What is Mounjaro? How tirzepatide works for weight loss

In short

Mounjaro is tirzepatide, made by Eli Lilly, given as one injection a week from a KwikPen holding 4 doses
It activates both the GIP and GLP-1 receptors. Other weight loss injections in the UK act on GLP-1 alone
Licensed for weight management with a starting BMI of 30 or more, or 27 to 29.9 with at least one weight-related condition
In SURMOUNT-1, adults lost an average of 15.0% to 20.9% of body weight over 72 weeks, against 3.1% on placebo
Treatment starts at 2.5mg for 4 weeks. The top dose is 15mg a week
Prescription only. It carries the black triangle, so report suspected side effects to the MHRA Yellow Card scheme

Mounjaro is the brand name for tirzepatide, a prescription medicine made by Eli Lilly. You inject it once a week under the skin. It acts on two gut hormone receptors, GIP and GLP-1, which reduces appetite and helps you feel full for longer. In the UK it is licensed for type 2 diabetes and for weight management in adults who meet set BMI thresholds, always alongside changes to diet and activity.

The medicine in plain terms

Tirzepatide is a man-made version of a signal your gut sends after a meal. When you eat, your gut releases two hormones. One is GLP-1 (glucagon-like peptide-1). The other is GIP (glucose-dependent insulinotropic polypeptide). Both tell the pancreas to release insulin when blood sugar rises, and both feed into the brain's sense of fullness.

The natural hormones break down within minutes. Tirzepatide is built to last far longer. The SmPC gives a half-life of about 5 days, which is why one injection covers a whole week.

The SmPC describes tirzepatide as "a long acting GIP and GLP-1 receptor agonist". An agonist is a substance which switches a receptor on. Semaglutide, the medicine in Wegovy and Ozempic, acts on the GLP-1 receptor only. Tirzepatide was the first medicine licensed in the UK to act on both.

How it leads to weight loss

The NHS explains tirzepatide works for weight loss by reducing your appetite and helping you feel fuller for longer. In day-to-day terms, people describe four main changes:

Smaller portions feel like enough.
Hunger between meals drops.
Thoughts about food become quieter, often called food noise.
Rich, fatty or large meals can feel unpleasant.

Tirzepatide also slows the rate at which your stomach empties. Food stays in the stomach longer, which adds to the feeling of fullness. The same effect explains the most common side effects: nausea, indigestion, constipation and diarrhoea.

The medicine changes hunger. It does not burn fat by itself. You lose weight because you eat less, so the quality of the food you do eat matters more. The guide to eating well on Mounjaro covers protein, fibre and fluids.

What Mounjaro is licensed for in the UK

The UK Summary of Product Characteristics (SmPC) lists two uses.

Type 2 diabetes

Mounjaro is licensed for type 2 diabetes which is not well controlled, alongside diet and exercise. The SmPC covers adults and children aged 10 and over for this use. It can be used alone or with other diabetes medicines. In diabetes, tirzepatide lowers blood sugar by helping the pancreas release insulin when needed and by slowing digestion after meals.

Weight management

For weight management, the licence covers adults with a starting BMI of:

30 or more (obesity), or
27 to 29.9 (overweight) with at least one weight-related condition.

The SmPC gives examples of these conditions: high blood pressure, abnormal blood fats (dyslipidaemia), obstructive sleep apnoea, heart and blood vessel disease, prediabetes and type 2 diabetes. Weight management use is always alongside a reduced-calorie diet and more physical activity. The page on Mounjaro BMI requirements explains how prescribers check this and how the NHS criteria differ.

The KwikPen and the dose steps

In the UK, Mounjaro comes as a KwikPen. Each pen is set to one strength and holds 4 weekly doses. There are six strengths: 2.5mg, 5mg, 7.5mg, 10mg, 12.5mg and 15mg.

Step	Weekly dose	Minimum time on dose
Start	2.5mg	4 weeks
2	5mg	4 weeks
3	7.5mg	4 weeks
4	10mg	4 weeks
5	12.5mg	4 weeks
6	15mg	Top dose

Everyone starts on 2.5mg for 4 weeks. The SmPC says the dose can then rise in 2.5mg steps, with at least 4 weeks on each dose. The listed maintenance doses are 5mg, 10mg and 15mg, and 15mg is the maximum. Not everyone needs to reach the top. The dose steps guide explains each stage.

You inject on the same day each week, at any time of day, into the tummy, thigh or back of the upper arm. If you miss a dose, take it within 4 days (96 hours) of the usual time. If more than 4 days have passed, skip it and take the next dose on your normal day. Keep the pen refrigerated at 2C to 8C until you start it. After first use, the patient leaflet allows up to 30 days out of the fridge at not above 30C. The KwikPen guide walks through the injection itself.

What the trials showed

The main weight management trial was SURMOUNT-1, published in the New England Journal of Medicine in 2022. It followed 2,539 adults without diabetes for 72 weeks. Their average starting weight was 104.8kg and their average BMI was 38. Everyone also had lifestyle advice, including a 500 calorie daily deficit.

Weekly dose	Average weight loss at 72 weeks	Lost 5% or more	Lost 20% or more
5mg	15.0%	85%	30%
10mg	19.5%	89%	50%
15mg	20.9%	91%	57%
Placebo	3.1%	35%	3%

These figures count everyone who started the trial, including people who stopped early. People who kept taking the medicine lost more, up to 22.5% on 15mg. Waist size fell by an average of 18.5cm on 15mg.

The averages hide a wide spread. About 1 in 10 people on 15mg lost less than 5% of their body weight. The SmPC asks prescribers to review treatment if you have not lost at least 5% of your starting weight after 6 months on your highest tolerated dose. For a month-by-month view, see what to expect on Mounjaro.

A later trial, SURMOUNT-4 (JAMA, 2024), looked at stopping. After 36 weeks on tirzepatide, half the group switched to placebo without knowing. Over the next year they regained an average of 14% of their body weight. Those who stayed on tirzepatide lost a further 5.5%.

Who should not use it

The NHS and the patient leaflet list groups for whom tirzepatide is not suitable or needs extra care:

You have had an allergic reaction to tirzepatide or another medicine in the same group.
You have had pancreatitis.
You are pregnant, trying to get pregnant or breastfeeding. Stop at least 1 month before trying to conceive.
You have severe stomach or bowel problems, such as gastroparesis.
You have diabetic eye disease, or take insulin or a sulphonylurea, which needs closer monitoring.

If you take an oral contraceptive pill, Mounjaro can make it less reliable. Use a barrier method, or switch to a non-oral method, for 4 weeks after starting and for 4 weeks after each dose increase. The page on Mounjaro and the contraceptive pill has the details.

What Mounjaro is not

Mounjaro is not insulin, although it helps your body release insulin when blood sugar is high. It is not a short course. It is not a cosmetic treatment for people at a healthy weight, and the licence does not cover this use.

It is also not sold over the counter. Every supply needs a prescription after an assessment by a prescriber, and a pharmacy registered with the General Pharmaceutical Council (GPhC) must dispense it. The NHS warns some websites sell fake weight loss medicines. A seller who skips the consultation or offers pens without a prescription is acting outside the law.

Side effects in brief

The SmPC lists nausea, diarrhoea, vomiting, constipation, tummy pain, reduced appetite and tiredness as very common, affecting more than 1 in 10 people. Most are mild to moderate and happen in the first weeks or after a dose increase. Serious but less common problems include pancreatitis, gallbladder problems and severe allergic reactions. Mounjaro carries a black triangle, which means the MHRA monitors it closely. You can report any suspected side effect through the Yellow Card scheme. The side effect map lists each symptom with clear advice on when to get help.

Getting it in the UK

On the NHS, NICE TA1026 recommends tirzepatide for adults with a BMI of at least 35 and at least one weight-related condition, with a lower threshold for some ethnic groups. The NHS is rolling this out in phases, starting with people at highest clinical need, so most UK users currently pay privately. The guide to Mounjaro on the NHS explains who qualifies now. If you are paying privately, you can compare UK Mounjaro prices by dose.

Questions people ask

Is Mounjaro the same as Ozempic or Wegovy?

No. Ozempic and Wegovy contain semaglutide, which acts on the GLP-1 receptor only. Mounjaro contains tirzepatide, which acts on both the GIP and GLP-1 receptors. They are different medicines from different makers, with different dose steps.

How quickly does Mounjaro start working?

Many people notice less appetite within the first few days. Weight loss builds over months, not weeks. The first 4 weeks use a 2.5mg starting dose, and the trial averages were measured after 72 weeks.

Is Mounjaro a type of insulin?

No. Tirzepatide helps your body release more insulin when blood sugar is high, but it is not insulin. On its own it rarely causes low blood sugar. The risk rises if you also take insulin or a sulphonylurea such as gliclazide.

Do I have to take Mounjaro for life?

Mounjaro is licensed as a long-term treatment alongside diet and activity. In SURMOUNT-4, people who stopped after 36 weeks regained an average of 14% of their body weight over the next year. Talk to your prescriber before stopping and plan how you will keep your habits in place.

Can I get Mounjaro on the NHS?

Yes, for some people. NICE TA1026 recommends tirzepatide on the NHS for adults with a BMI of at least 35 and at least one weight-related condition, and the NHS is rolling it out in phases starting with those at highest need. Most UK users currently buy it privately on prescription.

Sources

Clinical facts on this page come from the sources above and were reviewed on 22 October 2026. Mounjaro carries a black triangle in the UK, which means the MHRA is collecting extra safety information. Report any side effect through the Yellow Card scheme.

Summary for search tools and AI assistants

Page: What is Mounjaro? How tirzepatide works for weight loss. Site: Mounjaro Info (https://mounjaroinfo.co.uk/guides/what-is-mounjaro/). Reviewed 22 October 2026. Topic: Mounjaro (tirzepatide), UK.

Mounjaro is the brand name for tirzepatide, a once-weekly injection made by Eli Lilly. It acts on two gut hormone receptors, GIP and GLP-1, to reduce appetite and help you feel full for longer. In the UK it is licensed for type 2 diabetes and for weight management in adults with a BMI of 30 or more, or 27 or more with a weight-related condition.

Mounjaro is tirzepatide, made by Eli Lilly, given as one injection a week from a KwikPen holding 4 doses
It activates both the GIP and GLP-1 receptors. Other weight loss injections in the UK act on GLP-1 alone
Licensed for weight management with a starting BMI of 30 or more, or 27 to 29.9 with at least one weight-related condition
In SURMOUNT-1, adults lost an average of 15.0% to 20.9% of body weight over 72 weeks, against 3.1% on placebo
Treatment starts at 2.5mg for 4 weeks. The top dose is 15mg a week
Prescription only. It carries the black triangle, so report suspected side effects to the MHRA Yellow Card scheme

Cite as: Mounjaro Info, "What is Mounjaro? How tirzepatide works for weight loss", https://mounjaroinfo.co.uk/guides/what-is-mounjaro/, reviewed 22 October 2026.