug is approved for.

Sources: DailyMed: Egrifta WR label (opens in a new tab)

GHRP (growth hormone-releasing peptide) (GHRP)

A short, lab-made peptide that triggers growth hormone release by activating the ghrelin receptor (also called the growth hormone secretagogue receptor). Ghrelin is the body's own hormone for this receptor; it is made mainly in the stomach. This is a separate pathway from GHRH.

In practice: Studies sort growth hormone-related peptides by receptor; knowing whether a compound works through the ghrelin receptor or the GHRH receptor helps you read them.

Related terms: Growth hormone secretagogue, GHRH (growth hormone-releasing hormone), Growth hormone (GH), Ghrelin receptor agonist, GHS-R1a (growth hormone secretagogue receptor)

Sources: Dominikowski et al. (2026) (opens in a new tab); Kojima et al. (1999) (opens in a new tab)

GIP (glucose-dependent insulinotropic polypeptide) (GIP, gastric inhibitory polypeptide)

A hormone made by cells in the upper small intestine after eating. Like GLP-1, it is an incretin: it boosts insulin release from the pancreas when blood sugar is raised.

In practice: Tirzepatide (Mounjaro, Zepbound) activates the GIP receptor as well as the GLP-1 receptor, which is why its label names both hormones.

Sources: Nauck et al. (2018) (opens in a new tab); DailyMed: Zepbound label (opens in a new tab)

GLP-1 (glucagon-like peptide-1) (GLP-1, glucagon-like peptide 1)

A hormone released by cells in the lower gut after a meal. It helps the pancreas release insulin when blood sugar is high, curbs glucagon release, and acts in brain areas that control appetite.

In practice: GLP-1 is the natural hormone that semaglutide and similar medicines are built to mimic, so it appears throughout their labels and studies.

Sources: Nauck et al. (2018) (opens in a new tab); DailyMed: Wegovy label (opens in a new tab)

GLP-1 receptor agonist (GLP-1 RA, GLP-1 agonist)

A medicine that binds to and switches on the same receptor as the body's own GLP-1. Semaglutide, sold as Wegovy and Ozempic, is one example. It is built to last much longer in the body than natural GLP-1, mainly because it binds to the blood protein albumin.

In practice: This drug class is the reason many people meet the terms on this page; labels for these products describe how they work and list their warnings.

Sources: DailyMed: Ozempic label (opens in a new tab); DailyMed: Wegovy label (opens in a new tab)

Glucagon

A hormone made by the pancreas that raises blood sugar. It works in opposition to insulin. GLP-1 receptor agonists such as semaglutide lower glucagon release when blood sugar is high.

In practice: Glucagon is one of the three receptor targets of the investigational drug retatrutide, and glucagon effects appear in GLP-1 medicine labels.

Sources: NCI: glucagon (opens in a new tab); DailyMed: Ozempic label (opens in a new tab)

Growth hormone (GH) (GH, human growth hormone, hGH, somatotropin)

A protein hormone made by the pituitary gland at the base of the brain. It drives growth in children and helps control metabolism, including how the body uses glucose and fat.

In practice: Many peptides discussed online are described by their effect on GH, so the term shows up on study summaries and lab reports.

Sources: MedlinePlus: Growth Hormone Tests (opens in a new tab); NCI: growth hormone (opens in a new tab)

Growth hormone secretagogue (GHS, GH secretagogue)

Any substance that causes the pituitary gland to release growth hormone. The term usually refers to compounds acting on the ghrelin receptor, including growth hormone-releasing peptides and small-molecule drugs taken by mouth.

In practice: "Secretagogue" means "causes release." A secretagogue prompts the body to release its own hormone instead of supplying the hormone itself.

Sources: Kojima et al. (1999) (opens in a new tab); Sigalos et al. (2018) (opens in a new tab)

H

Half-life (elimination half-life, t½)

Half-life is the time it takes for the amount of a drug in the body to fall by half. With a 1-hour half-life, half is left after 1 hour and a quarter after 2 hours. The FDA label for semaglutide (Ozempic) lists a half-life of about 1 week.

In practice: Half-life helps explain how long a drug stays in the body. The Ozempic label says semaglutide remains in the blood for about 5 weeks after the last dose.

Related terms: Steady state, Pharmacokinetics

Sources: Merck Manual: Drug Metabolism (opens in a new tab); DailyMed: Ozempic label (opens in a new tab)

HbA1c (A1C) (A1C, hemoglobin A1C, glycated hemoglobin, glycohemoglobin)

A blood test that reflects average blood glucose over roughly the past three months. The result is the percentage of hemoglobin with glucose attached. Hemoglobin is the oxygen-carrying protein in red blood cells.

In practice: A1C is a main result reported in diabetes drug labels and trials, including those for semaglutide and tirzepatide.

Related terms: Biomarker, GLP-1 receptor agonist

Sources: NIDDK: the A1C test (opens in a new tab)

hs-CRP (high-sensitivity C-reactive protein) (hs-CRP, high-sensitivity CRP)

C-reactive protein is a protein the liver makes in response to inflammation. The high-sensitivity test picks up very small increases in CRP and is used to help estimate a person's risk of heart disease.

In practice: Studies of weight-loss and metabolic drugs often report hs-CRP as a marker of inflammation; it is a blood test, not a diagnosis by itself.

Related terms: Biomarker, C-reactive protein

Sources: MedlinePlus: C-Reactive Protein (CRP) Test (opens in a new tab)

I

IGF-1 (insulin-like growth factor 1, somatomedin C)

A hormone, made in the liver and other tissues, that carries out many of growth hormone's effects. Its blood level stays steadier through the day than growth hormone's, so it is a common way to track growth hormone activity in lab tests.

In practice: Labels and studies often report IGF-1 changes to show a drug's effect on the growth hormone system; the tesamorelin label lists IGF-1 increases.

Related terms: Growth hormone (GH), Biomarker

Sources: MedlinePlus: Growth Hormone Tests (opens in a new tab); DailyMed: Egrifta WR label (opens in a new tab)

Incretin (incretin hormone)

A gut hormone released after eating that increases insulin release. GIP and GLP-1 are the two known incretins. Because of them, glucose taken by mouth triggers a larger insulin response than the same glucose given into a vein.

In practice: GLP-1 and GIP medicines are often called incretin-based therapies, so the word shows up on labels, in studies, and in news coverage.

Sources: Nauck et al. (2018) (opens in a new tab)

Indication (approved use, Indications and Usage)

An indication is a disease or condition that a drug is approved to treat, prevent, or diagnose. FDA rules require each prescribing label to list its indications in a section called Indications and Usage.

In practice: Using a drug for something not listed as an indication is called off-label use.

Sources: 21 CFR 201.57 (opens in a new tab); FDA: off-label use (opens in a new tab)

Injection site rotation (site rotation)

Changing the exact spot used for each injection rather than using the same place repeatedly. Insulin labeling says to rotate sites within the same body region to lower the risk of lipodystrophy, and a 2023 expert review suggests at least 1 cm between successive injections.

In practice: Repeated injections into one spot are a main risk factor for lipohypertrophy.

Related terms: Lipohypertrophy

Sources: DailyMed: Humulin R label (opens in a new tab); Tian et al. (2023) (opens in a new tab)

Insulin syringe

A small syringe with a fine needle, marked in units instead of milliliters. Common barrel sizes are 0.3 mL, 0.5 mL, and 1 mL, which hold up to 30, 50, and 100 units on the U-100 scale.

In practice: The barrel size sets the largest volume one syringe can hold; the unit marks are the same U-100 scale on all three.

Related terms: Syringe units, U-100, Needle gauge, Dead space, Peptide calculator, U-100 syringe

Sources: McKee et al. (2023) (opens in a new tab); MedlinePlus: Giving an insulin injection (opens in a new tab)

International unit (IU)

A unit that measures a substance by its biological activity rather than its weight. Each substance has its own agreed definition of one unit, so IU cannot be converted to mg or mcg without a factor specific to that substance. For example, FDA uses 1 mcg = 40 IU for vitamin D only.

In practice: A unit of one product is not a unit of another; ISMP also advises writing "units" rather than "IU" to avoid misreading.

Related terms: Milligram, Microgram, Syringe units

Sources: NCI: international unit (opens in a new tab); FDA: vitamin unit conversions (opens in a new tab)

Intramuscular (IM)

An intramuscular injection puts a drug into a muscle, usually of the upper arm, thigh, or buttock. It is generally chosen over the subcutaneous route when a larger volume of drug is needed.

In practice: The route printed on a label matters: a product approved for injection under the skin is not automatically approved for muscle.

Sources: Merck Manual: Drug Administration (opens in a new tab)

Intranasal (nasal route)

Intranasal means given through the nose. The drug is turned into a fine mist so it can be breathed in and absorbed through the thin lining of the nasal passages.

Related terms: Route of administration, Bioavailability

Sources: Merck Manual: Drug Administration (opens in a new tab)

Investigational drug (investigational new drug, experimental drug)

A drug being studied in people. It may be a drug without FDA approval, or an approved drug being studied for a new use or in a different patient population.

In practice: “Investigational” describes the use being studied. It does not, by itself, tell you whether the drug is approved for other uses.

Sources: FDA: investigational new drug applications (opens in a new tab)

Investigational New Drug application (IND, IND application)

A submission to FDA for authorization to study a drug in people under investigational drug requirements. An IND is not FDA approval to market the drug.

In practice: An IND can cover an unapproved drug or research on an approved drug for a new use.

Sources: FDA: investigational new drug applications (opens in a new tab)

L

Lipohypertrophy (LH)

An area of enlarged, thickened fatty tissue under the skin linked to repeated injections, sometimes felt as a lump. Main risk factors include not rotating sites, injecting into a small area, and reusing needles. Medicine injected into these areas can be absorbed less and less predictably.

In practice: Injection site problems can change how much drug is absorbed, so labels advise avoiding these areas.

Related terms: Injection site rotation

Sources: Tian et al. (2023) (opens in a new tab); DailyMed: Humulin R label (opens in a new tab)

Lyophilized (freeze-dried, lyophilization)

Freeze-dried. Lyophilization removes water from a frozen product by turning the ice directly into vapor. Drugs are often made this way because they are unstable in liquid form, and the dry powder must be reconstituted before use.

In practice: A lyophilized vial holds powder, not liquid, so its concentration does not exist until a diluent is added.

Related terms: Reconstitution, Diluent

Sources: FDA: lyophilization (opens in a new tab)

M

Melanocortin (melanocortin peptides)

A family of peptide hormones cut from a larger precursor protein called POMC (pro-opiomelanocortin). The family includes ACTH and alpha-MSH. Melanocortins act through melanocortin receptors to help control skin pigmentation, feeding and energy balance, and steroid hormone production.

Related terms: Melanocortin receptor, Analog, Peptide

Sources: MeSH: Melanocortins (opens in a new tab)

Melanocortin receptor (MCR)

One of a family of receptors, labeled MC1R through MC5R, that respond to melanocortin hormones. FDA-approved examples that act on them: bremelanotide (Vyleesi) activates several subtypes, and setmelanotide (Imcivree) acts mainly at MC4R, which in the brain is involved in hunger, fullness, and energy use.

In practice: Both labels note that activating MC1R on pigment cells can increase skin pigmentation, which is why a drug's receptor targets matter.

Related terms: Melanocortin, Agonist, Receptor, MC1R, MC3R, MC4R

Sources: DailyMed: Vyleesi label (opens in a new tab); DailyMed: Imcivree label (opens in a new tab)

Microgram (mcg, µg)

A metric unit of mass equal to one-millionth of a gram, or one-thousandth of a milligram (1 mg = 1,000 mcg). Safety groups advise writing "mcg" rather than the symbol "µg," because a handwritten or misread "µg" can be mistaken for "mg."

In practice: Confusing micrograms with milligrams changes the amount by a factor of 1,000, so check which unit a label or order uses.

Related terms: Milligram, Peptide calculator

Sources: NCI: microgram (opens in a new tab); ISMP: error-prone abbreviations (opens in a new tab)

Milligram (mg)

A metric unit of mass equal to one-thousandth of a gram. One milligram equals 1,000 micrograms (mcg). Peptide and drug vials often state their total contents in milligrams.

In practice: A dose calculator needs the vial amount and the dose in the same unit; mixing up mg and mcg causes a 1,000-fold error.

Related terms: Microgram, Concentration, Peptide calculator

Sources: NIST: Metric (SI) Prefixes (opens in a new tab); NCI: milligram (opens in a new tab)

Milliliter (mL, cc, cubic centimeter)

A metric unit of volume equal to one-thousandth of a liter. One milliliter is the same as one cubic centimeter (1 mL = 1 cc). Medication safety groups advise writing "mL" instead of "cc," since "cc" can be misread as "u" for units.

In practice: Syringe barrels, diluent volumes, and concentrations are all expressed per milliliter.

Related terms: Concentration, Syringe units

Sources: NCI: milliliter (opens in a new tab); ISMP: error-prone abbreviations (opens in a new tab)

Mitochondrial-derived peptide (MDP)

A small peptide encoded by a short stretch of DNA inside mitochondria, the cell's energy-producing structures. Humanin, discovered in 2001, was the first; MOTS-c is encoded in the mitochondrial 12S rRNA gene. Research is early, and much of the human evidence so far is correlational.

In practice: These are research compounds. FDA has said it has not identified any human exposure data on drug products containing MOTS-c.

Related terms: Peptide, Signaling pathway, Humanin, MOTS-c

Sources: Kim et al. (2021) (opens in a new tab); FDA: compounding safety risks (opens in a new tab)

Multiple-dose vial (MDV, multi-dose vial)

A vial that contains a preservative and is made to be entered more than once. Under USP <797> and CDC guidance, once it is first punctured it should be dated and discarded within 28 days, unless the manufacturer states a different date.

In practice: The 28-day clock starts at first puncture, not at the printed expiration date.

Sources: CDC: injection safety (opens in a new tab); USP: <797> FAQs (opens in a new tab)

N

Needle gauge (gauge, G)

A number that describes a needle's thickness. The higher the gauge number, the thinner the needle. Needle length is listed separately; insulin syringe needles are commonly 5, 6, 8, or 12.7 mm (about 3/16 to 1/2 inch) long.

In practice: Syringe packages list both gauge and length, such as "31G x 6 mm," and the two numbers mean different things.

Related terms: Insulin syringe

Sources: McKee et al. (2023) (opens in a new tab)

Nootropic (smart drug, cognitive enhancer)

A term first used by Cornelius E. Giurgea in 1972–1973 for substances that mainly boost mental functions such as memory and learning, especially when those functions are impaired. Today the label, also written as "smart drugs," covers a mixed group of compounds.

In practice: The word describes what a substance is meant to do; on its own it does not show that a product has been tested or approved.

Sources: Malík et al. (2022) (opens in a new tab)

O

Off-label use (unapproved use of an approved drug)

Off-label use means using an FDA-approved drug in a way its label does not cover, such as for a different condition, dose, or patient group.

In practice: FDA approval applies to the uses on the label, not to every way a drug might be used.

Sources: FDA: off-label use (opens in a new tab)

Outsourcing facility (503B) (503B compounder)

An outsourcing facility is a site that compounds sterile drugs and has chosen to register with FDA under section 503B. It must meet federal manufacturing-quality (CGMP) rules and is inspected by FDA. It may fill patient-specific prescriptions or supply clinics with office stock.

In practice: 503B drugs are held to stricter quality rules than 503A drugs, but they still are not FDA-approved.

Sources: FDA: outsourcing facilities (opens in a new tab); FDA: compounding laws (opens in a new tab)

P

Peptide

A molecule made of two or more amino acids joined by peptide bonds. Peptides are short chains, typically about 2 to 50 amino acids long; longer chains are called polypeptides or proteins.

In practice: Most compounds that people run through a reconstitution calculator are peptides, so this word frames much of the other vocabulary on vial and prescription labels.

Sources: NHGRI: Peptide (opens in a new tab); NCI: peptide (opens in a new tab)

Peptide bond (amide bond)

The chemical link that joins one amino acid to the next in a peptide chain. It forms when the carboxyl group of one amino acid reacts with the amino group of another, releasing a molecule of water. It is a type of amide bond.

In practice: Drug labels describe peptide breakdown in the body as hydrolysis of these bonds; the Vyleesi label describes bremelanotide's metabolism this way.

Related terms: Amino acid, Peptide

Sources: StatPearls: Protein Structure and Function (opens in a new tab); DailyMed: Vyleesi label (opens in a new tab)

Pharmacokinetics (PK)

Pharmacokinetics is the study of how a drug moves into, through, and out of the body over time. It covers absorption, bioavailability, distribution, metabolism, and excretion.

In practice: The Pharmacokinetics section of a prescribing label is where half-life, bioavailability, and time to steady state are reported.

Related terms: Half-life, Bioavailability, Steady state

Sources: Merck Manual: Overview of Pharmacokinetics (opens in a new tab)

Placebo (sham)

A placebo is an inactive substance or treatment that looks like the real therapy and is given the same way. Researchers compare results in people who got the drug with results in people who got the placebo.

In practice: A "placebo-controlled" study shows whether a drug did better than an inactive look-alike.

Related terms: Clinical trial phases, Washout period

Sources: NCATS: Placebo - Toolkit (opens in a new tab)

Polypeptide

A longer chain of amino acids linked by peptide bonds. Proteins are built from one or more polypeptides. The size cutoffs between peptide, polypeptide, and protein are conventions that differ by source, but chains of more than about 50 amino acids are generally treated as polypeptides or proteins.

In practice: Sources draw the peptide/protein line differently, so the same molecule may be called either one in different papers.

Related terms: Peptide, Amino acid sequence, Peptide versus protein

Sources: NHGRI: Peptide (opens in a new tab); StatPearls: Protein Structure and Function (opens in a new tab)

Prescribing information (PI, package insert, drug label, USPI)

Prescribing information is the FDA-approved label written for health care professionals. It summarizes the key scientific facts needed to use a drug safely and effectively. It is also known as the package insert, and current versions are posted on Drugs@FDA and NIH's DailyMed.

In practice: It is the official source for a drug's approved uses, dosing, warnings, and pharmacokinetics.

Related terms: Indication, Boxed warning, Contraindication

Sources: FDA: prescription labeling (opens in a new tab); FDA: prescribing information (opens in a new tab)

R

Receptor

A molecule on the surface of a cell or inside it that binds a specific substance, such as a hormone, neurotransmitter, or drug, and then sets off a specific effect in the cell.

In practice: Drug labels explain how a medicine works by naming the receptor it acts on.

Related terms: Agonist, Antagonist, Signaling pathway

Sources: NCI: receptor (opens in a new tab); NCI: signal transduction (opens in a new tab)

Reconstitution (reconstitute, mixing)

Adding a measured volume of liquid (a diluent) to a dry powder drug so it dissolves into a solution. FDA labels for powder products state which diluent to use and how much.

In practice: The diluent volume chosen during reconstitution sets the concentration, which every later dose calculation depends on.

Sources: FDA: lyophilization (opens in a new tab); DailyMed: Egrifta WR label (opens in a new tab)

Research use only (RUO) (RUO, for research purposes, not for human consumption)

"Research use only" and similar phrases, such as "for research purposes" or "not for human consumption," say a product is not meant for use in people. FDA has warned that some unapproved GLP-1 products sold to the public carry these labels falsely.

In practice: An RUO label is not a sign of FDA approval, and the product has not been through FDA review for use in people.

Sources: FDA: unapproved GLP-1 drugs (opens in a new tab)

Route of administration

The route of administration is the way a drug enters the body, such as by mouth, by injection (into a vein, muscle, or the fat under the skin), through the nose, by inhalation, or on the skin. Each route has its own uses, advantages, and drawbacks.

In practice: A label names the approved route; the same drug can act differently, or not be approved, by another route.

Sources: Merck Manual: Drug Administration (opens in a new tab)

S

Satiety (fullness)

The reduced hunger that follows eating. A related term, satiation, is the feeling of fullness that builds during a meal.

In practice: GLP-1 medicine labels describe effects on appetite and food intake; satiety is the plain-language idea behind those statements.

Related terms: GLP-1 (glucagon-like peptide-1), Satiation

Sources: Rakha et al. (2022) (opens in a new tab)

Secretagogue

A substance that stimulates the release (secretion) of another substance, such as a hormone. For example, macimorelin (Macrilen), an FDA-approved drug used to diagnose adult growth hormone deficiency, triggers growth hormone release by activating growth hormone secretagogue receptors in the pituitary and hypothalamus.

Related terms: Agonist, Receptor

Sources: MeSH: Secretagogues (opens in a new tab); DailyMed: Macrilen label (opens in a new tab)

Sharps disposal

Safe disposal of needles, syringes, and other items that can puncture skin. FDA says to put used sharps right away into an FDA-cleared sharps container, made of puncture-resistant plastic. If one is not available, FDA says a heavy-duty plastic household container, such as a laundry detergent bottle, can be used.

In practice: Used needles can puncture skin and spread infection, so FDA advises a dedicated container rather than loose disposal.

Related terms: Insulin syringe, Sharps container

Sources: FDA: sharps disposal (opens in a new tab)

Signaling pathway (cell signaling, signal transduction)

A series of chemical steps in which molecules inside a cell work together to pass along a message. It usually starts when a substance such as a hormone binds a receptor, and it ends with a specific cell response, such as cell growth, division, or death.

In practice: Study summaries often describe a compound by the pathway it is thought to affect, which is a description of mechanism, not proof of a clinical benefit.

Related terms: Receptor, Downstream effect

Sources: NCI: signaling pathway (opens in a new tab); NCI: signal transduction (opens in a new tab)

Single-dose vial (SDV, single-use vial)

A vial meant for one patient and one use, usually without a preservative. CDC guidance says leftover contents should not be saved or combined for later use.

In practice: A single-dose vial label is a signal that leftover liquid is meant to be discarded, not stored.

Related terms: Multiple-dose vial, Sterile water for injection, Single-patient-use vial

Sources: CDC: injection safety (opens in a new tab); DailyMed: Sterile Water label (opens in a new tab)

Steady state (steady-state exposure)

Steady state is the point during regular, repeated dosing when drug levels in the blood stop climbing and stay in a consistent range from dose to dose. Labels often say how long this takes. The Ozempic label says steady state is reached after 4 to 5 weeks of once-weekly dosing.

In practice: Drugs with long half-lives take weeks to level off, which is one reason labels describe effects over weeks rather than days.

Related terms: Half-life, Pharmacokinetics

Sources: StatPearls: Elimination Half-Life of Drugs (opens in a new tab); DailyMed: Ozempic label (opens in a new tab)

Sterile water for injection (SWFI, WFI)

Sterile water with no preservative, antimicrobial agent, or added buffer. It is labeled only for diluting or dissolving drugs and comes in single-dose containers; any unused portion is discarded. The label warns that injecting it into a vein without a solute may cause hemolysis.

In practice: Because it has no preservative, a vial is meant for one use, unlike bacteriostatic water.

Sources: DailyMed: Sterile Water label (opens in a new tab)

Subcutaneous injection (SubQ, SC, under the skin)

A subcutaneous injection puts a drug into the fatty tissue just under the skin. The drug then passes into small blood vessels and is carried through the body. This route is often used for protein drugs that would be broken down in the digestive tract if swallowed.

In practice: FDA labels for semaglutide (Ozempic) and tirzepatide (Zepbound) describe the drug as injected subcutaneously.

Sources: Merck Manual: Drug Administration (opens in a new tab)

Synthetic peptide

A peptide made by chemical synthesis rather than taken from a natural source. The main method, solid-phase peptide synthesis, was developed by Merrifield in 1963. Some FDA-approved drugs are synthetic peptides; the Vyleesi label, for example, describes bremelanotide as a synthetic, cyclic peptide of seven amino acids.

In practice: A synthetic peptide can be a copy of a natural one or a modified analog, so the name on the label identifies which exact molecule it is.

Related terms: Peptide, Analog, Acetate salt

Sources: Wang et al. (2022) (opens in a new tab); DailyMed: Vyleesi label (opens in a new tab)

Syringe units (unit markings)

The scale on an insulin syringe. On a U-100 syringe, 100 units represents 1 mL, so 1 unit represents 0.01 mL. For non-insulin medicines, this scale tells you the volume drawn; the dose depends on the solution’s concentration.

In practice: Check the value of each small line on your syringe. One line is not always 1 unit. Syringe capacity and the spacing of its markings are separate from the volume represented by 1 U-100 unit.

Sources: MedlinePlus: Giving an insulin injection (opens in a new tab); FDA: syringe units and dosing errors (opens in a new tab)

T

Telomerase

An enzyme that adds DNA to telomeres, the ends of chromosomes. Telomeres lose a little DNA each time a cell divides, and telomerase helps offset that loss. Cancer cells usually have more telomerase than most normal cells.

In practice: Telomerase is a common topic in aging and cancer research, so it often appears in study summaries and product claims.

Sources: NCI: telomerase (opens in a new tab); MeSH: Telomerase (opens in a new tab)

Titration (dose escalation, dose titration)

Titration means adjusting a dose step by step over time. FDA labels for semaglutide (Ozempic) and tirzepatide (Zepbound) begin with a lower dose meant only for starting treatment, then raise it in steps after at least 4 weeks, partly to lower the risk of stomach and gut side effects.

In practice: In a titration schedule the dose changes, so the volume drawn into the syringe changes too, even from the same vial.

Sources: DailyMed: Ozempic label (opens in a new tab); DailyMed: Zepbound label (opens in a new tab)

Topical (cutaneous)

A topical drug is applied to the skin. Drugs given this way are usually meant to act locally, where they are applied, rather than throughout the body.

Related terms: Route of administration

Sources: Merck Manual: Drug Administration (opens in a new tab)

Triple agonist (triple-hormone-receptor agonist, tri-agonist)

A single molecule that activates three hormone receptors. Retatrutide, an investigational drug that is not FDA-approved, targets the GIP, GLP-1, and glucagon receptors and is being tested in clinical trials for obesity and related conditions.

In practice: Study reports and news use this term for retatrutide; "investigational" means it has no FDA-approved label, uses, or warnings yet.

Sources: Jastreboff et al. (2023) (opens in a new tab); Schnitzer et al. (2026) (opens in a new tab)

U

U-100 (U-100 insulin, 100 units/mL)

An insulin strength of 100 units per milliliter. A U-100 insulin syringe uses a matching scale: 100 syringe units represents 1 mL, and 1 syringe unit represents 0.01 mL.

In practice: For a non-insulin medicine, the U-100 syringe scale still measures volume. The dose in mg or mcg depends on the medicine’s concentration; “U-100” does not describe the strength of that medicine.

Sources: DailyMed: Humulin R label (opens in a new tab); MedlinePlus: Giving an insulin injection (opens in a new tab); FDA: syringe units and dosing errors (opens in a new tab)

V

Visceral fat (visceral adipose tissue, VAT, intra-abdominal fat)

Fat stored deep inside the abdomen, around the internal organs, rather than just under the skin. High amounts are linked to metabolic syndrome, heart disease, and some cancers.

In practice: The tesamorelin (Egrifta) label refers to reduction in visceral adipose tissue, and studies often report visceral fat separately from total weight.

Related terms: Body composition, GHRH analog

Sources: Iacobellis et al. (2005) (opens in a new tab); Shuster et al. (2012) (opens in a new tab)

W

Washout period (wash-out)

A washout period is a stretch with no treatment between two treatments in a study, so the effects of the first one can wear off before the next begins. It is common in crossover trials, where each person tries more than one treatment.

Related terms: Half-life, Placebo, Clinical trial phases

Sources: Sibbald et al. (1998) (opens in a new tab)