How the Mounjaro injection works: the science behind it

The dual-receptor mechanism: what GIP and GLP-1 actually do

Tirzepatide is described in clinical literature as a dual GIP and GLP-1 receptor agonist. Those two acronyms refer to gut hormones (glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1)) that the body releases naturally after eating. Their job, in simple terms, is to signal to the brain that food has arrived and to the pancreas that insulin should be released.

What the Mounjaro injection does is mimic both of those signals at once. GLP-1 receptor activation slows gastric emptying, meaning food moves from the stomach to the small intestine more slowly than usual. That alone tends to reduce appetite noticeably. GIP receptor activation works alongside it, contributing to how the brain processes satiety and, researchers believe, how the body handles fat storage, though the full picture of GIP's role is still being studied.

The NHS medicines page for tirzepatide explains the dual mechanism clearly and is a useful reference if you want to read the patient-level description directly. What matters practically is that the combination of both pathways appears to produce stronger appetite suppression than activating either receptor alone. If you are curious about how this compares to the pen's physical delivery, our page on how the Mounjaro pen works covers the injection device itself in detail.

There is a common assumption that these medicines work by speeding up metabolism. They do not, at least not primarily. The weight loss comes from eating less, not from burning more. Clearing that up early tends to set more realistic expectations.

What the SURMOUNT trial programme found

The clinical evidence behind how the Mounjaro injection works comes largely from the SURMOUNT programme, a set of trials involving thousands of adults. SURMOUNT-1, published in the New England Journal of Medicine, randomised 2,539 adults with obesity and no diabetes to tirzepatide or placebo over 72 weeks. At the 15 mg dose, participants lost an average of around 20–21% of their body weight. That is a figure that, before these medicines existed, was largely associated only with bariatric surgery.

SURMOUNT-5, also published in the NEJM in 2025, went a step further and directly compared tirzepatide to semaglutide 2.4 mg (the active ingredient in Wegovy) in adults with obesity. Tirzepatide produced greater average weight reduction. NICE cited this evidence in its appraisal of tirzepatide, TA1026, published in December 2024, when it recommended the medicine for eligible adults on the NHS.

It is worth being precise about what these trial results mean. They are averages from large populations under controlled conditions, with lifestyle support alongside the medicine. Individual results vary considerably. How soon you might notice a difference is a separate question, our page on how quickly Mounjaro works after injection goes into that timing in more detail.

The titration schedule and why it matters for how the medicine works

Mounjaro is available in six strengths: 2.5, 5, 7.5, 10, 12.5 and 15 mg. Treatment begins at 2.5 mg, a dose whose purpose is to let your system settle, not to produce significant weight loss immediately. Each pen contains four weekly doses. Your prescriber typically increases the dose in steps of roughly four weeks, moving up when you are tolerating the current level well.

This staged approach is directly connected to how the injection works. Because tirzepatide slows gastric emptying, the most common early side effects are gastrointestinal: nausea, occasional vomiting, loose stools or constipation. These tend to be most noticeable in the first days after a new dose and usually settle within one to two weeks. Starting low and going slowly gives the gut time to adapt, which is why your prescriber controls the titration rather than leaving it to you.

For a fuller picture of the injection experience from the first dose onward, this page on what to expect after your first Mounjaro injection covers the early weeks in practical terms. If you want to understand the broader context of what Mounjaro is used for, our page on how Mounjaro works for weight loss brings together the mechanism and the clinical outcomes in one place.

Mounjaro is a prescription-only medicine, and your dose is set by a prescriber who has reviewed your full health picture. Questions about whether to change dose, what to do after a missed injection, or how to manage side effects should go to your prescriber or the Patient Information Leaflet, not a general information page.

Getting the injection right: practical points from the SmPC

The once-weekly injection is given subcutaneously (under the skin) in the abdomen, thigh or upper arm. Rotating sites each week reduces the risk of skin reactions at any one spot. The pen is kept refrigerated at 2–8°C; it can be left at room temperature for a limited period, but for the exact window you should check the Patient Information Leaflet that comes with your pen, since that is the authoritative source for your specific product.

Alcohol swabs are a simple way to clean the injection site beforehand; sterile alcohol swabs are available if you need to restock. Used pens are sharps and should be disposed of safely, your pharmacy or GP surgery can advise on local collection points.

If your pen looks damaged before use, do not inject from it. Guidance on what to do with a damaged Mounjaro pen covers the steps to take in that situation. And if you are thinking about whether Mounjaro could be right for you, the Mounjaro overview page brings together eligibility, cost context and the consultation process. You can also see what Mounjaro typically costs through a private clinic if that is part of your decision. When you are ready, starting a free consultation is the next step, a GPhC-registered prescriber, not an algorithm, reviews your answers the same day.