Receptor agonist
A molecule that binds a receptor and switches it on. The GLP-1 drugs are agonists built to last far longer than the gut peptide they copy.
A receptor is a protein, usually on the surface of a cell, that changes shape when the right molecule binds to it. That change sets off a signal inside the cell. An agonist is a molecule that binds the receptor and produces that signal. An antagonist binds the same receptor and blocks it without switching it on. A partial agonist switches it on, but less than fully.
"GLP-1 receptor agonist" therefore describes what a drug does, not what it is made of. Semaglutide, liraglutide and tirzepatide are peptides. Orforglipron, sold as Foundayo, is a small molecule that also activates the GLP-1 receptor. Both kinds are agonists.
Why the body's own GLP-1 is not a drug
The gut releases GLP-1 after a meal. It stimulates insulin release when glucose is high, lowers glucagon, slows the stomach and acts on appetite centres in the brain. The trouble is speed. The Saxenda label states that endogenous GLP-1 has a half-life of 1.5 to 2 minutes, because the enzymes DPP-4 and neutral endopeptidase cut it almost as soon as it is released.
Every GLP-1 drug is an answer to that problem. The agonist has to hit the same receptor while resisting the enzymes and staying in the blood for hours or days.
| Drug | Receptors activated | Half-life | How it lasts |
|---|---|---|---|
| Native GLP-1 | GLP-1 | 1.5 to 2 minutes | It does not; DPP-4 cleaves it |
| Liraglutide (Saxenda, Victoza) | GLP-1 | About 13 hours | 97% sequence homology; C16 fatty acid binds albumin |
| Semaglutide (Wegovy, Ozempic) | GLP-1 | About 1 week | 94% homology; position 8 change resists DPP-4; C18 fatty diacid binds albumin |
| Tirzepatide (Zepbound, Mounjaro) | GIP and GLP-1 | About 5 days | Built on the GIP sequence; C20 fatty diacid binds albumin |
Single, dual and triple agonists
Semaglutide and liraglutide act on one receptor. Tirzepatide is a dual agonist: its label describes it as a GIP receptor and GLP-1 receptor agonist. Retatrutide, which is still investigational, adds a third target, the glucagon receptor. In its phase 2 trial of 338 adults, the 12 mg dose produced a mean 24.2 percent weight loss at 48 weeks against 2.1 percent on placebo. That is phase 2 data, and the drug is not approved.
Adding receptors is not automatically better. Each receptor brings its own effects and side effects, and the balance between them is set by how strongly the molecule activates each one. That balance is a design choice, which is why two "dual agonists" can behave quite differently.
What agonism does not tell you
Activating a receptor in a dish says nothing about dose, duration or outcome in a person. Those come from pharmacokinetics (how much reaches the receptor, and for how long) and pharmacodynamics (what happens once it does). Many research peptides are described by the receptor they are said to act on, with little or no human data on either. The compound pages label how much evidence sits behind each claim.
In the app, the compound library lists the receptor targets for each compound, and the dose log keeps doses, weight and side effects on one timeline so you can see what a given agonist is doing for you.
Related terms: half-life, pharmacodynamics, amino acid sequence, titration. Back to the glossary.
Questions people ask
What does GLP-1 receptor agonist mean?
A drug that binds the GLP-1 receptor and activates it, copying the effect of the gut peptide GLP-1. Semaglutide, liraglutide, tirzepatide and orforglipron are all GLP-1 receptor agonists.
Why not just use natural GLP-1?
It is broken down within minutes. The Saxenda label gives the half-life of endogenous GLP-1 as 1.5 to 2 minutes, compared with about 13 hours for liraglutide and about a week for semaglutide.
What is the difference between an agonist and an antagonist?
An agonist binds a receptor and switches it on. An antagonist binds the same receptor and blocks it without producing a signal.
Is tirzepatide a GLP-1 agonist?
Yes, and also a GIP receptor agonist. Its label describes it as a GIP receptor and GLP-1 receptor agonist, which is why it is called a dual agonist.
Is retatrutide approved?
No. It is investigational. Its phase 2 trial in 338 adults reported a mean 24.2 percent weight loss at 48 weeks on 12 mg against 2.1 percent on placebo.
Sources
- Saxenda prescribing information, Novo Nordisk
- Wegovy prescribing information, Novo Nordisk
- Zepbound prescribing information, Eli Lilly
- Mechanisms of action and therapeutic application of glucagon-like peptide-1 (Drucker), Cell Metabolism, 2018
- Retatrutide for obesity: a phase 2 trial (Jastreboff et al.), New England Journal of Medicine, 2023
