Few areas of peptide research get discussed as often, or as loosely, as the GLP-1 class. The names blur together, the headline numbers get quoted without their context, and the line between an approved medicine and an investigational compound often gets lost. This guide is the wider map: what these peptides are, how they work, what the trials have actually reported, and where each compound stands. It is written for a research audience, it is not medical advice, and the figures below are trial results rather than promises.
If you want the direct three-way comparison of the headline compounds, the semaglutide, tirzepatide, and retatrutide comparison goes deeper on that single question. This guide sets out the whole landscape around it.
What GLP-1 actually is
GLP-1, or glucagon-like peptide-1, is a hormone the gut releases after eating. It belongs to a family of gut hormones called incretins, and it is central to what researchers call the incretin effect: glucose taken by mouth triggers a larger insulin response than the same amount of glucose delivered intravenously, because eating releases these hormones and an intravenous drip does not.
GLP-1 signalling does several things at once, and this is the reason a single class ends up being studied across so many different questions:
- It amplifies glucose-dependent insulin release. The “glucose-dependent” part matters: it prompts insulin when blood glucose is high, not when it is already low.
- It slows gastric emptying, so a meal leaves the stomach more gradually.
- It acts on appetite pathways.
Native GLP-1 is broken down within minutes by an enzyme called DPP-4. That short half-life is the whole engineering problem the class solves: the research compounds are analogues built to resist that breakdown and stay active for far longer, which is what turns a fleeting gut signal into something measurable over a week.
How the class evolved
The class did not start with the compounds that dominate the conversation now. The first GLP-1 receptor agonist to reach the clinic, exenatide, was derived from exendin-4, a peptide found in the venom of the Gila monster that happens to resemble human GLP-1 closely enough to activate the same receptor while resisting rapid breakdown. Liraglutide followed as a once-daily analogue, then dulaglutide and semaglutide as once-weekly ones. The trajectory since has run toward longer dosing intervals and, more recently, toward engaging more than one receptor at a time. Read that way, the single-dual-triple story is the latest chapter of a two-decade progression, not a sudden arrival, and it helps to know the earlier chapters because the older compounds carry the longest safety records in the class.
The one idea that organises the whole class
Almost every meaningful distinction between the well-known GLP-1 compounds comes down to a single design choice: how many receptors the molecule activates. All of them act on the GLP-1 receptor. What separates them is what else they touch.
- Single agonist (GLP-1 only). Semaglutide targets the GLP-1 receptor and nothing else.
- Dual agonist (GLP-1 + GIP). Tirzepatide adds a second incretin receptor, GIP. The structural basis for that dual activity is set out in the published characterisation of tirzepatide.
- Triple agonist (GLP-1 + GIP + glucagon). Retatrutide adds a third target, the glucagon receptor, as described in a systematic review of the retatrutide trials.
Hold on to that one-versus-two-versus-three framing. Nearly every other pattern in the data, and most of the confusion, traces back to it.
What each receptor is understood to contribute
In preclinical and clinical research, the three receptors are associated with different effects. This is why adding a receptor is not simply “more of the same”.
- GLP-1 is the shared foundation: glucose-dependent insulin release, slower gastric emptying, reduced appetite.
- GIP is the second incretin receptor. Adding it (tirzepatide) has been linked to improved insulin sensitivity and to effects on lipid handling that look distinct from GLP-1 alone.
- Glucagon is retatrutide’s third target. Alongside its familiar role in raising blood glucose, glucagon signalling is associated with increased energy expenditure and with a marked reduction in liver fat, which is why retatrutide is also being studied specifically in metabolic liver disease.
The interesting research question is not “which receptor is best” but how the combination behaves, because these signals interact rather than simply add up.
What the trials have reported
The broad pattern in the published data is that adding receptor targets has tended to track with larger average weight reductions. The compounds were mostly studied in separate trials, so these are parallel results rather than a single race.
- Semaglutide reported roughly 15% average weight reduction in its obesity programme.
- Tirzepatide reported roughly 20 to 21%.
- Retatrutide reported roughly 24% at 48 weeks in Phase 2, and its Phase 3 TRIUMPH programme has since reported reductions of roughly 28%.
The single most useful data point is the one direct comparison. In SURMOUNT-5, a head-to-head trial, tirzepatide produced a mean weight reduction of 20.2% against 13.7% for semaglutide over 72 weeks, and improved several cardiometabolic measures more than semaglutide did. There is not yet an equivalent published head-to-head between retatrutide and the other two, so retatrutide’s higher figures come from its own trials rather than a direct contest.
Where each compound stands on approval
This is the practical difference the receptor count does not capture, and it is the part most often skipped over.
| Compound | Receptors | Reported trial weight reduction | Approval status (2026) |
|---|---|---|---|
| Semaglutide | GLP-1 | ~15% | Approved for type 2 diabetes and weight management |
| Tirzepatide | GLP-1 + GIP | ~20 to 21% | Approved for type 2 diabetes and weight management |
| Retatrutide | GLP-1 + GIP + glucagon | ~24 to 28% | Investigational; Phase 3 ongoing; not approved |
Semaglutide and tirzepatide are approved medicines with years of post-approval use behind them. Retatrutide is not: its Phase 3 results are still reading out through 2026, with a regulatory filing expected around late 2026 or 2027. Any figure quoted for retatrutide describes an investigational compound.
The wider class: beyond the headline three
Semaglutide, tirzepatide, and retatrutide dominate the discussion, but they are not the whole class, and a complete picture holds several groups at once. There is a mature core of older single agonists still in use, including liraglutide, dulaglutide, and exenatide; these matter for research precisely because they carry the longest real-world records. Then there is the approved dual agonist, tirzepatide. And there is a moving frontier of investigational multi-agonists: survodutide, a GLP-1 and glucagon dual agonist studied in obesity and liver disease, is one example alongside retatrutide. When someone says “GLP-1”, it is worth asking which of these they mean, because the evidence behind each is at a very different stage.
Injectable, oral, and what is changing
Most of the compounds above are injectables, usually given once weekly. The route of administration is the other axis that is moving. An oral formulation of semaglutide already exists, and small-molecule oral GLP-1 agonists such as orforglipron are in development. This matters because a tablet that needs no cold chain and no injection is a different proposition from a weekly injectable, even when the underlying receptor target is the same. For research purposes the receptor pharmacology and the delivery format are separate variables, and a result is easier to read when they are kept separate.
How to read a GLP-1 trial without getting misled
Most of the confusion in this area does not come from the science. It comes from comparing numbers that were never meant to be compared. A few habits help:
- Check whether a comparison is head-to-head. Apart from SURMOUNT-5, most of the figures above come from separate trials with different populations, doses, and durations. Cross-trial numbers are directionally informative, not a leaderboard.
- Read the duration. A 15% reduction at 68 weeks and a 20% reduction at 72 weeks are not measured at the same point, and these compounds are still changing the curve when many trials end.
- Separate “approved” from “studied”. A trial result for an investigational compound is a research finding, not a label claim, and the two are governed very differently.
- Watch the denominator. Average reductions hide a wide spread between individuals, and an average is not a promise for any single case.
Tolerability and the length of the record
Larger reported effect is only one axis. The class shares a broadly similar tolerability profile, with gastrointestinal effects (nausea, and slowed digestion) the most commonly reported, usually most noticeable as a dose is increased. Beyond that headline, the compounds differ in how much long-term data exists at all. The longest post-approval record of the three belongs to the single agonist, simply because it has been in use the longest. A compound with a larger reported effect and a shorter record is not straightforwardly “better”; it is a different point on a trade-off, and the evidence on the newest compound is still maturing.
Where the research is heading
The class is still moving quickly, which is part of why summaries date fast. A few directions worth watching:
- Retatrutide’s remaining readouts. Its Phase 3 programme spans obesity, type 2 diabetes, sleep apnoea, and liver and cardiovascular outcomes, so the picture will keep filling in through 2026.
- The glucagon axis. Because glucagon signalling is linked to energy expenditure and to liver fat, the triple agonist is being studied specifically in metabolic liver disease, not only weight.
- Beyond a single mechanism. The broad research trajectory has moved from one receptor to two to three, and the open question is how far that combination logic extends before the trade-offs outweigh the gains.
How this guide connects to the rest
This page is the overview. The focused pieces go deeper on one question each:
- The three-way comparison of semaglutide, tirzepatide, and retatrutide is the detailed head-to-head of the compounds above.
- When you are evaluating any research compound, the identity and purity on the vial matter as much as the molecule itself. How to read a peptide Certificate of Analysis covers how to verify that.
The short version
GLP-1 peptides are engineered analogues of a gut hormone that acts on insulin, gastric emptying, and appetite at the same time. The class is organised by how many receptors a compound activates: one (semaglutide), two (tirzepatide), or three (retatrutide). In the trials to date, that ladder has tracked with progressively larger average weight reductions, and the one head-to-head trial put tirzepatide ahead of semaglutide. The largest reductions reported so far belong to retatrutide, which is also the one compound of the three that remains investigational. Read the approval status and the trial design as carefully as the headline number, because in this class they carry most of the meaning.