GLP-1 Peptides: The Complete Class Guide

Something fundamentally changed in metabolic medicine over the past few years, and if you're not paying attention to the GLP-1 receptor agonist data, you should be.

For decades, the approach to weight management was essentially willpower-based — eat less, move more, and if that doesn't work, try harder. The research community knew this was insufficient, but the pharmacological tools available were modest at best. Then the incretin-based therapies arrived, and the clinical data was unlike anything the field had seen before.

This article is the class-level guide. It covers what GLP-1 peptides are, how they work, what defines this class today, and how the major compounds compare against each other. For trial-level depth on any single compound, the deep-dive articles linked throughout this piece are where to go.

The incretin system: a quick primer

GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide) are hormones your gut releases after eating. They tell your brain you're full, they tell your pancreas to release insulin, and they slow gastric emptying so you absorb nutrients more gradually.

The insight that changed everything was this: if you give people a synthetic version of these hormones that lasts much longer than the natural version, the metabolic effects are profound — larger and more consistent than anything the pharmacology of diet drugs had produced before.

What counts as a GLP-1 peptide

A GLP-1 peptide is a synthetic analogue of glucagon-like peptide-1 designed to bind the GLP-1 receptor and produce the same biological effects as the natural hormone — but with a much longer duration of action. Native GLP-1 is broken down in about two minutes; modern GLP-1 peptides have been engineered for half-lives ranging from 13 hours to 7 days.

The first commercially significant GLP-1 peptide was an early first-generation GLP-1. Novo Nordisk launched it for type 2 diabetes in 2010 and for chronic weight management in 2014. A 31-amino-acid analogue of native GLP-1 with a palmitic acid chain attached at lysine-26, it binds albumin in the bloodstream, extending its half-life to about 13 hours and enabling daily injection. This early first-generation GLP-1 was the proof of concept that targeting the GLP-1 receptor could move weight in a way diet drugs hadn't — modest by today's standards, and enormous at the time.

The class has since branched into four lineages, each represented by a flagship compound:

Single GLP-1 receptor agonists — first-generation GLP-1 is the modern leader, binding only the GLP-1 receptor
Dual GIP/GLP-1 agonists — second-generation GLP-1 (dual GIP/GLP-1 agonist), adding the second incretin receptor
Triple agonists — third-generation GLP-1 (triple agonist), adding the glucagon receptor for energy expenditure
Combinations — amylin + first-generation GLP-1 combination, pairing a GLP-1 agonist with a different satiety hormone, in this case amylin

Each lineage represents a different bet on how to amplify the metabolic signal beyond what GLP-1 alone can do.

The four compounds defining the class today

First-Generation GLP-1

This is the one most people have heard of. Approved for type 2 diabetes (FDA-approved 2017) and for chronic weight management (2021), with a daily oral option as well. Once-weekly injection at the obesity dose. The STEP trials established it as a leading weekly GLP-1 across the populations they studied, and SELECT (NEJM 2023) showed a 20% relative risk reduction in heart attacks and strokes — the first GLP-1 to prove the cardiovascular benefit at scale, not just the weight one.

If you want the full picture on STEP, SELECT, the muscle loss conversation, and what happened to compounded first-generation GLP-1 as the FDA shortage ended, the first-generation GLP-1 deep dive covers it.

Second-Generation GLP-1

The first compound to hit two incretin receptors at once — GIP and GLP-1. Eli Lilly's, approved for type 2 diabetes (FDA-approved 2022) and chronic weight management (2023). The SURMOUNT trials studied it in obesity at 72 weeks. And in May 2025, the direct head-to-head against first-generation GLP-1 everyone had been asking for finally ran: SURMOUNT-5 (NEJM, May 11 2025) reported greater average weight loss with the second-generation compound than with the first-generation comparator over 72 weeks, with a larger share of participants reaching the higher weight-loss thresholds.

The second-generation GLP-1 deep dive covers SURMOUNT-5 in detail and the GIP receptor debate.

Third-Generation GLP-1 (investigational, Phase 3)

The first one to go three-for-three on receptors — adds glucagon to the GIP/GLP-1 base. That third receptor is the interesting one: glucagon tells your liver to burn stored fat and turns up your resting metabolic rate, which is a different lever than the appetite suppression GLP-1 and GIP work through. Phase 2 (NEJM) studied it over 48 weeks and reported the largest average weight loss seen in the class to date — in less time than the second-generation compound needed. Phase 3 is running now; if those observations hold, FDA submission follows.

The third-generation GLP-1 deep dive covers the glucagon mechanism, Phase 2 body composition data, and the direct mechanism comparison vs the second-generation compound.

Amylin + First-Generation GLP-1 Combination (investigational, FDA review)

The first one to combine GLP-1 with a completely different satiety hormone — amylin. Novo Nordisk's bet that you could pair a first-generation GLP-1 with a long-acting amylin analogue in a single weekly injection and get more weight loss than the GLP-1 alone. The REDEFINE program tested it across obesity. REDEFINE 1 (obesity without diabetes, 68 weeks) reported greater weight loss than first-generation GLP-1 alone — validating the amylin half of the equation. Novo Nordisk filed an FDA New Drug Application on December 18, 2025; first regulatory decision is expected in late 2026.

The bigger question — could the combination beat the second-generation compound head-to-head? — got its answer in February 2026 with REDEFINE 4, and the answer was no. Over 84 weeks the second-generation compound came out ahead; the combination's primary endpoint of non-inferiority was not met. It came in second. But REIMAGINE 2 (type 2 diabetes, 68 weeks) read out the same month and went the other direction: the combination outperformed first-generation GLP-1 on both weight loss and HbA1c reduction. That gap is meaningful in T2D specifically, where endogenous amylin secretion is already impaired alongside insulin.

The amylin + GLP-1 combination deep dive covers amylin biology, the full REDEFINE and REIMAGINE program, and where the compound actually lands given the head-to-head loss.

How they compare

The lineages side by side:

| Generation | Mechanism | Key trials | Status | |---|---|---|---| | Early first-generation GLP-1 | GLP-1 (daily) | SCALE | FDA-approved 2014 | | First-generation GLP-1 | GLP-1 (weekly) | STEP 1, SELECT | FDA-approved 2021 | | Second-generation GLP-1 | GLP-1 + GIP | SURMOUNT-1, SURMOUNT-5 | FDA-approved 2023 | | Amylin + first-generation GLP-1 combination | GLP-1 + amylin | REDEFINE program | FDA NDA Dec 2025, decision late 2026 | | Third-generation GLP-1 | GLP-1 + GIP + glucagon | Phase 2 (NEJM) | Phase 3 underway |

Reading the pattern carefully: the most potent compounds cluster closely together, and the headline ranking matters less than the pattern — each additional mechanism tends to add incremental weight loss in trials, and the differences between the top compounds (second-generation, third-generation, and the amylin combination) are smaller than the gap between them and single-agonist first-generation GLP-1.

Which one is "best" is the wrong question. The right questions are: which mechanism matches the metabolic phenotype, which has cardiovascular outcomes data, which has lean mass preservation evidence, which has the long-term safety record. First-generation GLP-1 leads on cardiovascular outcomes (SELECT). The second-generation compound leads on direct head-to-head weight loss (SURMOUNT-5, REDEFINE 4). The third-generation compound leads on early-trial magnitude. The amylin combination leads on diabetes-specific glycemic plus weight effects (REIMAGINE 2). For the trial-level details that justify each of those claims, the deep dives are the place to go.

What's still being worked out

The long-term durability data is being collected. What happens after five or ten years on these compounds? The cardiovascular outcomes trials (SELECT for first-generation GLP-1, SURPASS-CVOT for the second-generation compound) are providing the first real answers, and the early data is encouraging — but we need more time.

The weight regain question is unresolved. Current data suggests that stopping GLP-1 therapy leads to weight regain in most people. SURMOUNT-4 and STEP-4 confirmed that approximately half the weight comes back within a year of discontinuation. Whether the next-generation combinations (the amylin combination, the third-generation compound) provide more durable maintenance is an active research question.

The muscle mass question is real. Across the class, a substantial fraction of weight lost tends to be lean mass — consistent with any rapid caloric restriction. Resistance training during treatment substantially mitigates this; 2024 evidence in first-generation GLP-1 trials showed structured resistance training preserved lean mass even at peak weight loss rates. For older adults already at risk of sarcopenia, this consideration matters.

The cost and access question is the largest non-clinical issue. List prices exceed $1,000 per month for most compounds. Insurance coverage varies enormously. The compounded first-generation GLP-1 ecosystem that grew during the FDA-recognized shortage has largely closed as supply normalized — that's a substantial story covered in the first-generation GLP-1 deep dive.

What's coming next

The pipeline is dense. Oral formulations of the next-generation compounds are in development. Additional triple agonists beyond the third-generation compound are in early trials. Next-generation experimental GLP-1 candidates — including an oral GLP-1 + amylin agent and another triple agonist — have produced encouraging Phase 2 data. The class's next decade will likely be defined by combinations and dosing-route diversification rather than entirely new mechanisms.

The mechanistic story has settled around three core levers: appetite suppression (GLP-1, amylin, central pathways), nutrient partitioning (GIP), and energy expenditure (glucagon). Most next-generation compounds combine two or three of these. The remaining engineering questions are about ratios, kinetics, and delivery routes.

Frequently asked questions

What are GLP-1 peptides?

Synthetic versions of the gut hormone GLP-1 (glucagon-like peptide-1), engineered to last much longer than the natural form. Native GLP-1 is destroyed in about two minutes; pharmaceutical GLP-1 peptides have half-lives from 13 hours to 7 days. They bind the same receptors and produce the same biological effects — appetite suppression, insulin enhancement, gastric emptying delay — at clinically useful durations.

Are the well-known GLP-1 medications GLP-1 peptides?

Yes. The best-known GLP-1 medications are first-generation GLP-1 receptor agonists — 31-amino-acid analogues of native GLP-1 — with both weekly injectable and daily oral forms. The newer second-generation GLP-1 compounds are dual GLP-1/GIP agonists — they activate the GLP-1 receptor plus the GIP receptor.

Which GLP-1 shows the most weight loss in trials?

In direct head-to-head data, the second-generation compound outperformed first-generation GLP-1 (SURMOUNT-5). The amylin combination outperformed first-generation GLP-1 alone (REDEFINE 1) but fell short of the second-generation compound in REDEFINE 4 (84 weeks). The third-generation compound's Phase 2 data is the largest reported so far, but Phase 3 data hasn't read out yet. The rankings between the top compounds are tighter than the headlines suggest.

What's the difference between GLP-1, GIP, and glucagon receptor agonism?

GLP-1 reduces appetite and enhances insulin secretion. GIP also enhances insulin but additionally appears to improve fat metabolism and contribute to lean mass preservation. Glucagon increases energy expenditure — your body burns more calories at rest. The reason multi-receptor agonists outperform single GLP-1 agonists is that each receptor addresses a different lever of metabolic regulation.

Are GLP-1 peptides safe?

They have the most extensive safety data of any peptide drugs in modern use. The dominant adverse events are gastrointestinal (nausea, vomiting, diarrhea, constipation), most pronounced during dose escalation, generally manageable. Boxed warnings about thyroid C-cell tumors derive from rodent studies; no human signal has emerged at scale. Muscle loss is real but mitigated by resistance training. The compounded grey market is a different story — the FDA has issued explicit warnings about unapproved versions sold through non-pharmaceutical channels.

Are compounded GLP-1s still legal?

Largely no, as of 2026. The FDA-recognized shortage that allowed 503A and 503B pharmacies to produce compounded versions has ended for both first-generation and second-generation GLP-1. The legal window for mass compounding has closed for most patients. Narrow exceptions remain (specific clinical needs documented by a prescriber), but the era of widely available compounded GLP-1 is over. The first-generation GLP-1 deep dive covers the compounding saga in detail.