Glossary

85 terms you meet on peptide and GLP-1 pages, each in one to three plain sentences, with a link to the page where it matters most.

#

503A: The section of the Food, Drug, and Cosmetic Act that lets a licensed pharmacist in a state-licensed pharmacy, or a licensed physician, compound a drug for an individual patient's prescription. State boards of pharmacy are the main overseers, and the drugs are not FDA-approved. Read more
503B: The section of the Food, Drug, and Cosmetic Act that created outsourcing facilities: compounders that register with FDA, follow manufacturing rules (CGMP) and are inspected by FDA on a risk-based schedule. They may make batches without a prescription for each patient, but their drugs are still not FDA-approved. Read more

A

Accelerated approval: An FDA approval based on an endpoint that is reasonably likely to predict a real clinical benefit, such as a lab measure, rather than the benefit itself. The company must run a confirmatory trial, and FDA can withdraw the approval if that trial fails. Read more
Adverse event: Any unwanted medical event in a person taking a drug, whether or not the drug caused it. An adverse reaction is an adverse event that is reasonably likely to have been caused by the drug. Read more
Agonist: A molecule that binds to a receptor and switches it on, mimicking the body's own signal. Semaglutide is a GLP-1 receptor agonist; tirzepatide is an agonist at both the GIP and GLP-1 receptors. Read more
Amino acid: The building block of peptides and proteins. Twenty standard amino acids, linked in chains in a specific order, make up the peptides and proteins in the body. Read more
Amylin: A hormone released by the pancreas together with insulin after meals. It slows stomach emptying and signals fullness. Pramlintide is an approved amylin analogue; cagrilintide, in CagriSema, is an investigational one. Read more

B

Bacteriostatic water: Sterile water for injection with 0.9% benzyl alcohol added as a preservative, so one vial can be punctured repeatedly. It is a prescription diluent and must never be used in newborns. Read more
Beyond-use date: The date after which a compounded preparation, or an opened multi-dose vial, should not be used. It is set by the pharmacy or by USP standards and is separate from the manufacturer's expiry date. Read more
BLA (Biologics License Application): The application a company submits to FDA to market a biological product, such as a protein, vaccine or antibody. Since 2020, FDA treats amino acid chains longer than 40 amino acids as proteins regulated this way. Read more
BMI (body mass index): Weight in kilograms divided by height in meters squared. A BMI of 25 to 29.9 is classed as overweight and 30 or more as obesity, and weight-management drug labels use these cutoffs to define who the drug is for. Read more
Boxed warning: FDA's strongest label warning, printed in a black-bordered box at the top of the prescribing information. GLP-1 drugs carry one about thyroid C-cell tumors seen in rodents. Read more
Bulk drug substance: The raw active ingredient, usually a powder, that a pharmacy uses to compound a drug. Under 503A it must have a USP monograph, be a component of an approved drug, or be on FDA's bulks list. Read more

C

Cash price: What a drug costs when paid for without insurance. It can differ from the list price and from what an insured patient pays at the counter. Read more
Category 1 (503A): FDA's list of bulk substances nominated for the 503A bulks list with enough information to evaluate. Under a long-standing FDA interim policy, FDA does not intend to act against pharmacies compounding with them while the evaluation runs. Read more
Category 2 (503A): FDA's list of nominated bulk substances that raise significant safety risks. Compounding with them is not covered by FDA's long-standing category policy. Read more
Category 3 (503A): FDA's list of bulk substances nominated without enough information for FDA to evaluate them. Compounding with them is not covered by FDA's long-standing category policy. Read more
CGMP (current good manufacturing practice): FDA's regulations for how drugs are made, tested and released, covering facilities, equipment, procedures and records. Drug manufacturers and 503B outsourcing facilities must follow them; 503A pharmacies are not subject to them. Read more
COA (certificate of analysis): A lab report on one batch of a substance, listing each test, its method, its acceptance limit and the result. It describes one sample and does not make an unapproved product legal or safe for human use. Read more
Compounding: A pharmacist or physician combining, mixing or altering ingredients to make a drug for a patient, such as a version of a drug in a different strength. Compounded drugs are not FDA-approved, and FDA does not verify their safety, effectiveness or quality before they are sold. Read more
Concentration: How much drug is dissolved in each unit of liquid, usually written in mg per mL. The same dose in milligrams is a different volume, and a different number of syringe units, at a different concentration. Read more
Contraindication: A condition or circumstance in which a drug should not be used because the risk outweighs any benefit. Contraindications are listed in their own section of the drug label. Read more
Counterion: A charged molecule, such as trifluoroacetate (TFA) or acetate, that is bound to a peptide as a salt after manufacture. Counterions add weight to the powder without being peptide, which is one reason net peptide content is lower than total weight. Read more

D

Diluent: The liquid used to dissolve a powdered drug before injection, such as sterile water or bacteriostatic water. The drug's label or pharmacy sheet specifies which diluent to use and how much. Read more
Double-blind: A trial design in which neither the participants nor the researchers assessing them know who is receiving the drug and who is receiving placebo. It reduces the chance that expectations influence the results.

E

Endotoxin: A toxic fragment of the outer wall of certain bacteria that causes fever, chills and, in large amounts, shock. It remains after the bacteria are dead, so a product can pass a sterility test and still contain it; it is measured in endotoxin units (EU). Read more
Endpoint: The outcome a clinical trial measures to decide whether a drug works, such as percent change in body weight at 72 weeks. The primary endpoint is chosen before the trial starts.
Enforcement discretion: A regulator choosing, under a stated policy, not to act against certain conduct that does not fully meet the law. It does not approve a product or change the law, and it can be ended. Read more

F

FDA approval: FDA's decision, after reviewing trial data and manufacturing information, that a specific drug is safe and effective for a specific use and may be marketed with an FDA-reviewed label. It applies to a product and an indication, not to a molecule in general. Read more

G

Gastroparesis: Slowed emptying of the stomach without a physical blockage, causing nausea, vomiting and fullness. GLP-1 drugs slow stomach emptying, and tirzepatide's label says it is not recommended for people with severe gastrointestinal disease, including severe gastroparesis. Read more
GHRH (growth hormone-releasing hormone): A hormone made in the hypothalamus that signals the pituitary gland to release growth hormone. Sermorelin is its first 29 amino acids, and tesamorelin is an approved GHRH analogue. Read more
GIP (glucose-dependent insulinotropic polypeptide): A gut hormone released after eating that boosts insulin release when blood sugar is high. Tirzepatide activates the GIP receptor as well as the GLP-1 receptor. Read more
GLP-1 (glucagon-like peptide-1): A gut hormone released after eating that prompts insulin release, slows stomach emptying and signals fullness. Drugs such as semaglutide are modified versions of it that last for days instead of minutes. Read more
Glucagon receptor: The receptor for glucagon, a pancreatic hormone that raises blood sugar and affects how the body uses energy. Retatrutide, an investigational drug, activates the glucagon, GIP and GLP-1 receptors. Read more
Growth hormone: A protein hormone made by the pituitary gland that drives growth in children and affects muscle, fat and bone in adults. Much of its effect works through IGF-1, which the liver makes in response to it. Read more

H

Half-life: The time it takes for the amount of a drug in the blood to fall by half. Semaglutide's half-life is about a week and tirzepatide's about five days, which is why both are injected once weekly. Read more
HbA1c: A blood test showing the percentage of hemoglobin with sugar attached, which reflects average blood sugar over the past two to three months. Diabetes drug trials use the change in HbA1c as a main measure of effect. Read more
HHS interim peptide policy: An October 2026 policy saying HHS does not intend to act against 503A compounding of BPC-157, TB-500, MOTS-c, KPV, epitalon and semax when ten conditions are met, until a final rule decides each one. None of the six is FDA-approved. Read more
HPLC (high-performance liquid chromatography): A lab method that separates the components of a dissolved sample so each can be detected as a peak. On a COA, HPLC purity is usually the main peak's share of the total peak area; it does not show how much peptide is in a vial. Read more

I

IGF-1 (insulin-like growth factor 1): A growth factor made mainly by the liver in response to growth hormone. It is the usual blood marker of growth hormone activity, and tesamorelin's label calls for monitoring it. Read more
IND (Investigational New Drug application): The application a sponsor files with FDA before testing an unapproved drug in people. Being under an IND makes a drug legal to use inside the registered trial, not outside it. Read more
Injection site reaction: Redness, itching, swelling, pain or bruising where a drug was injected. It is one of the most common adverse reactions listed on injectable peptide labels. Read more
Insulin syringe: A small syringe marked in units rather than milliliters, made for U-100 insulin. On a U-100 syringe, 100 units equals 1 mL, so each unit mark is 0.01 mL whatever drug is inside. Read more
Intramuscular: Injected into a muscle, such as the thigh or upper arm, with a longer needle than a subcutaneous shot. Most peptide drugs are not given this way; their labels specify subcutaneous injection. Read more
Investigational: A drug being tested in clinical trials that FDA has not approved. It is legal to use only inside a registered trial. Read more
ISO/IEC 17025: The international standard for the competence, impartiality and consistent operation of testing and calibration laboratories. Accreditation to it covers a specific list of tests, called the scope, which can be checked on the accreditation body's register. Read more
IU (international unit): A unit of biological activity defined separately for each substance against a reference standard, used for insulin, some hormones and vitamins. An IU of one substance has no fixed relationship to an IU of another, or to the unit marks on a syringe. Read more

L

Label (prescribing information): The FDA-reviewed document for an approved drug that states what it is approved for, the dosing, contraindications, warnings and the adverse reactions seen in trials. It is published on DailyMed and Drugs@FDA. Read more
Lot number: The code that identifies one manufacturing batch of a product. A COA is valid only for the lot number printed on it, which should match the vial exactly. Read more
Lyophilized: Freeze-dried: water removed from a frozen solution under vacuum, leaving a powder or cake. Many peptides are supplied this way and must be reconstituted with a diluent before injection. Read more

M

Mass spectrometry: A lab method that measures the mass of molecules. On a COA it confirms identity by showing the measured mass matches the expected mass of the named peptide; on its own it does not confirm the amino acid order or the amount. Read more
mcg (microgram): One millionth of a gram, or one thousandth of a milligram: 1 mg equals 1,000 mcg. Mixing up mg and mcg is a thousand-fold dosing error. Read more
MedWatch: FDA's program for reporting adverse events and product problems with drugs. Patients and clinicians can report online or by calling 1-800-FDA-1088. Read more
Meta-analysis: A statistical method that pools the results of several studies on the same question to produce one combined estimate. It is only as reliable as the studies it combines.
mg (milligram): One thousandth of a gram. Most peptide doses and vial strengths are written in milligrams, while syringes are marked in mL or units. Read more
mg/mL: Milligrams of drug per milliliter of liquid, the usual way to state concentration. A 5 mg vial mixed with 2 mL of diluent is 2.5 mg/mL. Read more
mL (milliliter): One thousandth of a liter, the unit of liquid volume on most syringes and vials. On a U-100 insulin syringe, 1 mL is 100 units. Read more
MTC/MEN 2 thyroid warning: The boxed warning on GLP-1 drugs about thyroid C-cell tumors seen in rodents. The drugs are contraindicated in anyone with a personal or family history of medullary thyroid carcinoma (MTC) or multiple endocrine neoplasia syndrome type 2 (MEN 2), an inherited cancer syndrome. Read more

N

NDA (New Drug Application): The application a company submits to FDA to market a new drug, containing trial results, manufacturing details and the proposed label. Most peptide drugs of 40 amino acids or fewer, such as semaglutide, are approved through an NDA. Read more
Net peptide content: The share of a peptide powder's total weight that is actually peptide, once water and counterions are counted. It is different from purity, and a 99% pure powder can contain noticeably less peptide than its weight. Read more

O

Off-label: Use of an approved drug for a condition, dose, route or group of patients not on its label. Prescribers may generally do this when they judge it appropriate, but FDA has not found the drug safe and effective for that use. Read more

P

Pancreatitis: Inflammation of the pancreas, typically causing severe, persistent abdominal pain that can spread to the back, often with vomiting. GLP-1 drug labels list acute pancreatitis as a warning. Read more
PCAC (Pharmacy Compounding Advisory Committee): A panel of outside experts that advises FDA on which bulk substances should be allowed for compounding. Its votes are recommendations; only an FDA rule changes the bulks list. Read more
Peptide: A short chain of amino acids, typically 2 to 50, shorter than a protein. Many hormones are peptides, and many drugs, from semaglutide to oxytocin, are modified copies of them. Read more
Phase 1 trial: The first testing of a drug in people, usually in a small group, to study safety, side effects and how the body handles different doses. Read more
Phase 2 trial: A trial in a larger group of patients, often a few hundred, to see whether the drug has the intended effect and which doses to carry forward, while continuing to study safety. Read more
Phase 3 trial: Large trials, often hundreds to thousands of patients, that confirm a drug's benefit and safety against placebo or another treatment. Their results are the main basis for an FDA approval. Read more
Placebo: An inactive treatment made to look like the real drug, given to a comparison group in a trial. Comparing against placebo separates the drug's effect from the effect of being treated and observed.
Prior authorization: An insurer's requirement that the prescriber get the plan's approval before it will cover a drug. Plans often require it for weight-management drugs, with criteria such as BMI. Read more
Purity: On a peptide COA, the share of the peptide material that is the target peptide, usually measured by HPLC. It says nothing about how much peptide is in the vial or about endotoxin and microbes. Read more

R

RCT (randomized controlled trial): A trial in which participants are assigned by chance to the drug or to a comparison such as placebo. Randomization is what lets differences in outcome be attributed to the drug.
Receptor: A protein on or in a cell that a specific hormone or drug binds to, triggering a response. Peptide drugs work by binding receptors such as the GLP-1 receptor. Read more
Reconstitution: Dissolving a freeze-dried powder in a measured volume of diluent to make an injectable solution. The volume added sets the concentration, and so how many syringe units each dose is. Read more
Research use only: A label meaning a product may be shipped only for laboratory or animal research, under 21 CFR 312.160, and is not for use in humans. FDA warning letters have found that the label does not change a product's status when it is marketed for people. Read more

S

Savings card: A manufacturer program that lowers what a patient pays for a brand-name drug, usually for people with commercial insurance. These cards generally cannot be used with Medicare or Medicaid. Read more
Secretagogue: A substance that makes a gland release a hormone. Growth hormone secretagogues include GHRH analogues such as sermorelin and ghrelin-mimicking peptides such as ipamorelin. Read more
Sterility test: A test that incubates samples from a batch to check for growth of bacteria or fungi, described in USP chapter 71. A pass shows the tested samples grew nothing; it cannot prove every vial is sterile and does not detect endotoxin. Read more
Subcutaneous: Injected into the fat layer just under the skin, usually in the abdomen, thigh or upper arm. Nearly all peptide drugs, including semaglutide and tirzepatide, are given this way. Read more
Systematic review: A review that searches for every study on a question using stated methods, assesses their quality and summarizes the findings. It may or may not include a meta-analysis.

T

Telehealth: Health care delivered remotely by video, phone or messaging. A telehealth prescription still requires a prescriber licensed in the patient's state and a licensed pharmacy to dispense it. Read more
Titration: Raising a dose in planned steps over weeks to reduce side effects. Tirzepatide's label, for example, starts at 2.5 mg weekly and increases in 2.5 mg steps every four weeks. Read more

U

U-100: Insulin at 100 units per mL, and the matching syringe scale on which 100 units equals 1 mL. When a non-insulin drug is drawn into a U-100 syringe, the unit marks measure volume only: 1 unit is 0.01 mL. Read more
Unapproved new drug: A drug sold or marketed for use in people without FDA approval and without a legal exemption such as lawful compounding. Selling one is a violation of federal law. Read more
Unit (syringe): A mark on an insulin syringe. On a U-100 syringe each unit is 0.01 mL, so the number of units for a dose depends on the drug's concentration in mg/mL. Read more

W

WADA (World Anti-Doping Agency): The body that publishes the Prohibited List used in most international sport. Its class S0 bans any substance with no approval for human therapeutic use, and it names BPC-157 as an example. Read more