Peptide Therapy Glossary (US Terms)

If you spend any time reading about peptide therapy in the US, you run into a wall of shorthand: 503A, 503B, Category 1, RUO, COA, secretagogue, GLP-1, bulks list, telehealth, compounding. A lot of it is genuine regulatory vocabulary, and a lot of it gets used loosely — or deliberately blurred — in marketing. This glossary defines the terms that actually change what a sentence means, in plain language, grouped by what they’re about. It’s a reference page, so skim to what you need.

A note on dates: the regulatory terms below describe a landscape that is actively moving in 2026. Where a definition depends on current status, it’s flagged, and the full picture lives on the 2026 FDA reclassification page and are peptides legal in the US.

Regulatory and legal terms

These are the terms that decide whether a peptide is legitimately available, and they’re the ones most often overstated by clinics and vendors.

FDA-approved. A drug the FDA has formally reviewed and authorized for a specific use, after clinical trials demonstrating safety and efficacy. Very few peptides discussed in “peptide therapy” are FDA-approved as such. Semaglutide and tirzepatide are; most wellness peptides (BPC-157, CJC-1295, ipamorelin, MOTS-C, and so on) are not. “Not FDA-approved” doesn’t automatically mean illegal — but it means there’s no approved product called by that peptide’s name.

Compounding. The practice of a licensed pharmacy preparing a medication for a patient rather than dispensing a mass-manufactured drug. Compounding is how many non-approved peptides reach patients legally — when the substance is eligible. The eligibility rules are where 503A, 503B, and the Category system come in.

503A pharmacy. A traditional compounding pharmacy operating under Section 503A of the federal Food, Drug, and Cosmetic Act. It compounds medications for an individual, named patient in response to a specific prescription. This is the route most relevant to individual peptide therapy.

503B outsourcing facility. A larger compounder registered under Section 503B that manufactures batches under stricter, FDA-overseen quality standards (closer to commercial drug manufacturing) and can supply clinics in bulk without a patient-specific prescription. The deeper 503A-vs-503B breakdown is on compounded peptides: 503A vs 503B.

Bulks list (503A Bulk Drug Substances List). The FDA’s list of bulk ingredients that 503A pharmacies are allowed to compound with. The list sorts substances into categories. Getting a peptide onto this list (in the “may compound” category) is the regulatory milestone that would clearly authorize legal compounding of it.

Category 1. On the bulks list, the “may compound” tier — substances under FDA evaluation with adequate supporting data, which licensed pharmacies may compound when a physician prescribes them. This is the status clinics often claim for peptides that don’t yet have it.

Category 2. The “significant safety concerns” tier — substances not eligible for compounding unless the FDA issues a specific authorizing regulation. In 2023-2024 the prior administration moved many popular peptides here, effectively halting their legal compounding.

Note: In April 2026 the FDA removed a group of peptides (including BPC-157, TB-500, MOTS-C, and others) from Category 2. This is widely misreported. Removal from Category 2 is not the same as placement in Category 1, and it does not by itself authorize compounding. A Pharmacy Compounding Advisory Committee review was scheduled for July 23-24, 2026, with federal rulemaking to follow. Any clinic telling you these peptides are “now Category 1” or “legal again” as of mid-2026 is ahead of the facts. Current as of June 19, 2026; expected to change.

PCAC (Pharmacy Compounding Advisory Committee). The independent FDA advisory panel that reviews whether specific substances should be recommended for the 503A bulks list. A PCAC review is a required step before a peptide can be formally added — its recommendation isn’t the final word, but nothing gets added without going through it.

Rulemaking. The formal federal process (notice, public comment, final rule) the FDA must complete to actually change a substance’s regulatory status. It’s why “the FDA signaled a change” and “the change is in effect” are different things, sometimes months apart.

Gray market. The informal supply channel where unapproved peptides are sold — often as “research” product — outside the prescription-and-pharmacy system. Not the same as the black market (it’s not always explicitly illegal), but it sits outside the quality controls that make a medicine trustworthy.

Supply, sourcing, and quality terms

These terms tell you what you’re actually buying and how much to trust it.

RUO (“research use only”). A label stating the product is sold for laboratory research, not for human use. Crucially, RUO is a legal disclaimer, not a quality grade. It lets a vendor sell an unapproved substance while formally disclaiming human-use claims. “Research use only” product is not prescription-grade medicine, regardless of how clinical the website looks. More on this on research peptides explained.

Research chemical / research peptide. Marketing shorthand for gray-market peptides sold under the RUO framing. The phrase signals the product hasn’t been manufactured or tested to medical standards.

COA (Certificate of Analysis). A lab document reporting a product’s identity, purity, and content. A genuine, third-party COA tied to the specific batch you received is meaningful. A generic PDF reused across every order, or one with no traceable independent lab, is close to decoration. Vendors know “comes with a COA” reassures buyers, so the phrase is common; the substance behind it varies wildly.

Purity (%). The share of a sample that is the intended peptide versus impurities, fragments, or contaminants. High stated purity on an untraceable product is just a number on a page. Purity claims mean something only when they’re independently verified.

Reconstitution. Mixing a freeze-dried (lyophilized) peptide powder with a liquid to make an injectable solution. This page defines the term only; it does not provide reconstitution math or dosing — those are clinical decisions, and fixed internet recipes applied to unverified product are a known hazard.

Lyophilized. Freeze-dried. Many peptides ship as a lyophilized powder for stability and are reconstituted before use.

Pharmacy-grade / prescription-grade. Informal terms for product made and dispensed through the licensed pharmacy system, as opposed to RUO/gray-market vials. The distinction matters: same-named molecule, very different assurance of what’s actually in the vial.

Pharmacology and “how it works” terms

These describe what peptides do in the body — useful for reading evidence claims critically.

Peptide. A short chain of amino acids — the same building blocks as proteins, just fewer of them. Many function as signaling molecules, telling cells to do something (release a hormone, repair tissue, modulate metabolism). The mechanics of that signaling are covered on how peptide therapy works.

Endogenous. Made by the body itself. Several therapy peptides are synthetic copies of molecules you already produce (MOTS-C, sermorelin-like compounds). “Your body already makes it” is true but often used to imply safety — endogenous describes origin, not whether injecting a manufactured copy past your own regulation is safe or effective.

Secretagogue. A substance that prompts a gland to secrete something. Growth-hormone secretagogues (sermorelin, CJC-1295, ipamorelin) don’t supply growth hormone — they nudge the pituitary to release more of its own. This is a different mechanism from injecting the hormone directly, and it’s a frequent point of marketing confusion.

Agonist. A molecule that activates a receptor, mimicking the body’s natural signal. GLP-1 drugs are receptor agonists. Antagonist is the opposite — it blocks a receptor.

Half-life. How long a compound stays active in the body, often expressed as the time for its level to halve. Half-life drives how often something would be dosed and is why two versions of a “similar” peptide (e.g., with and without a DAC modification) behave differently.

AMPK. A cellular “low fuel” sensor that, when activated, shifts cells toward burning fuel for energy. It comes up constantly in metabolic-peptide marketing (MOTS-C is described as activating it) because exercise activates it too — which is why some peptides get called “exercise mimetics.”

Exercise mimetic. A compound said to reproduce some of exercise’s molecular effects. The label is informative but routinely abused: activating one pathway exercise uses is not replacing exercise, which drives many adaptations no peptide touches.

GLP-1 and weight-loss terms

The weight-loss side of the field has its own vocabulary, and the brand-vs-molecule confusion is constant.

GLP-1 (glucagon-like peptide-1). A natural gut hormone that signals fullness and helps regulate blood sugar. “A GLP-1” colloquially means a GLP-1 receptor agonist drug — a medication that mimics that hormone. It’s a class, not a single product. The full overview is on the GLP-1 weight-loss guide.

Semaglutide. A specific GLP-1 drug. Sold as Ozempic (diabetes branding) and Wegovy (weight-management branding) by Novo Nordisk — same molecule, two brand names for two approved uses.

Tirzepatide. A related but distinct molecule that acts on two receptors (GLP-1 and GIP), hence “dual agonist.” Sold as Mounjaro (diabetes) and Zepbound (weight management) by Eli Lilly.

Retatrutide. An investigational triple agonist (GLP-1, GIP, and glucagon) still in trials as of 2026 — not an approved product. Worth knowing because it’s marketed gray-market well ahead of any approval.

Compounded GLP-1. A pharmacy-made version of semaglutide or tirzepatide. This has its own distinct legal track from the wellness peptides — tied to drug-shortage status and 503A/503B enforcement — and that status shifted through 2025-2026. The current picture is on compounded GLP-1 legal status.

Titration. The clinical practice of starting a medication low and adjusting upward over time under supervision. The term appears here for comprehension only — specific titration schedules are medical decisions, not something to copy from a website.

Access and practical terms

Telehealth. Remote medical care by video or messaging. Many peptide and GLP-1 programs operate this way: a licensed provider evaluates you remotely and, if appropriate, sends a prescription to a pharmacy. Legitimate telehealth still involves a real evaluation — “fill a form, get a vial, no assessment” is the warning sign, not telehealth.

Off-label. Prescribing an approved drug for a use other than its approved indication. Legal and common in medicine, but it shifts the evidence and oversight picture and should be a conscious, disclosed choice.

Indication. The specific medical condition a drug is approved (or studied) to treat. A peptide can be “studied” for many things and “approved” for none of them — the gap between those two words is where a lot of overclaiming happens.

Stack. Informal term for combining multiple peptides or supplements at once. Common in biohacker culture; relevant here because stacking makes it nearly impossible to attribute any effect (or side effect) to a single compound.

How to use this glossary

The single most valuable habit this vocabulary supports is catching the quiet substitutions in marketing: a peptide “removed from Category 2” sold as “now legal to compound,” an RUO vial framed as “pharmaceutical grade,” a class name like “GLP-1” used to borrow a specific brand’s credibility, or “your body makes it” deployed to imply safety. When the terms are defined cleanly, those moves become visible. For the moving regulatory parts, keep the 2026 reclassification page and are peptides legal in the US handy, since those are the definitions most likely to change as the year unfolds.