Prescription / GLP-1how glp-1 medicines work

How GLP-1 Medicines Work

What GLP-1 is as a natural hormone, what receptor agonist drugs actually do in the body, and how dual and triple agonists differ from the original.

Written bySebastian Hunter, Founder and EditorI'm a 42-year-old senior advisor and distance runner who came to peptides as a sceptic. I'm not a doctor: I read the research, run my own experiment and keep honest notes. Read full bio

Published September 27, 2026

The short version

GLP-1 is a hormone your gut makes after meals. It normally survives only a couple of minutes before an enzyme breaks it down. Drugs like semaglutide and tirzepatide copy its effect on appetite, insulin and digestion. They are re-engineered to resist that breakdown, so one injection lasts about a week. Tirzepatide adds a second hormone receptor, and retatrutide, still investigational, adds a third.

GLP-1 is a hormone your own gut releases after eating. The drugs mimic its receptor-binding effect, not the hormone itself.
Natural GLP-1 is broken down within minutes. The drugs are chemically modified to last about a week.
One receptor (semaglutide, liraglutide), two (tirzepatide: GIP and GLP-1), or three and still investigational (retatrutide: GIP, GLP-1 and glucagon).
The same core actions repeat across all of them: appetite signalling in the brain, glucose-dependent insulin release, less glucagon, slower stomach emptying.

GLP-1 is a hormone your own gut makes every time you eat. The drugs built around it (semaglutide, tirzepatide, liraglutide and others) copy one narrow piece of what that hormone does. They keep doing it far longer than the hormone itself ever could.

This page walks through the mechanism underneath the injection. It covers what the natural hormone does, why it does not last, and how the drugs get around that. It also covers what changes when a drug hits two or three hormone receptors instead of one.

For the approved product details behind any one drug, see semaglutide, tirzepatide, liraglutide, orforglipron, and the investigational retatrutide. This page is the mechanism underneath all of them.

GLP-1 is a hormone your gut already makes

Glucagon-like peptide-1 is released from cells in the intestine after a meal. The FDA-approved labels for semaglutide and liraglutide both describe it the same way. GLP-1 is a physiological regulator of appetite and caloric intake, and the GLP-1 receptor it acts on is present in several areas of the brain involved in appetite regulation.

It is part of the body’s own after-meal signalling. The drugs introduce nothing foreign: they are built to activate the same receptor this hormone already uses.

What activating that receptor actually does

Across the approved labels for semaglutide, tirzepatide and liraglutide, the same core set of actions repeats:

Appetite and satiety signalling in the brain. The receptor sits in brain regions involved in regulating food intake. Animal studies cited in semaglutide’s label show the drug distributes to and activates neurons in those food-intake regions.
Glucose-dependent insulin release. The labels state that these drugs stimulate insulin secretion in a glucose-dependent manner. The boost scales with how high blood glucose already is rather than firing at a fixed rate. That is also why the hypoglycaemia risk from the GLP-1 mechanism itself is low on its own, compared with insulin or a sulfonylurea.
Reduced glucagon secretion. Glucagon is the hormone that raises blood glucose. The labels state these drugs reduce its secretion, again glucose-dependently, which adds to the insulin effect in lowering blood glucose.
Slowed gastric emptying. Food moves out of the stomach more slowly. Tirzepatide’s label adds that this effect is largest after the first dose and diminishes with continued treatment. So it is unlikely to be the main mechanism behind weight loss that continues over months.

None of the labels claim these four effects fully explain weight loss or the cardiovascular benefits seen in trials. Semaglutide’s label states that the exact mechanism behind its cardiovascular risk reduction has not been established. That is a disclosed gap between what the drug measurably does and what is fully explained.

Why the natural hormone lasts minutes and the drug lasts a week

This is what makes a once-weekly injection possible at all.

Native GLP-1 does not last long in the body. An enzyme called dipeptidyl peptidase-4 (DPP-4) breaks it down almost as soon as it is released, much of it before it has even left the gut Holst 2007 - Holst JJ. The physiology of glucagon-like peptide 1. Physiol Rev. 2007;87(4):1409-39., in the references below. In one study of GLP-1 injected under the skin, 30 minutes later roughly 80 to 90 percent of what was circulating was already the cut-down, inactive fragment Deacon 1995 - Deacon CF, Nauck MA, Toft-Nielsen M et al. Both subcutaneously and intravenously administered glucagon-like peptide I are rapidly degraded from the NH2-terminus in type II diabetic patients and in healthy subjects. Diabetes. 1995;44(9):1126-31., in the references below. That works as a signal tied to one meal. It is far too fast for one injection to do anything a week later.

The approved drugs are re-engineered versions built to survive:

Liraglutide, the earlier once-daily GLP-1 drug, uses a related version of the same idea. Its label describes it as stable against breakdown by DPP-4 and a second enzyme, neutral endopeptidase. The label states this stability, together with self-association that slows absorption and extensive plasma protein binding, produces a plasma half-life of 13 hours after injection. That is much longer than the natural hormone but shorter than semaglutide’s week. It is a once-daily rather than once-weekly drug, with a smaller version of the same fix.

One receptor, two receptors, three receptors

Every drug discussed so far activates one receptor: GLP-1’s own. The newer drugs add more.

Tirzepatide is a dual agonist. Its FDA label states that tirzepatide is a GIP receptor and GLP-1 receptor agonist. It selectively binds and activates both, the targets for native GIP and native GLP-1 respectively. GIP, glucose-dependent insulinotropic polypeptide, is a second gut hormone with its own receptor and its own overlapping effects on insulin secretion. The label states tirzepatide enhances first- and second-phase insulin secretion and reduces glucagon levels, both glucose-dependently. That is the same glucose-dependent pattern as the single-receptor drugs, now driven by two receptors instead of one.

Retatrutide, still investigational, is a triple agonist. Its phase 2 trial publication in the New England Journal of Medicine describes it as an agonist of the GIP, GLP-1 and glucagon receptors Jastreboff 2023 - Jastreboff AM, Kaplan LM, Frías JP et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity - A Phase 2 Trial. N Engl J Med. 2023;389(6):514-526., in the references below. That is a third receptor beyond what tirzepatide activates.

Glucagon itself is best known for raising blood glucose, which sounds like the opposite of what an obesity or diabetes drug wants. The trial studied retatrutide’s weight and safety outcomes as a combined triple-receptor molecule. It does not establish how much the glucagon-receptor component contributes versus the GIP and GLP-1 components shared with tirzepatide. Retatrutide is not FDA-approved for any indication, and nothing here claims it is available, proven safe, or proven effective. See the retatrutide page for its trial status in full.

Orforglipron activates the same single receptor as semaglutide, by a different chemical route. It is a small molecule rather than a peptide. That is a separate design choice from the one-versus-two-versus-three receptor question. See the orforglipron page for what the distinction means in practice.

Why this matters beyond the mechanism itself

The chemistry above is also why dosing is not something to improvise. A molecule engineered for a week-long half-life behaves nothing like the natural hormone it is derived from, and the approved titration schedules exist because of it. See the semaglutide half-life calculator for how that week-long elimination plays out between doses, and each drug’s own dosing page for its approved schedule.

Common questions

What is GLP-1?

GLP-1, or glucagon-like peptide-1, is a hormone released by cells in the gut after a meal. It is a physiological regulator of appetite and caloric intake. It also affects insulin secretion, glucagon secretion and how fast the stomach empties. GLP-1 receptor agonist drugs are built to activate the same receptor this natural hormone acts on.

Why does natural GLP-1 only last a couple of minutes?

An enzyme called dipeptidyl peptidase-4, or DPP-4, cleaves the native hormone almost as soon as it is released, so it lasts only minutes in the blood. That works as a meal-by-meal signal but is far too fast for a once-weekly injection without re-engineering the molecule.

How do drugs like semaglutide last a week if the natural hormone only lasts minutes?

The approved labels describe two changes. First, a fatty acid chain is attached to the molecule. It binds tightly to albumin, a blood protein, which shields the drug from being filtered out by the kidneys and slows its breakdown. Second, the molecule is modified at a specific position to resist cleavage by DPP-4, the enzyme that clears the natural hormone in minutes. Together, these give an elimination half-life of approximately one week, according to semaglutide's label.

What does a GLP-1 receptor agonist actually do in the body?

Per the FDA-approved labels, it selectively binds and activates the GLP-1 receptor, which sits in several brain regions involved in appetite regulation. Animal studies show it reaches and activates neurons in food-intake regions specifically. It also stimulates insulin secretion and reduces glucagon secretion, both in a glucose-dependent manner, and it delays gastric emptying.

What does glucose-dependent mean for insulin release?

It means the insulin-boosting effect scales with how high blood glucose already is, rather than firing at a fixed rate regardless of it. According to the labels, that is why these drugs carry a much lower intrinsic hypoglycaemia risk on their own than insulin or a sulfonylurea. The risk rises mainly when they are combined with one of those other drug classes.

What is the difference between tirzepatide and semaglutide?

Semaglutide activates one receptor, GLP-1. Tirzepatide's label states it is a GIP receptor and GLP-1 receptor agonist that selectively binds and activates both. It enhances first- and second-phase insulin secretion and reduces glucagon, both glucose-dependently. GIP is a second gut hormone with its own receptor. Adding it is the mechanistic difference between a single and a dual agonist.

What is retatrutide and how is it different from tirzepatide?

Retatrutide is described in its phase 2 trial publication as an agonist of the GIP, GLP-1 and glucagon receptors, a third receptor beyond what tirzepatide activates. It is investigational and not FDA-approved for any use. This describes its studied mechanism, not a claim that it is available or proven safe and effective.

Why would adding a glucagon receptor to a weight-loss drug make sense?

Glucagon itself is best known for raising blood glucose, which sounds like the opposite of what a diabetes or obesity drug wants. Retatrutide's phase 2 publication describes it as a triple-receptor agonist studied specifically for obesity. It adds glucagon-receptor activity to the GIP and GLP-1 actions already established in tirzepatide. The trial measured weight and safety outcomes. It is not established here whether or how much the glucagon component adds to the effect. That is a question for the primary literature and an approved label, not this page.

Does slowed gastric emptying explain the whole appetite effect?

No. The labels describe it as one of several mechanisms, alongside direct action on brain appetite circuits and the hormone effects on insulin and glucagon. Tirzepatide's label also notes that the gastric-emptying delay is largest after the first dose and diminishes with continued treatment. That argues against slowed digestion being the main driver of sustained weight loss.

Is a GLP-1 receptor agonist the same thing as insulin?

No. Insulin lowers blood glucose directly and can cause hypoglycaemia on its own. A GLP-1 receptor agonist increases insulin release only when glucose is already elevated. It also reduces glucagon and slows gastric emptying, a different and broader mechanism than injecting insulin.

References (6 sources)

Bibliographic detail is fetched from PubMed, not written by me - so a citation here cannot drift from the paper it names. Study design comes from PubMed's own tags rather than my judgement.

1.Holst JJ. The physiology of glucagon-like peptide 1. Physiol Rev. 2007;87(4):1409-39.ReviewPMID 17928588
2.Deacon CF, Nauck MA, Toft-Nielsen M et al. Both subcutaneously and intravenously administered glucagon-like peptide I are rapidly degraded from the NH2-terminus in type II diabetic patients and in healthy subjects. Diabetes. 1995;44(9):1126-31.StudyPMID 7657039
3.Jastreboff AM, Kaplan LM, Frías JP et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity - A Phase 2 Trial. N Engl J Med. 2023;389(6):514-526.Randomised trialPMID 37366315
4.WEGOVY (semaglutide) injection - full prescribing information, including Mechanism of Action (12.1) and Pharmacokinetics (12.3).- DailyMed, National Library of Medicinelabel
5.MOUNJARO (tirzepatide) injection - full prescribing information, including Mechanism of Action (12.1) for the dual GIP/GLP-1 agonist.- DailyMed, National Library of Medicinelabel
6.SAXENDA (liraglutide) injection - full prescribing information, including Mechanism of Action (12.1) and its stated 13-hour half-life.- DailyMed, National Library of Medicinelabel