How tirzepatide works: GIP and GLP-1 dual agonism
Tirzepatide is the first medication to activate two incretin receptors at once — here's what that does in the body.
The hormones tirzepatide imitates
After you eat, the gut releases incretin hormones — chiefly glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP). These hormones tell the pancreas to release insulin in proportion to the meal, an effect called the “incretin effect.” In type 2 diabetes this response is blunted. Tirzepatide is a synthetic peptide engineered to activate both incretin receptors, restoring and amplifying these post-meal signals.
What activating the GLP-1 receptor does
GLP-1 receptor activation drives several effects: it stimulates glucose-dependent insulin release (so insulin rises mainly when blood glucose is high), suppresses glucagon (the hormone that raises blood sugar), slows the rate at which the stomach empties, and acts on appetite centers in the hypothalamus and brainstem to increase satiety. Because insulin release is glucose-dependent, the risk of hypoglycemia from GLP-1 action alone is low.
The role of the second receptor
GIP is the other major incretin. On its own its therapeutic role was historically uncertain, but combined GIP/GLP-1 agonism appears to produce greater improvements in glucose control and weight than GLP-1 agonism alone in head-to-head data. Proposed contributions of GIP-receptor activation include additional insulinotropic effects, influences on adipose-tissue lipid handling and energy metabolism, and possible central effects that may help with nausea tolerance. The exact contribution of each receptor in humans is still an active research question.
Why dual agonism matters
By engaging both receptors with one molecule, tirzepatide lowers blood glucose and reduces body weight to a degree that, in trials, exceeded a leading GLP-1-only therapy (see SURPASS-2). In the obesity program, mean weight reductions reached roughly a fifth of body weight at the highest dose (see SURMOUNT-1). The dual mechanism is the leading explanation for these results, though weight loss also depends on dose, duration and lifestyle.
What this does and doesn't mean
Mechanism explains how a drug can work, not whether it is right for any individual. Tirzepatide carries risks and contraindications (see thyroid warning and GI effects), and only a licensed clinician can determine appropriateness. Compounded tirzepatide is not FDA-approved and is not the same product as brand-name Mounjaro or Zepbound.
What researchers are still working out
Even as the clinical results are clear, the mechanism is not fully mapped. Scientists are still quantifying how much of tirzepatide's effect comes from the GIP arm versus the GLP-1 arm, why combined agonism appears to outperform either alone, and how central (brain) versus peripheral (gut, pancreas, fat) actions each contribute. There is also active debate about whether GIP-receptor agonism or, paradoxically, partial antagonism best explains the weight effect, since both hypotheses have experimental support. Receptor signaling is complex: the same receptor can trigger different downstream pathways depending on how a drug binds it (so-called biased agonism), which may explain why tirzepatide behaves differently from native GIP. For readers, the practical point is humility — the drug works in trials, but “we know exactly why” would overstate the current science. This is one reason next-generation molecules are tested empirically in humans rather than designed purely from theory.
Primary sources
- Coskun T, Sloop KW, Loghin C, et al. LY3298176, a novel dual GIP and GLP-1 receptor agonist. Mol Metab. 2018;18:3-14.
- Jastreboff AM, Aronne LJ, Ahmad NN, et al. Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). N Engl J Med. 2022;387(3):205-216.
- Frías JP, Davies MJ, Rosenstock J, et al. Tirzepatide versus semaglutide once weekly in type 2 diabetes (SURPASS-2). N Engl J Med. 2021;385(6):503-515.
- U.S. Food and Drug Administration. Mounjaro and Zepbound (tirzepatide) prescribing information. Eli Lilly and Company.
Citations are provided for educational reference. This article summarizes published research in plain language and is not medical advice. Always consult a licensed clinician.
Common questions
Is tirzepatide a GLP-1 drug?
It activates the GLP-1 receptor but also the GIP receptor, making it a dual GIP/GLP-1 receptor agonist rather than a GLP-1-only medication like semaglutide.
Does tirzepatide cause hypoglycemia?
On its own the risk is low because its insulin effect is glucose-dependent. Risk rises when it is combined with insulin or sulfonylureas. See our hypoglycemia explainer.
Is compounded tirzepatide the same as Mounjaro or Zepbound?
No. Only brand-name Mounjaro and Zepbound are FDA-approved. Compounded tirzepatide is prepared by licensed pharmacies and is not FDA-approved.