The Complete Peptide & GLP-1 Drug Glossary: Semaglutide, Tirzepatide, Compounding, and Regulatory Terms Explained

The language of GLP-1 medications, peptide science, and compounding pharmacy is dense — and the stakes of misunderstanding it are high. Patients are making treatment decisions based on social media posts that confuse Ozempic with Wegovy, conflate "503A" with "503B," and invent drug classes like "GLP-3" that don't exist. Prescribers, journalists, and even some formulary managers routinely misapply regulatory terminology in ways that have real-world consequences.

This glossary was built to fix that. Whether you're a health-curious patient researching your options, a clinician navigating the compounding pharmacy landscape, or a journalist covering the obesity pharmacotherapy space, the terms below will give you the precision you need.

What Are GLP-1 Receptor Agonists?

Glucagon-Like Peptide-1 (GLP-1) is a hormone secreted by L-cells in the distal small intestine after you eat. It does several important things simultaneously: it stimulates insulin secretion from pancreatic beta cells (but only when blood glucose is elevated), suppresses glucagon release, slows gastric emptying, and signals to the hypothalamus to reduce appetite and promote satiety.

The catch? Native GLP-1 has a half-life of roughly two minutes in the bloodstream, rapidly degraded by an enzyme called DPP-4 (Dipeptidyl Peptidase-4). That makes it useless as a drug in its native form.

GLP-1 receptor agonists (GLP-1 RAs) are engineered analogs — structurally modified versions of GLP-1 that resist DPP-4 degradation and have dramatically longer half-lives. The current generation includes:

• Semaglutide (half-life ~1 week): the active ingredient in Ozempic, Wegovy, and Rybelsus
• Liraglutide (half-life ~13 hours): the active ingredient in Victoza and Saxenda
• Tirzepatide (half-life ~5 days): the active ingredient in Mounjaro and Zepbound — though calling tirzepatide a "GLP-1 drug" is an oversimplification (more on that below)
• Dulaglutide (Trulicity), exenatide (Byetta/Bydureon), and others with varying durations

Incretins: GLP-1 Is Not the Whole Story

GLP-1 is an incretin — a gut-derived hormone that amplifies insulin secretion in response to food. But it isn't the only one. GIP (Gastric Inhibitory Polypeptide), secreted by K-cells in the duodenum, is equally important. Together, GLP-1 and GIP account for the incretin effect: the observation that oral glucose triggers far more insulin secretion than intravenous glucose. This effect explains 50–70% of postprandial insulin secretion in healthy individuals.

Why does this matter? Because tirzepatide (Mounjaro/Zepbound) is not a GLP-1 receptor agonist alone — it is a dual GIP/GLP-1 receptor agonist, a single molecule that activates both receptors simultaneously. Its GIP activity is central to its superior weight-loss efficacy compared to GLP-1-only drugs. Calling Mounjaro a "GLP-1 drug" erases half of its mechanism.

Key Drug-by-Drug Reference

Semaglutide: Three Brands, One Molecule

Semaglutide is perhaps the most misunderstood drug in this space — not because it's complicated, but because Novo Nordisk markets it under three distinct brand names with different indications and doses:

Ozempic ≠ a generic term for GLP-1 weight-loss drugs. Ozempic is FDA-approved for diabetes — not obesity. It became culturally synonymous with weight loss because physicians prescribe it off-label for that purpose. The weight-management-approved version is Wegovy, which uses a higher dose (2.4 mg vs. 2 mg max for Ozempic). These are not interchangeable, and conflating them creates insurance, access, and clinical confusion.

Rybelsus is notable for a different reason: it is the only oral GLP-1 receptor agonist approved in the US. It uses an absorption enhancer called SNAC to enable GI peptide absorption, which is normally impeded by stomach acid and digestive enzymes. Unlike injectable semaglutide (approved via the FDA's Biologics License Application pathway), Rybelsus was approved via the New Drug Application (NDA) pathway due to its formulation characteristics.

Tirzepatide: The Dual Agonist

Tirzepatide (Mounjaro for diabetes, Zepbound for obesity) is Eli Lilly's dual GIP/GLP-1 receptor agonist. Clinical trials (SURMOUNT-1 through SURMOUNT-4) have shown weight loss of up to 22.5% of body weight at the highest dose — exceeding the efficacy of any prior approved anti-obesity medication. The role of GIP activity in this efficacy is an active area of research; mechanistically, GIP receptor activation in adipose tissue and the central nervous system appears to amplify the effects of GLP-1 receptor signaling.

Retatrutide, still investigational as of 2026, goes one step further: it is a triple agonist targeting GLP-1, GIP, and glucagon receptors simultaneously. Early data suggests weight loss potentially exceeding tirzepatide's results.

Liraglutide: The First Obesity Approval

Before semaglutide, liraglutide (Novo Nordisk) was the gold standard. Its weight-management brand, Saxenda (3 mg daily), was the first GLP-1 receptor agonist approved specifically for obesity and remains approved for adolescents ≥ 12 years old — a population for whom the weekly semaglutide options are also approved.

The Regulatory Layer: FDA Pathways and Compounding Rules

Understanding the drug approval and compounding framework is essential — particularly now that compounded semaglutide exists in a legally contested gray zone.

NDA vs. BLA: Two Pathways for Two Drug Types

• NDA (New Drug Application): Used for small-molecule drugs. Think metformin, orlistat, phentermine.
• BLA (Biologics License Application): Used for biologics — drugs derived from living organisms. Injectable semaglutide and tirzepatide are biologics approved via BLA.

A common error: saying "Ozempic's NDA" or treating GLP-1 injectable drugs as if they follow the small-molecule pathway. They don't. This distinction matters because biosimilar GLP-1 drugs (if and when they emerge) follow a different regulatory pathway than generic small-molecule drugs.

503A vs. 503B: Two Types of Compounding Pharmacies

This distinction is frequently collapsed into a single bucket labeled "compounding pharmacy" — a category error with real regulatory consequences.

503A Pharmacies are traditional compounding pharmacies that prepare drugs for individual patient prescriptions. Key characteristics:

• Patient-specific formulations required (no large batches)
• Subject to state pharmacy board oversight
• Exempt from FDA's CGMP (Current Good Manufacturing Practice) requirements
• May use bulk drug substances on the 503A Bulks List

503B Outsourcing Facilities are FDA-registered facilities that can compound in large batches for office use, without individual patient prescriptions. Key characteristics:

• Must comply with FDA CGMP standards
• Subject to FDA inspection
• May use substances on the 503B Bulks List (formally, the 503B Bulks List is distinct from the 503A list)

Drug Shortage and Compounding Legality

Under Sections 503A and 503B of the FDCA (Federal Food, Drug, and Cosmetic Act), compounding pharmacies are permitted to compound drugs that appear on FDA's Drug Shortage list — even if those drugs are normally protected from compounding by intellectual property or FDA exclusivity rules. This was the legal basis for a large-scale compounded semaglutide market that emerged when Ozempic and Wegovy faced supply shortages.

Two critical caveats:

1. Compounded semaglutide is not FDA-approved. Compounded drugs are not bioequivalence-tested against the branded reference product. They are not generic equivalents.
2. The shortage exemption applies only during the shortage. Once FDA removes a drug from the shortage list, the legal basis for compounding under the shortage exemption dissolves. The FDA's enforcement posture has been evolving; practitioners and patients should follow current FDA guidance.

The Bulks List and the Shortage List are distinct: bulk drug substances on the 503B Bulks List are permitted for compounding regardless of shortage status (subject to FDA evaluation). These terms should not be used interchangeably.

Mechanisms Worth Understanding

How GLP-1 Drugs Work in the Brain

The appetite-suppressing effects of GLP-1 receptor agonists are not purely gastrointestinal. GLP-1 receptors are expressed in the hypothalamus — specifically in the arcuate nucleus, a key node in appetite regulation — as well as in the brainstem. Activation of these central receptors reduces food-seeking behavior and increases satiety independent of gastric emptying effects.

This central mechanism also appears to contribute to weight loss plateau: after 6–12 months of GLP-1 therapy, weight loss typically slows even on continued treatment. This is thought to involve adaptive metabolic responses, including reductions in resting metabolic rate and lean mass loss.

The Glucose-Dependent Difference

One mechanistic feature that distinguishes GLP-1 receptor agonists from older diabetes drug classes is glucose-dependent insulin secretion: GLP-1 RAs stimulate insulin release only when blood glucose is elevated. This makes their risk of hypoglycemia (low blood glucose) very low when used as monotherapy — in sharp contrast to sulfonylureas (which stimulate insulin secretion regardless of glucose level) or exogenous insulin.

This glucose-dependent mechanism also explains why GLP-1 receptor agonists don't cause hypoglycemia in non-diabetic patients using them for weight loss.

Amylin and Peptide YY: The Supporting Cast

GLP-1 doesn't act alone in satiety signaling. Two other gut peptides work in concert:

• Amylin, co-secreted with insulin from pancreatic beta cells, slows gastric emptying and suppresses glucagon. Pramlintide (Symlin) is a synthetic amylin analog approved as adjunct therapy with insulin. Cagrilintide, a long-acting amylin analog being developed by Novo Nordisk, is in trials as a combination with semaglutide (under the name cagrisema) for obesity.
• Peptide YY (PYY) is co-secreted with GLP-1 from L-cells after meals. It suppresses appetite by acting on hypothalamic Y2 receptors, providing an additive satiety signal.

Common Misuse and Misconceptions

These are the errors that cause the most real-world confusion in clinical, journalistic, and patient contexts:

"GLP-3" Is Not a Drug Class

There is no established GLP-3 receptor agonist drug class as of 2026. References to "GLP-3 drugs" in media or social media are factually incorrect. Glucagon-like peptide-3 exists as a peptide fragment but has no approved pharmacological use. Anyone using "GLP-3" to describe a drug class is either misinformed or describing something that doesn't exist in approved pharmacotherapy.

Ozempic Is Not a Generic Weight-Loss Term

Using "Ozempic" as shorthand for any GLP-1 weight-loss drug — or for "semaglutide for weight loss" — is imprecise and clinically misleading. Ozempic is specifically Novo Nordisk's branded injectable semaglutide for type 2 diabetes and cardiovascular risk reduction. Wegovy is the weight-management formulation at a higher dose. The distinction affects FDA indications, insurance coverage, and off-label prescribing considerations.

Tirzepatide Is Not "Just a GLP-1 Drug"

Tirzepatide (Mounjaro/Zepbound) is a dual GIP/GLP-1 receptor agonist. Its GIP receptor activity is not incidental — it is mechanistically central to its superior efficacy over GLP-1-only agents. Describing tirzepatide as a "GLP-1 drug" is like describing a car with two engines as "a single-engine vehicle."

Compounded ≠ Generic

A compounded semaglutide preparation is not equivalent to Ozempic or Wegovy. It has not undergone the FDA bioequivalence testing required for generic approval. Compounded drugs made under shortage exemptions may vary in quality, concentration, and excipients depending on the compounding pharmacy. "Compounded semaglutide" and "generic Ozempic" are not the same thing.

GLP-2 Is Not a Weight-Loss Hormone

GLP-2 is co-secreted with GLP-1 from L-cells, but its primary action is promoting intestinal mucosal growth and nutrient absorption — the opposite effect on caloric efficiency. Teduglutide (Gattex) is a GLP-2 receptor agonist approved for short bowel syndrome, a condition where nutrient absorption is severely impaired. It is not used for weight loss.

503A ≠ 503B

These are meaningfully different regulatory categories with different manufacturing standards, oversight regimes, and permitted activities. Conflating them obscures important quality and legal distinctions in the compounded drug landscape.

Key Terms at a Glance

A Note on Regulatory Status (2026)

The compounding landscape for GLP-1 drugs has changed substantially over the past two years and continues to evolve. FDA shortage designations, state pharmacy board actions, and federal enforcement guidance all affect what compounded semaglutide products are legally available and under what conditions. Always verify current FDA.gov guidance before making clinical or procurement decisions based on compounding status.

Similarly, the pipeline for next-generation peptide therapies — including triple agonists, amylin combination products, and potentially oral GIP/GLP-1 agents — is advancing rapidly. The regulatory terms and drug classes covered here reflect the approved and advanced-stage investigational landscape as of early 2026.

This glossary is intended for educational and informational purposes. It does not constitute medical advice. Consult a licensed healthcare provider and refer to FDA-approved prescribing information for clinical decisions.