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How Mounjaro Works

Whether you are considering Mounjaro for the first time or trying to understand why it works the way it does, this guide explains, in plain English, what Mounjaro actually does in the body, why it is different from earlier weight-loss medicines, and what UK patients can realistically expect. LetsLoseWeight is an independent comparison site; we are not affiliated with any pharmacy or provider, and we do not sell or prescribe medication.

What is Mounjaro?

Mounjaro is the brand name used by Eli Lilly for tirzepatide, a prescription-only injection licensed by the Medicines and Healthcare products Regulatory Agency (MHRA) in the UK for two uses:

  • Weight management — alongside a reduced-calorie diet and increased physical activity, in adults with obesity or with overweight plus a weight-related health condition.
  • Type 2 diabetes — to help control blood sugar.

Tirzepatide is given once a week as a small subcutaneous injection, usually in the abdomen, thigh or upper arm. The dose is increased gradually over several months to reduce the risk of side effects.

In December 2024, NICE Technology Appraisal TA1026 recommended tirzepatide for use within the NHS for weight management, with phased rollout through primary care over three years. Outside the NHS, Mounjaro is available privately from regulated UK online pharmacies — see our Mounjaro UK guide for how access works.

How Mounjaro works in the body

Mounjaro is a dual GIP and GLP-1 receptor agonist — meaning it mimics two different hormones the body naturally produces in the gut after a meal:

  • GLP-1 (glucagon-like peptide-1)
  • GIP (glucose-dependent insulinotropic polypeptide)

Both hormones are part of the body's normal appetite and blood sugar control system. They are released from the small intestine after eating, tell the brain you have eaten, slow down how quickly food leaves the stomach, and prompt the pancreas to release insulin. Mounjaro activates the receptors for both of these hormones at the same time and sustains the effect for the full week between injections.

1. It reduces appetite

Tirzepatide acts on appetite centres in the brain — particularly in the hypothalamus — to reduce hunger between meals and the feeling of wanting to eat for reasons other than hunger. Many people on Mounjaro describe a quieter relationship with food, with food noise or constant snacking thoughts becoming less prominent.

2. It increases fullness after eating

By slowing the rate at which the stomach empties, Mounjaro increases the feeling of fullness during and after a meal. Smaller portions feel more satisfying, and the satisfied feeling lasts longer.

3. It lowers blood sugar in a glucose-dependent way

When blood sugar is high, Mounjaro stimulates the pancreas to release insulin and reduces the release of glucagon (a hormone that raises blood sugar). Because it only acts when blood sugar is elevated, the risk of low blood sugar (hypoglycaemia) is low when tirzepatide is used on its own — though that risk can rise when it is combined with other diabetes medicines such as sulfonylureas or insulin.

4. It supports steady weight loss alongside diet and exercise

The clinical-trial evidence for tirzepatide in weight management comes mainly from the SURMOUNT-1 trial, published in the New England Journal of Medicine in 2022. In adults with obesity but without type 2 diabetes, average weight loss after 72 weeks was around 15% with the 5mg dose, around 19.5% with 10mg, and around 20.9% with 15mg, compared with around 3.1% with placebo. All participants also followed lifestyle interventions including a reduced-calorie diet and increased physical activity. (Jastreboff et al., NEJM 2022)

Those figures are averages from trial conditions. Real-world results vary, and Mounjaro is licensed as an adjunct to lifestyle changes — not a replacement for them.

Why Mounjaro is described as different

Earlier weight-loss medicines like Wegovy (semaglutide) act on one hormone receptor — GLP-1 only. Tirzepatide is the first medicine licensed in the UK to act on two appetite-regulating hormone receptors simultaneously. This dual action is one of the reasons head-to-head trials have shown larger average weight losses for tirzepatide compared with semaglutide (Aronne et al., SURMOUNT-5, NEJM 2025).

It is worth noting that "more weight loss on average" does not mean "more weight loss for everyone." Individual response varies. Some people lose more on semaglutide than on tirzepatide, and the right choice depends on factors such as tolerability, side effects, eligibility, cost, and other medical conditions. See our independent Mounjaro vs Wegovy comparison for a fuller picture.

What Mounjaro is not

Mounjaro is not a quick fix or a willpower replacement. The clinical trials, NICE guidance, and the MHRA licence all describe it as a treatment to be used alongside lifestyle change. Without the dietary and activity component, weight tends to return when the medication stops, as documented in trial extensions where participants regained a substantial portion of the lost weight after stopping the drug.

It is also a prescription-only medicine. In the UK it can only be supplied by a registered pharmacy after a prescription has been issued by a qualified prescriber following a clinical assessment. Buying tirzepatide without a prescription — for example from social-media sellers or unregulated international websites — carries significant risks of counterfeit product, incorrect dosing, and no clinical safety net.

Side effects to be aware of

The most commonly reported side effects of tirzepatide are gastrointestinal: nausea, diarrhoea, constipation, and reduced appetite. These tend to be most pronounced when the dose is first started or stepped up, and usually settle within a few weeks. Less common but more serious risks — including pancreatitis and gallbladder disease — are listed in the Mounjaro Summary of Product Characteristics on the electronic Medicines Compendium (EMC).

For a fuller list and how to manage side effects, see our Mounjaro side effects guide.

Who Mounjaro is for

The licensed indication and the NHS eligibility criteria are different. The full eligibility picture is covered in our who can take weight loss injections guide, but in summary:

  • Privately, the licensed indication covers adults with a BMI of 30 or above (obesity), or a BMI of 27 or above plus a weight-related health condition.
  • On the NHS, NICE TA1026 sets a higher threshold: BMI of at least 35 with at least one weight-related comorbidity, with a 2.5 reduction in the BMI threshold for people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean ethnic backgrounds.

Frequently asked questions

How quickly does Mounjaro start working?
Most people notice reduced appetite within the first one to two weeks. Visible weight changes typically follow over the first one to three months as the dose is gradually increased. Full clinical effect is usually assessed after several months on a maintenance dose.

How long is Mounjaro taken for?
Privately, treatment can continue long-term as long as it remains clinically appropriate. NHS-prescribed tirzepatide under TA1026 is not capped at two years (unlike NICE's earlier guidance for semaglutide), but ongoing benefit must be reviewed.

What happens if I stop taking it?
Weight regain is common after stopping, particularly without continued lifestyle support. The STEP 1 trial extension on semaglutide showed people regained around two-thirds of the weight lost within a year of stopping. Comparable long-term withdrawal data for tirzepatide is more limited but follows a similar pattern in published trials.

Is Mounjaro the same as Ozempic?
No. Ozempic and Mounjaro are different medicines from different manufacturers. Ozempic contains semaglutide (the same active ingredient as Wegovy) and is licensed only for type 2 diabetes in the UK, not for weight management. Mounjaro contains tirzepatide and is licensed for both weight management and type 2 diabetes.

Next steps

Sources

This guide is for general information only and is not a substitute for professional medical advice. Talk to a qualified prescriber about whether Mounjaro is suitable for you.

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How Mounjaro Works: GLP-1/GIP Mechanism Explained | LetsLoseWeight