Medically reviewed by: Dr Abdullah
Reading time: 7 minutes · Guidance verified: August 2026
Most explanations stop at “it mimics a gut hormone.” That is true of many treatments in this class and explains almost nothing about this one. How does Mounjaro work at the level that actually matters is a question about two receptors, one engineered molecule, and a signalling quirk that separates tirzepatide from every single-hormone alternative.
Two hormones, not one
After you eat, the gut releases hormones called incretins. Two matter here.
GLP-1, glucagon-like peptide-1, is released from L-cells in the lower intestine. It prompts insulin release, suppresses glucagon, slows stomach emptying and acts on appetite centres in the brain.
GIP, glucose-dependent insulinotropic polypeptide, comes from K-cells higher up in the small intestine. It is the more powerful insulin trigger of the two in healthy physiology, and it also acts on fat tissue and on separate regions of the brain.
Together these account for the incretin effect: oral glucose produces far more insulin than the same amount of glucose given intravenously. In type 2 diabetes that effect is blunted. Tirzepatide engages both pathways at once, which is why it is described as a dual incretin agonist rather than a GLP-1 medicine.
The molecule is built on GIP, not GLP-1
This surprises people. Tirzepatide is a 39 amino acid synthetic peptide whose backbone is derived from native GIP, with targeted substitutions engineered in to allow it to also bind the GLP-1 receptor. It is not a GLP-1 drug with GIP bolted on. It is the reverse.
Two pieces of engineering make it usable as a weekly injection:
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Aib residues. Non-standard aminoisobutyric acid at positions 2 and 13 blocks the enzyme DPP-4, which ordinarily destroys native incretins within minutes.
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A C20 fatty diacid chain attached via a linker at lysine 20. This binds tightly to albumin in the blood, roughly 99%, creating a circulating reservoir that releases the drug slowly.
The result is a plasma half-life of around five days, against roughly five minutes for native GIP. That single change is what turns an incretin into a once-weekly medicine, and it is why steady state takes about four weeks to reach after starting or changing dose.
Imbalanced and biased: the part most explanations skip
Tirzepatide is not a balanced 50/50 dual agonist, and this is where how does Mounjaro work becomes genuinely distinct. At the GIP receptor, it binds with affinity comparable to the body’s own GIP. At the GLP-1 receptor, affinity is roughly five times weaker than native GLP-1. The molecule is therefore GIP-forward, with GLP-1 activity acting as a secondary arm.
More interesting is what happens after binding. Receptors of this type can signal down two main routes: the Gs and cAMP pathway, which drives the useful metabolic effects, and beta-arrestin recruitment, which pulls the receptor inside the cell and shuts signalling down.
At the GLP-1 receptor, tirzepatide strongly favours cAMP generation and recruits very little beta-arrestin. This is called biased agonism. Because less beta-arrestin means less receptor internalisation, the receptor stays available on the cell surface rather than desensitising during sustained exposure. Weaker binding, but longer-lasting signalling, and a different tolerability profile as a result.
How does Mounjaro work in the pancreas?
Both receptors are present on pancreatic beta cells, and both contribute. Blocking the GIP receptor experimentally reduces the insulin response to tirzepatide, which confirms the GIP arm is doing real work rather than sitting redundant.
Two effects follow:
Insulin secretion increases, but only when glucose is high. This is the critical safety feature. The mechanism is glucose-dependent, so as blood sugar falls towards normal the stimulus fades. That is why tirzepatide on its own carries a low intrinsic risk of hypoglycaemia, and why the risk rises when it is combined with insulin or a sulfonylurea.
Glucagon is suppressed. Glucagon is the hormone that tells the liver to release stored glucose. Reducing it lowers the amount of sugar the liver pushes into the bloodstream between meals, again in a glucose-dependent way.
How does Mounjaro work for weight loss?
Weight reduction comes from several mechanisms operating together, not one.
Appetite signalling in the brain. GLP-1 and GIP receptors are both expressed in the hypothalamus and hindbrain, regions that regulate hunger, satiety and food reward. Acting on both is thought to reduce food intake more than acting on either alone, and appears to blunt the intrusive preoccupation with food that many patients describe.
Slower gastric emptying. Food leaves the stomach more slowly, so fullness after a meal lasts longer and arrives sooner. This effect is strongest after the first doses and lessens over time.
Effects on fat tissue. GIP receptors are present on adipocytes, where activation appears to influence how fat is stored and how sensitive tissue is to insulin. This arm is unique to dual agonists and is still being characterised.
The honest caveat: the full mechanism is not completely understood. What is established is that engaging two incretin pathways produces greater average weight reduction in trials than engaging one, and that the medicine is licensed as an adjunct to a reduced-calorie diet and increased physical activity, not as a substitute for either.
How does Mounjaro work differently from single-hormone treatments?
Medicines that act on the GLP-1 receptor alone use one lever. Tirzepatide uses two, and the second is not simply more of the same.
GIP receptors sit in places GLP-1 receptors do not, most notably on fat cells and in distinct brain regions. Adding that arm changes where the drug acts, not just how hard it acts. The biased signalling at the GLP-1 receptor adds a further difference, because a receptor that internalises less remains responsive for longer during continuous exposure.
That combination is the working explanation for why dual agonism has produced larger average reductions in body weight and HbA1c in trials than single-receptor agonists. It is a mechanistic difference, not a dose difference, and it is the reason the two classes are not interchangeable milligram for milligram.
Why the side effects follow from the mechanism
The common adverse effects are not incidental. Nausea, vomiting, diarrhoea and constipation are direct consequences of slowed gastric emptying and altered gut motility, which is precisely the mechanism producing the benefit. This is why the dose is escalated stepwise with a minimum interval at each level, and why symptoms typically ease as the body adapts.
Delayed gastric emptying is also why anyone taking tirzepatide should tell an anaesthetist before a procedure involving general anaesthesia or deep sedation, because residual stomach contents raise aspiration risk.
Full tolerability information is set out on our Mounjaro treatment page. In the UK, tirzepatide carries Black Triangle status, meaning it is under additional MHRA monitoring. Suspected adverse reactions should be reported through the MHRA Yellow Card scheme.
What this means in UK practice
NICE recommends tirzepatide for managing overweight and obesity in adults under technology appraisal TA1026, alongside a reduced-calorie diet and increased physical activity, for people with a BMI of at least 35 kg/m² and at least one weight-related comorbidity. A lower BMI threshold, usually reduced by 2.5 kg/m², applies to people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds.
National commissioning guidance phases publicly funded access by cohort and requires structured wraparound care as part of treatment. NICE also sets a review point: if less than 5% of initial weight has been lost after six months on the highest tolerated dose, continuation is reconsidered.
You can read more about Mounjaro treatment at Rightangled, or compare it against the wider range of weight management options. Patients in Europe are served by Medetone, and patients in the United States by Medetone US.
Summary
So, how does Mounjaro work? A GIP-based peptide, engineered to resist enzymatic breakdown and to bind albumin for weekly dosing, activates two incretin receptors at once. It triggers insulin only when glucose is raised, suppresses glucagon, slows the stomach, and acts on appetite pathways in the brain and on fat tissue. It does this with a signalling bias that keeps GLP-1 receptors responsive rather than desensitised. Two levers instead of one. If you want the practical detail on dosing and supply, see Mounjaro (tirzepatide) at Rightangled.
“Patients who understand that the nausea and the appetite change come from the same mechanism tend to stay the course. It stops feeling like a side effect happening to them and starts feeling like evidence the medicine is doing something.”
Dr Abdullah, Medical Director at Rightangled
Every weight management consultation at Rightangled is reviewed by GPhC-registered independent prescribers as soon as the consultation is submitted. Treatment is supplied only where it is clinically appropriate, and individual responses vary.
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References
1. National Institute for Health and Care Excellence. Tirzepatide for managing overweight and obesity, TA1026. NICE technology appraisal TA1026
2. Medicines and Healthcare products Regulatory Agency. Drug Safety Update, GLP-1 receptor agonists: reminder of the potential side effects. October 2024. MHRA Drug Safety Update on GLP-1 receptor agonists
3. Willard FS, et al. Tirzepatide is an imbalanced and biased dual GIP and GLP-1 receptor agonist. JCI Insight, 2020. JCI Insight receptor pharmacology study
This article is for information only and is not a substitute for individual medical advice. Tirzepatide is a prescription-only medicine. Suitability, dosing and expected outcomes vary between individuals and must be assessed by a prescriber. Always read the patient information leaflet supplied with your medicine. Last reviewed September 2026.
