How Mounjaro works: the mechanism of action behind tirzepatide

Two receptors, not one: what GIP and GLP-1 actually do

GLP-1 (glucagon-like peptide-1) is a hormone released by the gut after eating. It signals to the brain that you're full, slows the rate at which food leaves the stomach, and prompts the pancreas to release insulin in a glucose-dependent way. That last point matters: insulin release only increases when blood sugar is actually rising, which reduces the risk of hypoglycaemia.

GIP (glucose-dependent insulinotropic polypeptide) is a second gut hormone that amplifies insulin release and, crucially, also acts on fat tissue. Research suggests GIP signalling plays a role in how the body stores and mobilises fat, a pathway that earlier GLP-1-only medicines don't directly address. According to the NHS medicines page on tirzepatide, Mounjaro activates both receptors, which is why it is classified as a dual agonist rather than a single GLP-1 agonist like semaglutide.

There is a misconception worth clearing up gently here: some people assume Mounjaro simply works harder because the doses are higher than other injectables. The real reason its results differ in trials is the dual-pathway biology, not the number on the pen. The 2.5 mg starting dose barely touches weight, its job is to help your system adjust before the therapeutic doses begin, including the Mounjaro 2.5 mg dose that most people move on to next.

What the SURMOUNT-1 trial showed about this mechanism in practice

The most direct evidence for tirzepatide's mechanism comes from SURMOUNT-1, a 72-week phase 3 trial published in the New England Journal of Medicine involving 2,539 adults with obesity who did not have type 2 diabetes. Participants at the highest dose (15 mg) achieved an average body-weight reduction of around 20–21%, with some analyses reaching approximately 22.5%. Lower doses produced meaningful but smaller reductions, and if you want to understand what to expect at an earlier stage of treatment, the Mounjaro 5 mg dose guide covers the first of the main therapeutic steps in detail.

NICE reviewed this evidence in its appraisal of tirzepatide, published in December 2024 (NICE TA1026), and found the dual-agonist mechanism clinically distinct enough from semaglutide to support a separate recommendation. The head-to-head SURMOUNT-5 trial, published in the New England Journal of Medicine in 2025, subsequently showed that tirzepatide produced greater average weight reduction than semaglutide 2.4 mg over 72 weeks in adults with obesity, the first direct comparison of the two approaches at scale.

For a wider look at how tirzepatide compares to other treatments available in the UK, the tirzepatide overview covers the full clinical picture.

What this mechanism means for how treatment feels week to week

Understanding the mechanism helps make sense of the treatment experience. Appetite reduction tends to be the most noticed effect, most people find food feels less compelling, portion sizes that once felt modest now feel ample, and the urge to snack between meals fades. This is the GLP-1 pathway working on satiety signalling in the brain and the slower gastric emptying reducing how quickly hunger returns.

The GI side effects that some people notice early on (nausea, loose stools, indigestion) are also a direct consequence of this mechanism, particularly the gastric-emptying slowdown. They tend to be most noticeable in the first days after a dose increase and settle for most people within a week or two. They are not a sign something has gone wrong; they are the body adjusting to a new hormonal signal. If side effects feel unmanageable, the right conversation is with a prescriber, not a decision to stop abruptly. The downsides of Mounjaro page covers this in more detail.

Because the mechanism affects digestion timing, the practical rules around injection day, storage and what to eat matter more than people sometimes expect. If you are sourcing your treatment online, our e-Mounjaro service page explains how the process works and what to expect, and the Mounjaro Patient Information Leaflet and the Mounjaro SmPC on the eMC are the authoritative references for those specifics.

Who the mechanism is licensed to treat, and what a prescriber weighs up

Tirzepatide's UK licence covers adults with a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related condition such as high blood pressure, high cholesterol, obstructive sleep apnoea or type 2 diabetes. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. A BMI figure alone does not determine suitability; the prescriber looks at the full clinical picture, including other medicines you take, relevant medical history and any conditions that would make this mechanism less appropriate.

Some contraindications relate directly to the mechanism: people with a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia type 2 should not take tirzepatide. A history of pancreatitis also requires careful clinical discussion, the MHRA has highlighted acute pancreatitis as a known, infrequent but serious risk with GLP-1 medicines, and prescribers factor this in at assessment.

If you are considering treatment and want to understand whether tirzepatide's mechanism is matched to your situation, the full Mounjaro guide sets out the eligibility picture, and you can explore what private treatment involves on the weight-loss treatments overview. A prescriber at our clinical team reviews every consultation personally (not a checklist, not an algorithm) the same day it comes in. When you are ready, you can speak to our prescribers through a free consultation.