For most of this drug class's history, a GLP-1 medication meant exactly what the name said: a molecule that engaged the GLP-1 receptor, and nothing else. Then tirzepatide arrived engaging two, and the drugs currently in development engage three. Read the coverage and you would think this was a race to see who can stack the most receptors into one injection.
It is better understood as a design direction, and the reasoning behind it is more interesting than a scoreboard. Each additional target was chosen for a specific reason, one of them was chosen despite prevailing evidence suggesting it would not work, and one of them looks, at first glance, like the drug is contradicting itself.
This article works through what those receptors actually are. If you want the ground floor first, our explainer on what GLP-1 is covers the original hormone.
GIP, the other incretin
GIP stands for glucose-dependent insulinotropic polypeptide. It is secreted by K cells in the upper segment of the small intestine, which is a meaningful detail: GIP is released early, near the top of the gut, while GLP-1 comes from L-cells further along.
GLP-1 and GIP are the two main incretins. They are companion hormones with overlapping jobs, both released in response to eating, and together they enhance glucose-stimulated insulin secretion. They are also, tidily, taken apart by the same enzyme: DPP-4 rapidly converts both GLP-1 and GIP into inactive metabolites.
That shared vulnerability is why both had to be engineered before either could become a medication. Any molecule intended to act at either receptor for longer than a few minutes has to survive DPP-4, which is the design constraint that shaped this entire class.
The incretin effect, in one experiment
The reason these hormones were ever worth studying comes from a comparison that is easy to picture.
Give someone a measured amount of glucose to drink, and measure the insulin response. Then give the same person the same amount of glucose delivered directly into a vein, and measure again. The oral route produces the larger insulin response, even though the pancreas is dealing with the same quantity of glucose either way.
The difference is the gut. Glucose that arrives through the intestine triggers hormone release along the way, and those hormones prime the pancreas before the glucose fully lands. Glucose injected into a vein skips all of that. The gap between the two is what physiologists call the incretin effect, and GLP-1 and GIP together are what produce it.
That is the finding that made incretins worth a career. Eating is not simply a delivery mechanism for calories; it is a signaling event, and the signals do real work.
The wrinkle: GIP does not behave the same way in type 2 diabetes
Here is the part that explains why the medications took the path they did.
In healthy people, GIP is the main incretin hormone. It carries more of the load than GLP-1 does. On that basis alone, GIP looks like the obvious target, the one anyone would go after first.
But the insulin response to GIP is strongly reduced in type 2 diabetes. The hormone is still there, the receptor is still there, and the effect largely is not. For a field whose first goal was better diabetes treatment, that was close to disqualifying, and GIP spent years on the shelf while GLP-1, whose effect held up better, became the basis of an entire drug class.
Which is what makes tirzepatide interesting as a design decision rather than merely as a product. Combining GIP with GLP-1 was not the obvious move it looks like in hindsight; the prevailing evidence pointed away from it. Tirzepatide is a dual agonist of the GIP receptor and the GLP-1 receptor, and it is the version of that idea available in a pharmacy today, in Mounjaro and Zepbound. Our Mounjaro tracker guide covers the practical side of taking it, and the tirzepatide half-life article covers what happens between shots.
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Glucagon, and the paradox worth spelling out
The third receptor is the one that makes people stop and reread the sentence, because it appears to point in the wrong direction.
Glucagon is insulin's counterweight. Its everyday job is to push blood sugar back up when it falls, which is useful between meals and unhelpful right after one, so one of GLP-1's own jobs is to suppress it, as our guide to the GLP-1 hormone itself describes. That much is straightforward.
And yet newer investigational molecules deliberately activate the glucagon receptor. Both statements are true, and the apparent contradiction dissolves once you look at why.
Glucagon receptor activation is not being pursued for its effect on blood sugar. It is being pursued because it increases energy expenditure. The design logic is a pairing: GLP-1 and GIP receptor activation reduce calorie intake, glucagon receptor activation increases energy output, and the hypothesis is that addressing both sides of the equation does more than addressing one. Reduced intake plus increased output, from a single molecule.
Retatrutide is the example people have heard of, a triple agonist of the GLP-1, GIP and glucagon receptors, and it is investigational rather than available. We keep the specifics in one place: our retatrutide guide covers what has been reported, the realistic timeline, and why gray-market vials sold online are a bad idea.
A mechanism, it is worth saying plainly, is a hypothesis. Pairing two effects in a diagram is not the same as demonstrating that the pairing helps people, and regulators assess evidence rather than reasoning.
The receptor map, in one table
Three receptors, three different reasons for engaging them.
| Receptor | The hormone behind it | Why a medication engages it |
|---|---|---|
| GLP-1 | Glucagon-like peptide-1, from L-cells in the intestine | Glucose-dependent insulin release, less glucagon, slower gastric emptying, satiety |
| GIP | Glucose-dependent insulinotropic polypeptide, from K cells in the upper small intestine | The other main incretin; the two together produce the incretin effect |
| Glucagon | Glucagon, from the pancreas | Increased energy expenditure, to complement reduced intake |
Receptors are not the only direction the field is exploring, either. Some combinations pair semaglutide with cagrilintide, an amylin analog, which is a separate hormone family and a separate story; our CagriSema guide covers that one.
What more receptors mean for you today
If you are on a GLP-1 right now, the honest summary is short.
More receptors is not automatically better for you. It is a different pharmacology, not an upgrade path. The medication that produced the most striking results in a study population is not necessarily the one that suits your body, your other conditions or your tolerance, and there is no version of that judgment that can be made from a mechanism diagram.
Side effect profiles differ. Engaging different receptors produces different experiences, and tolerability is often what decides whether someone stays on a medication at all. This is exactly the kind of thing a log answers and memory does not, which is the argument our guide to tracking side effects makes at length.
Switching is a clinical decision with a real transition. Moving between medications in this class usually means restarting a titration ladder and living through a few weeks where the old medication is still clearing while the new one builds. Our article on switching GLP-1 medications covers what to track through it.
And these are prescriber conversations. Not forum conversations, and certainly not app conversations. Which receptors your medication engages is a good thing to understand, because it makes the rest of your experience legible: why the fullness arrives, why the timeline runs in weeks rather than days, why one medication felt different from another. It is not a basis for choosing one.
The direction of travel is clear enough. The field started with one hormone, found a second worth reconsidering, and is now testing whether a third can add something from the other side of the energy equation. What that means for any individual person is still, as it should be, a question answered one appointment at a time.
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Frequently asked questions
What is GIP and how is it different from GLP-1?
GIP is glucose-dependent insulinotropic polypeptide, secreted by K cells in the upper segment of the small intestine. GLP-1 comes from L-cells further along the gut. Both are incretins, both are released in response to eating, both help drive glucose-stimulated insulin secretion, and both are rapidly converted to inactive metabolites by the DPP-4 enzyme. They are companions rather than duplicates, and they arrive from different parts of the intestine.
What is the incretin effect?
It is the observation that glucose taken by mouth produces a larger insulin response than the same amount of glucose delivered directly into a vein. The difference comes from gut hormones released by eating, chiefly GLP-1 and GIP, which prime the pancreas before the glucose has fully arrived. That gap between the two routes is the incretin effect, and it is why these hormones became interesting to researchers in the first place.
Why does tirzepatide target GIP as well as GLP-1?
Tirzepatide is a dual agonist of the GIP receptor and the GLP-1 receptor, engaging both incretin pathways with one molecule. Combining them was not the obvious step it can look like in hindsight, because the insulin response to GIP is strongly reduced in type 2 diabetes, which kept GIP out of favor for years. Whether a dual approach suits you is a question for your prescriber.
If GLP-1 lowers glucagon, why do newer drugs activate the glucagon receptor?
It looks contradictory and it is worth being precise about. GLP-1 suppresses glucagon secretion after a meal, when raising blood sugar is not what the body needs. Glucagon receptor activation in investigational drugs is being pursued for a different reason: it increases energy expenditure. The goal is to pair reduced intake, driven by GLP-1 and GIP, with increased output, rather than to raise blood sugar.
Does hitting more receptors automatically mean better results?
No, and it is worth resisting that assumption. More receptors means a different pharmacology, not a guaranteed improvement for any particular person. Side effect profiles differ between these medications, tolerability differs, and the medication that suits someone else may not suit you. Which one fits your situation is a prescriber conversation informed by your history, not something to settle from a mechanism diagram.