--- title: "Peptide and GLP-1 glossary: the terms, not the math | Peptyn" description: "Plain definitions of the peptide and GLP-1 vocabulary, from lyophilized to 503A vs 503B, with sources. It defines every term and stops before the arithmetic." canonical: "https://peptyn.orlyn.ai/articles/peptide-glossary" last-updated: "2026-08-16" --- # Peptide and GLP-1 glossary: the terms, not the math *Plain definitions of the peptide and GLP-1 vocabulary, from lyophilized to 503A vs 503B, with sources. It defines every term and stops before the arithmetic.* Educational · Not medical advice · 18+ You are holding a vial or reading a prescriber's note and you hit a word you do not recognise. This glossary defines the vocabulary of peptides and GLP-1 medications and then stops at the point where a decision is required. That point, where words end and arithmetic begins, belongs to your prescriber and your pharmacist. Published 15 August 2026 · 13 min read · 13 sources Entries are alphabetical, numbers first. Claims about regulation, labelling, and pharmacokinetics carry a numbered source at the bottom; entries without a marker are plain definitions. ## 503A compounding pharmacy A 503A pharmacy is a traditional compounding pharmacy: a licensed pharmacist in a state-licensed pharmacy, or a physician, compounding for an identified individual patient. Section 503A of the FD&C Act sets the conditions for this, and one of them is that the drugs must be compounded based on the receipt of valid patient-specific prescriptions.[11] Day-to-day oversight sits with state boards of pharmacy rather than the FDA, though the FDA does conduct surveillance and for-cause inspections of these pharmacies. Drugs compounded under 503A conditions are not subject to current good manufacturing practice (CGMP) requirements.[6] ## 503B outsourcing facility An outsourcing facility is the category the Drug Quality and Security Act added in 2013, and registering as one is voluntary.[6][11] A 503B facility registers with the FDA, is inspected by the FDA on a risk-based schedule, and its preparations are subject to CGMP requirements.[6] Unlike a 503A compounder, it may distribute compounded drugs either against a patient-specific prescription or in response to an order from a health care provider that is not for an identified individual patient, such as office stock.[11] The distinction that matters to a reader is that one, not the polish of the vial. 503A is per-patient and state-supervised; 503B is batch-capable and FDA-supervised under manufacturing-grade quality rules. Neither category means "FDA approved": compounded drugs are not FDA-approved, which means the FDA does not verify their safety, effectiveness, or quality before they are marketed.[6] ## Active pharmaceutical ingredient (API) The API is the substance in a preparation that is actually doing the pharmacological work, as opposed to the water, preservative, and buffers around it. Regulators treat the identity of the API as the thing that defines the drug, which is why a change in chemical form is not a cosmetic detail. See *salt form*. ## Bacteriostatic water and sterile water Bacteriostatic Water for Injection USP is water with benzyl alcohol added as a bacteriostatic preservative, which is what allows it to be supplied in a multiple-dose container "from which repeated withdrawals may be made to dilute or dissolve drugs for injection." Labels list the benzyl alcohol content as 0.9% or 1.1% depending on the product, so the exact figure is a label detail, not a universal constant.[1] Sterile Water for Injection has no preservative. Which one belongs with a given preparation is specified by the prescription or the compounding instructions. It is not an interchangeable choice, and this glossary is not the place to decide it. ## Beyond-use date (BUD) The beyond-use date is the date, or hour and date, after which a preparation must not be used. Two different things get called a BUD, and conflating them is the most common error in this vocabulary: - **The compounder's BUD.** For a compounded sterile preparation, the compounding pharmacy assigns a BUD based on the environment it was prepared in, the sterility of the starting components, the sterilisation method, storage temperature, and stability. USP General Chapter 797 is the standard that governs how that date is assigned. If you cannot find one on your preparation, the compounder is the right person to ask. - **The in-use discard date.** Separately, the CDC relays the USP 797 recommendation that once a multi-dose vial is opened, meaning needle-punctured, "the vial should be dated and discarded within 28 days unless the manufacturer states another date for that opened vial," and that "the beyond-use-date should never exceed the manufacturer's original expiration date."[2] The 28-day figure is a general recommendation for multi-dose vials. It is not a promise about any particular compounded peptide, and it does not override a shorter date assigned by whoever made the preparation. What a log is good for here is recording the date you first punctured a vial, because that date is not printed anywhere and memory is unreliable. ## Compound In this context, a compound is any medication or active substance, whether FDA-approved (like semaglutide), compounded from a bulk substance, or investigational (under study, not approved). The word is a category label. It implies nothing about legality, efficacy, or safety. ## Cycle A cycle is a defined period during which a compound is taken, bounded by a start date and an end date. Whether a protocol is continuous or cyclical, and what the boundaries are, is a design question for the person who wrote it. The useful discipline is recording the boundaries, because a start date and a stop date are what make a later observation interpretable at all. ## DailyMed DailyMed is the National Library of Medicine's database of drug labelling. It holds labelling submitted to the FDA by companies, and for an approved prescription product that means the Prescribing Information: boxed warnings, indications, dosage and administration, contraindications, warnings and precautions, adverse reactions, drug interactions, and use in specific populations.[12] When you want to know what the approved label actually says rather than what a forum says it says, this is the primary source. One thing to keep straight, because it is easy to get backwards: DailyMed is not a list of approved drugs. Alongside approved products it also carries labelling for products the FDA regulates but has not approved, including dietary supplements, medical foods, and unapproved prescription and nonprescription products.[12] A label being present there is not an approval. What it will not contain is a label for a compounded preparation or a research-grade vial, and it does not carry compounding standards. ## Diluent The diluent is the liquid used to bring a lyophilized powder into solution. Bacteriostatic and sterile water are the common ones. The diluent is specified by the prescription or the compounding instructions; it is a material specified for that preparation, not a free choice. ## GIP Glucose-dependent insulinotropic polypeptide. Along with GLP-1 it is one of the two incretin hormones. Both are inactivated by the enzyme dipeptidyl peptidase-4 (DPP-4), and both stimulate insulin secretion after an oral glucose load, which is what is meant by the incretin effect.[10] The term turns up because some medications act at this receptor as well as the GLP-1 receptor. Tirzepatide is described as a GIP analogue that activates both the GLP-1 and GIP receptors.[10] ## GLP-1 Glucagon-like peptide-1, an incretin hormone. A GLP-1 receptor agonist is a medication that activates the receptor GLP-1 acts on. The actions described in the literature include stimulating insulin secretion, inhibiting glucagon production from pancreatic alpha cells when blood sugar is high, delaying gastric emptying, and increasing satiety through direct action on the hypothalamus.[10] Two things the term does not tell you. It does not distinguish an approved product from a compounded one, and it does not tell you how many receptors a given product acts on: single-receptor, dual, and triple agonists all get called GLP-1 medications in casual use. Those are specific facts about a specific product, and the label is where they live. ## Half-life Half-life is the time it takes for the concentration of a drug in the blood to fall by half. Semaglutide has an elimination half-life of approximately one week, and remains in circulation for roughly five weeks after the last dose.[3] Tirzepatide has a half-life of about five days.[4] The practical consequence is that a long-acting medication does not reach a stable level immediately. For semaglutide, steady-state exposure is typically reached after four to five weeks of weekly administration.[3] This is reference material for understanding why a medicine's behaviour early in a protocol differs from its behaviour later. It is not an input to a calculation you should be doing. ## Letter of medical necessity A document from your prescriber explaining why a particular medication is needed for your specific condition. Some insurers require one before they will cover a medication. It is not a prescription; it is an advocacy document, and your prescriber's office is who prepares it. ## Lyophilized Lyophilized means freeze-dried. A lyophilized preparation has had almost all its water removed under vacuum, leaving a solid powder or cake that is more stable to store and ship than a solution. It has to be brought back into solution before injection. Compounded peptides are commonly supplied in this form, which is why so much of this vocabulary is about liquids and vials rather than pills. ## MedWatch MedWatch is the FDA's safety information and adverse event reporting programme. The agency describes it as its medical product safety reporting programme for health professionals, patients and consumers, and says it receives reports from the public and, where appropriate, publishes safety alerts for FDA-regulated products.[5] You do not have to be a clinician to file one. If something unexpected happens, reporting it is a contribution to a dataset, not a complaint into the void. ## Multi-dose vial A container designed for multiple withdrawals over time. Multi-dose vials typically contain an antimicrobial preservative to help limit the growth of bacteria, which is what makes repeated entry acceptable in the first place. The limit of that protection is the part worth knowing: the CDC states that the preservative "does not have an effect on viruses nor does it provide complete protection against bacterial contamination."[2] Every puncture is an opportunity for contamination, which is why the in-use discard convention exists and why the date of first puncture is worth recording. See *beyond-use date*. ## Peak and trough Peak is the highest concentration of a drug in the blood after a dose. Trough is the lowest, immediately before the next one. The gap between them is much wider for a short-acting compound than for a weekly one, which is why weekly GLP-1 medications produce a comparatively flat concentration curve. These are pharmacokinetic descriptors used to characterise how a drug behaves over time. Neither is a number you measure at home. ## Peptide A short chain of amino acids joined by covalent bonds. The biochemistry literature describes a peptide as a short string of roughly 2 to 50 amino acids; past about 20 the unbranched chain is called a polypeptide, and longer chains again are proteins.[13] There is no sharp line, which is why the word gets used loosely. The word is a structural description and nothing more. Semaglutide and tirzepatide are peptides. So is insulin. So is a vial sold as a laboratory reagent. Nothing about approval, quality, or legality follows from the word itself, which is worth remembering when it is used as a selling point. ## Prior authorization A requirement from an insurer that your prescriber satisfy certain criteria before the plan will pay for a medication. Your prescriber's office submits clinical information; the insurer approves or denies; a denial has a formal appeals process. Prior auth is a coverage hurdle, not a clinical judgement about you. ## Reconstitution Reconstitution is the act of dissolving a lyophilized powder in a diluent to produce an injectable solution. It is the step where most of this glossary's terms meet at once: a lyophilized compound, a specified diluent, a multi-dose vial, and a beyond-use date that starts running. How much diluent goes into a given vial is a preparation instruction that comes with the prescription. It determines the concentration of the resulting solution, and therefore it sits directly upstream of every dosing decision. That is precisely why you will not find a number for it here. ## Research use only (RUO) RUO is a labelling statement, not a licence. Products sold with "research use only" or "not for human consumption" on the label are marketed as laboratory reagents rather than medicines. The important correction: that label does not determine the product's legal status. The FDA determines intended use from the totality of the evidence, including how the product is marketed. In a February 2024 warning letter to an online seller of semaglutide and tirzepatide, the FDA wrote that "despite statements on your product labeling marketing your products as 'research chemicals only' and 'not for human consumption,' evidence obtained from your website establishes that your products are intended to be drugs for human use," and classified them as unapproved new drugs and misbranded drugs.[8] So the accurate reading of RUO is narrow: it tells you how a seller has chosen to describe a product. It is not a statement that the product is lawful, that it is what the label says it is, or that anything about its manufacture has been verified. ## Salt form Medications can exist as different chemical forms, including salts of a base molecule. This is ordinarily an unremarkable formulation detail, but for semaglutide it specifically is not. The FDA's position is that the salt forms circulating in some compounded products, "including semaglutide sodium and semaglutide acetate, are different active ingredients than are used in the approved drugs," that the agency "does not have information on whether these salts have the same chemical and pharmacologic properties as the active ingredient in the approved drug," and that it is not aware of a lawful basis for their use in compounding.[7] A related StatPearls review notes adverse effects associated with cases involving these salts.[10] If you keep records, recording the exact form printed on the label is worth the ten seconds, because "semaglutide" and "semaglutide sodium" are not two spellings of one thing. ## Stack, stacking Running more than one compound concurrently. The record-keeping problem with a stack is attribution: when several variables change at once, an observed effect cannot be assigned to any one of them. Logging which compound started or stopped on which date is the only thing that makes a later attribution even arguable. ## Step therapy Also called fail-first. An insurance requirement that you try a preferred medication before the plan will cover an alternative. The steps are set by the insurer as a coverage policy, not by a clinician as a treatment recommendation. ## Subcutaneous Subcutaneous means into the layer just beneath the dermis and epidermis, rather than into muscle (intramuscular) or a vein (intravenous). Subcutaneous tissue has relatively few blood vessels, so medication injected there is absorbed slowly and steadily.[9] Most GLP-1 medications and peptides are given by this route, which is the reason the slow, sustained profile described under *half-life* is possible at all. Approved product labelling addresses which body areas are appropriate and how to rotate between them. That is label and prescriber territory, and the label that came with your medication is the authority on it. ## Titration Titration is the process of adjusting a dose in steps over time, generally starting low and increasing, with the aim of reaching an effective level while limiting side effects. The schedule (how large a step, how long between steps, where it stops) is set by the prescribing information and your prescriber. The FDA has received adverse event reports that may be related to patients prescribed compounded semaglutide or tirzepatide in doses beyond what is in the approved label, which it notes can mean more product in a single dose, doses taken more frequently, or the amount increased more quickly.[7] What is useful on your side is a record of when a change happened, so that "when did this start" has an answer that is not a guess. ## Washout A washout is a period during which a medication is stopped and allowed to clear before something else begins. Because clearance is governed by half-life, a washout for a long-acting compound is measured in weeks rather than days. Whether one is needed, and how long it runs, is a clinical decision tied to the specific medicines involved and the reason for the switch. It is never a self-directed one. ## Where this glossary stops The entries above define the landscape. What they deliberately do not contain: - Unit conversions (mg to mL, units to mL, or any arithmetic chain) - Reconstitution volumes or resulting concentrations - Dose amounts, dose schedules, or titration timelines - Compound-to-purpose recommendations - Where to obtain anything - Whether a dose is safe, correct, or typical for you That is not squeamishness. The FDA has received multiple reports of adverse events, some requiring hospitalisation, that may be related to dosing errors with compounded injectable semaglutide products. Those errors resulted from patients measuring and self-administering incorrect doses, and in some cases from health care professionals miscalculating doses.[7] Arithmetic performed by a website that has never seen your prescription is a known failure mode, not a convenience. Peptyn's compound library uses this vocabulary. The library is neutral reference material and this glossary is the vocabulary behind it. Neither one will do the arithmetic: the app records the mix and the dose you enter, exactly as you enter them, and works nothing out on your behalf. ## References 1. BACTERIOSTATIC WATER (bacteriostatic water injection, solution), Hospira Inc. Prescribing information, DESCRIPTION section. DailyMed, U.S. National Library of Medicine. 2. Preventing Unsafe Injection Practices. Injection Safety, U.S. Centers for Disease Control and Prevention. 3. Semaglutide. StatPearls, NCBI Bookshelf. Pharmacokinetics: elimination half-life and steady state. 4. Tirzepatide. StatPearls, NCBI Bookshelf. Pharmacokinetics: elimination. 5. MedWatch: FDA Safety Information and Adverse Event Reporting Program. U.S. Food and Drug Administration. 6. Compounding and the FDA: Questions and Answers. U.S. Food and Drug Administration. Sections on who may compound, who inspects compounding facilities, and applicable quality standards. 7. FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight Loss. U.S. Food and Drug Administration. Sections "Salt forms should not be used to compound semaglutide" and "Dosing concerns with compounded semaglutide and tirzepatide." 8. Warning Letter, US Chem Labs (MARCS-CMS 669074), 7 February 2024. U.S. Food and Drug Administration, Center for Drug Evaluation and Research. 9. Medication Routes of Administration. StatPearls, NCBI Bookshelf. Subcutaneous route. 10. Glucagon-Like Peptide-1 Receptor Agonists. StatPearls, NCBI Bookshelf. Mechanism of action; salt forms of semaglutide. 11. Compounding Laws and Policies. U.S. Food and Drug Administration. Sections on FD&C Act 503A and 503B, patient-specific prescriptions, and outsourcing facilities. 12. About DailyMed. DailyMed, U.S. National Library of Medicine. DailyMed Overview: products included and Prescribing Information contents. 13. Biochemistry, Peptide. StatPearls, NCBI Bookshelf. ## Keep reading - [The 28-day rule, and when it applies](https://peptyn.orlyn.ai/articles/28-day-rule-glp1-vial) Where the number comes from in USP 797, why most GLP-1 labels set a different one, and which date only you can record. - [Is BPC-157 legal in 2026?](https://peptyn.orlyn.ai/articles/bpc-157-legal-status-2026) Three questions that get collapsed into one, what Category 2 actually meant, and what the July 2026 vote did and did not do. - [Compounded GLP-1s: the 2026 timeline](https://peptyn.orlyn.ai/articles/compounded-glp1-2026-regulatory-changes) Every date traced to an FDA or Federal Register document, and why your own log is what carries across a formulation change. This article is educational. It does not recommend doses, schedules, or products, and it is not medical advice. Every factual claim above is linked to its source. Questions about your own protocol belong with the prescriber who wrote it. --- Source: