Glossary

Peptide posts and seller pages are full of jargon. Here are short, plain definitions of the terms you are most likely to run into.

Adverse event: Any unwanted health problem that happens to someone during a study or while using a product. It may or may not be caused by the product.
Agonist: A molecule that switches on a receptor, the way a key turns a lock. For example, a GLP-1 agonist switches on the GLP-1 receptor.
Amino acid: A small building block. Chains of amino acids make up peptides and proteins. The human body uses 20 standard amino acids.
Amylin: A hormone made in the pancreas alongside insulin. It helps you feel full and slows how fast the stomach empties. Cagrilintide is a lab made version.
Analog: A lab made molecule built to resemble a natural one, often changed so it lasts longer or works more strongly. Semaglutide is an analog of GLP-1.
Bacteriostatic water: Sterile water with a preservative added that slows the growth of bacteria. It is sold for mixing with some injectable drugs.
Batch or lot number: A code that identifies one production run of a product. A test report only applies to the batch whose number it shows.
Bioavailability: How much of a substance actually reaches the bloodstream after you take it. Many peptides have low bioavailability when swallowed because the gut breaks them down.
Case report: A published description of what happened to one patient, or a few. It can raise a question, but it cannot prove cause and effect.
Category 2: An FDA list of bulk drug ingredients that may present significant safety risks when used in compounding, kept under FDA's interim compounding policies.^[1]
Certificate of Analysis: A lab report listing the tests run on one batch, the results and the limits they had to meet. In regulated drug making, an authentic one is issued for each batch and signed by the quality team.^[2]
Compounding: When a licensed pharmacist or facility mixes or alters drug ingredients to make a medicine for a patient's needs. Compounded drugs are not FDA approved.^[3]
503A: The section of federal law covering pharmacies that compound drugs for specific patients with a prescription.^[3]
503B: The section of federal law covering outsourcing facilities. They must follow federal manufacturing quality rules and can supply compounded drugs to hospitals and clinics without a prescription for each patient.^[3]
Double blind: A study where neither the volunteers nor the research team know who is getting the real treatment. This reduces bias.^[4]
Endotoxin: A fever causing piece of the outer wall of certain bacteria. It can remain even after the bacteria are dead, so drug standards call for a separate endotoxin test for injectable drugs.^[5]
FDA approval: FDA's decision, after reviewing evidence from the maker, that a drug's benefits outweigh its risks for a specific use. Approval covers one product, its uses and its labeling.
GIP: Glucose dependent insulinotropic polypeptide. A gut hormone released after eating that helps the body release insulin. Tirzepatide acts on both GIP and GLP-1 receptors.
GLP-1: Glucagon like peptide 1. A gut hormone that helps release insulin, slows the stomach and reduces appetite. Drugs like semaglutide copy its effects.
Glucagon: A hormone made by the pancreas that raises blood sugar. Some newer weight loss drugs, such as retatrutide, also act on the glucagon receptor.
Growth hormone secretagogue: A substance that makes the pituitary gland release more of the body's own growth hormone. Ipamorelin and CJC-1295 are examples.
Half life: The time it takes for the amount of a substance in the body to drop by half. A long half life means a substance stays active longer.
HPLC: High performance liquid chromatography. A lab test that separates the molecules in a sample and shows them as peaks on a graph. It is used to measure purity.
IGF-1: Insulin like growth factor 1. A hormone, made mostly in the liver in response to growth hormone, that drives growth of many tissues.
Immunogenicity: The chance that a substance triggers the immune system to react against it, for example by making antibodies. Impurities and clumped peptide can raise this risk.
In vitro: Latin for "in glass." Research done on cells or molecules in a dish or test tube, not in a living body.
In vivo: Latin for "in the living." Research done in a whole living animal or person.
Intramuscular: Given by injection into a muscle.
Investigational new drug: An unapproved drug being tested in people. Before such testing starts, the sponsor files an IND application with FDA, and trials can begin 30 days later unless FDA raises concerns.^[6]
Lyophilized: Freeze dried. Many peptides are sold as a dry powder made this way because they last longer dry than in liquid.
Mass spectrometry: A lab test that weighs molecules. It helps confirm a peptide's identity by checking that its mass matches what is expected.
Meta analysis: A study that combines the numbers from several similar studies to get one overall answer. It is only as good as the studies it includes.
mg vs mcg: Milligram (mg) and microgram (mcg) are units of weight. One milligram equals 1,000 micrograms. Mixing them up means a thousandfold error.
Net peptide content: The share of a peptide powder's total weight that is actually the peptide, after leaving out water, salts and counter ions.
Off label: Use of an FDA approved drug for a condition, dose or route that is not in its approved labeling. Doctors may prescribe this way, but FDA has not found the off label use safe and effective.^[7]
Peptide: A short chain of amino acids, usually fewer than about 50. Many hormones in the body are peptides.
Phase 1: The first test of a treatment in people, usually a small group. The main goal is to check safety and side effects.^[4]
Phase 2: A larger trial, often a few hundred people, that tests whether a treatment works and further checks safety.^[4]
Phase 3: A large trial, often 1,000 or more people, that confirms whether a treatment works, watches for side effects and compares it with standard care.^[4]
Placebo: A look alike product with no active ingredient, used to compare against the real treatment.^[4]
Preclinical: Research done before any testing in people, such as lab and animal studies.
Protein: A long chain of amino acids folded into a specific shape. Proteins are larger than peptides.
Purity: In peptide testing, usually the share of what an HPLC test detects that is the intended peptide. It does not measure water, salts or counter ions.
Randomized controlled trial: A study where people are put into groups by chance, with one group getting the treatment and another getting a placebo or standard care. It is the strongest single way to test a treatment.^[4]
Receptor: A protein on or inside a cell that a specific molecule fits into, like a lock for a key. When switched on, it triggers a response in the cell.
Reconstitution: Mixing a freeze dried powder with a liquid to make a solution. This site does not give instructions for doing it.
Research use only: A label meaning a product is not meant for medical use in people. FDA treats it as a warning, not an approval, and the label does not exempt a product from the rules.^[8]
Sterile water: Water treated to contain no living germs. Unlike bacteriostatic water, it has no preservative.
Sterility: Being free of living germs such as bacteria and fungi. It requires its own test and is not shown by a purity result.
Subcutaneous: Given by injection into the fatty layer just under the skin.
Systematic review: A study that finds and judges all the research on a question using a set, documented method.
U-100 syringe: An insulin syringe marked in units, where each unit mark is one hundredth of a milliliter. A unit is a volume, not a fixed amount of drug, which has led to dosing errors.^[9]
WADA Prohibited List: The World Anti-Doping Agency's list of substances and methods banned in sport. It is updated at least once a year, and several peptides sold online are on it.^[10]

Sources

U.S. Food and Drug Administration. Certain Bulk Drug Substances for Use in Compounding that May Present Significant Safety Risks. Content current as of April 22, 2026. FDA list
U.S. Food and Drug Administration. Q7 Good Manufacturing Practice Guidance for Active Pharmaceutical Ingredients, Guidance for Industry (Revision 1). September 2016. FDA guidance (PDF)
U.S. Food and Drug Administration. Compounding Laws and Policies. FDA page
National Institutes of Health. NIH Clinical Research Trials and You: Glossary of Common Terms. NIH page
U.S. Food and Drug Administration. Q6A Specifications: Test Procedures and Acceptance Criteria for New Drug Substances and New Drug Products: Chemical Substances. December 2000. FDA guidance page
U.S. Food and Drug Administration. Investigational New Drug (IND) Application. FDA page
U.S. Food and Drug Administration. Understanding Unapproved Use of Approved Drugs "Off Label". FDA page
U.S. Food and Drug Administration. Distribution of In Vitro Diagnostic Products Labeled for Research Use Only or Investigational Use Only, Guidance for Industry and FDA Staff. November 2013. FDA guidance (PDF)
U.S. Food and Drug Administration. FDA alerts health care providers, compounders and patients of dosing errors associated with compounded injectable semaglutide products. July 26, 2024. FDA alert
World Anti-Doping Agency. The Prohibited List (2026 List in force). WADA page

Medical disclaimer

This page is educational information only. It is not medical advice and does not replace care from a licensed medical provider.