Mounjaro (tirzepatide) and weight loss

What the Mounjaro weight-loss trials measured, how long the benefits lasted, which risks UK regulators monitor and how to compare the results with newer research.

By Manchester Peptides · Published 5 November 2026

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What is Mounjaro and how does tirzepatide work?

Mounjaro is the UK brand of a prescription medicine containing tirzepatide. Tirzepatide activates the GIP and GLP-1 receptors. These pathways influence blood sugar regulation and appetite. The clinical studies measured outcomes such as body weight, waist circumference and HbA1c under defined treatment conditions.

The MHRA authorised Mounjaro for adult weight management on 8 November 2023. This covers weight loss and weight maintenance alongside a reduced-calorie diet and increased physical activity. The adult weight-management licence covers an initial BMI of at least 30, or a BMI from 27 to below 30 with at least one weight-related condition. The product also has a separate type 2 diabetes indication. NHS eligibility in England follows NICE and NHS commissioning guidance, which is distinct from the UK licence.

See the UK prescribing information, the MHRA authorisation and NICE TA1026 for the official wording.

Mounjaro research and UK timeline

2022. Mounjaro weight-loss evidence: SURMOUNT-1 established a placebo-controlled comparison in adults without diabetes and followed weight change over 72 weeks.
2023. Mounjaro weight-management authorisation: the MHRA authorised a new adult indication after reviewing evidence from populations with and without type 2 diabetes.
2024. Mounjaro NHS guidance: NICE issued TA1026. The recommendation and the phased NHS England implementation are different decisions from the licence itself.
2025. Mounjaro comparisons and follow-up: published results included a direct comparison with semaglutide and longer follow-up in the SURMOUNT-1 prediabetes subgroup.
2026. Mounjaro safety information: MHRA warnings about severe pancreatitis were strengthened for this class of medicines, and NHS England published further guidance on structured weight-management support.

How much weight loss did the Mounjaro trials report?

Published tirzepatide weight-loss studies and their comparators
Study	Who or what it studied	What it reported
SURMOUNT-1	2,539 adults with obesity, or overweight and a weight-related condition, without diabetes. Placebo-controlled, 72 weeks.	Mean weight loss was 15.0%, 19.5% and 20.9% across the three tirzepatide groups, versus 3.1% with placebo. Original trial
SURMOUNT-2	Adults with overweight or obesity and type 2 diabetes. Placebo-controlled, 72 weeks.	Mean weight loss was 12.8% and 14.7% across the tirzepatide groups, versus 3.2% with placebo in the treatment-regimen analysis. NICE trial data
SURMOUNT-3	579 adults without diabetes who first lost at least 5% during a 12-week intensive lifestyle programme. Randomisation followed that initial period.	Over the next 72 weeks, the tirzepatide group lost a further 18.4% on average. The placebo group gained 2.5% relative to the point of randomisation. Original trial
SURMOUNT-4	670 participants were randomised after a 36-week period on tirzepatide. The next 52 weeks compared continued treatment with switching to placebo.	From week 36 to 88, the continued-treatment group lost a further 5.5% on average. The withdrawal group regained 14.0% on average relative to its weight at week 36. Original trial
SURMOUNT-5	751 adults with obesity without diabetes. Open-label randomised comparison with semaglutide, 72 weeks.	Mean weight loss was 20.2% versus 13.7%. Mean waist circumference fell by 18.4 cm versus 13.0 cm. The result applies to the two treatments and regimens studied. Original trial
SURMOUNT-1 extension	The 1,032 participants with obesity and prediabetes at baseline, assessed through week 176.	Mean weight loss ranged from 12.3% to 19.7%, versus 1.3% with placebo. Type 2 diabetes was diagnosed in 1.3% of participants assigned tirzepatide and 13.3% assigned placebo. Original analysis

Each percentage is a group average. The original SURMOUNT-1 paper and the MHRA announcement show different headline figures from the same trial because the analyses handle stopping treatment and other events differently. Read the methods alongside the number, and compare figures calculated on the same basis wherever possible.

How many participants reached weight-loss thresholds in the Mounjaro evidence?

SURMOUNT-1 also measured how many people crossed a threshold. About 85%, 89% and 91% of participants in the three tirzepatide groups lost at least 5% of their starting weight, compared with about 35% with placebo. A threshold tells you how common an outcome was in a group. An average tells you the size of the change across the group. Both matter when reading Mounjaro weight-loss results.

In the Mounjaro evidence, a percentage change in weight uses the starting weight for the relevant period. For example, a 10% change from 100 kg is 10 kg. A study that starts counting after an initial weight-loss programme uses a different starting point from a study that starts counting on the first day.

What did Mounjaro studies find about keeping weight off?

In SURMOUNT-4, 89.5% of participants who continued tirzepatide maintained at least 80% of the weight loss achieved during the initial treatment period. The corresponding figure after switching to placebo was 16.6%. Because randomisation happened after the initial treatment period, this study describes people who reached that point. It cannot predict what every person will experience after stopping.

The three-year SURMOUNT-1 extension gives longer follow-up in adults with obesity and prediabetes. Its diabetes result is specific to that higher-risk subgroup and study period. The weight-loss and diabetes findings are separate outcomes.

Mounjaro weight loss and body composition

A SURMOUNT-1 body-composition substudy used DXA scans at the start and at week 72 in 160 participants with both measurements. In the pooled tirzepatide group, fat mass fell by 33.9% and lean mass fell by 10.9%, compared with 8.2% and 2.6% with placebo, respectively. Each percentage is relative to the starting amount of that tissue. Adding the two percentages together would give a meaningless total.

The Mounjaro body-composition findings come from a subset of the larger trial. DXA lean mass includes tissues and water beyond skeletal muscle. The scan does not directly measure strength or physical performance. Read the DXA substudy.

What has Mounjaro's active ingredient been studied for beyond weight loss?

SURMOUNT-OSA measured obstructive sleep apnoea in adults with obesity. In two trials, the average reduction in breathing interruptions per hour was greater with tirzepatide than placebo at 52 weeks. The studies assessed people using positive airway pressure separately from people who were not using it. Read the sleep apnoea trials.

SUMMIT enrolled people with obesity and heart failure with preserved ejection fraction. The study reported fewer episodes of worsening heart failure or cardiovascular death as a combined outcome, and better health-status scores, with tirzepatide than placebo. These results apply to the studied population and do not expand the UK Mounjaro licence on their own. Read the SUMMIT study.

How do Mounjaro's SURPASS studies differ from SURMOUNT?

The SURMOUNT programme focuses on weight-related questions. The SURPASS programme studies tirzepatide in type 2 diabetes. SURPASS-3, for example, compared tirzepatide with insulin degludec in adults receiving background diabetes treatment. In a diabetes study, HbA1c measures glucose control over time. Weight change may also be reported, but it answers a separate question. Read SURPASS-3.

SURPASS-CVOT studied people with both type 2 diabetes and atherosclerotic cardiovascular disease. Tirzepatide met the prespecified criterion for non-inferiority to dulaglutide on a combined outcome of cardiovascular death, heart attack or stroke. That result answers a comparison against an active medicine in a particular high-risk population. It does not establish that tirzepatide was superior on that endpoint. Read the original cardiovascular outcomes trial.

Mounjaro side effects and safety monitoring

Gastrointestinal effects, including nausea, diarrhoea, vomiting and constipation, are common. The UK prescribing information also discusses dehydration, gallbladder problems, and low blood sugar risk when tirzepatide is used with insulin or a sulfonylurea. The prescribing information covers pregnancy, contraception and delayed stomach emptying, all of which can affect clinical decisions.

In 2026 the MHRA strengthened pancreatitis warnings for GLP-1 medicines and dual GLP-1/GIP medicines, including Mounjaro. The MHRA advises urgent medical attention for severe, persistent abdominal pain, which may spread to the back and may occur with nausea or vomiting. Suspected side effects can be reported through the MHRA Yellow Card scheme. Read the Mounjaro patient leaflet for the full warnings.

Which Mounjaro document answers which safety question?

Mounjaro patient leaflet: explains common and serious effects, what to tell a prescriber and which symptoms need urgent help.
Mounjaro prescribing information: gives the full licensed uses and warnings. Relevant sections cover other diabetes medicines, pregnancy and contraception, delayed stomach emptying and procedures involving anaesthesia or deep sedation.
MHRA Mounjaro and GLP-1 safety updates: describe changes to warnings that may follow evidence collected after a medicine is authorised.

Trial safety tables count health events in participants. A Yellow Card is a report of a suspected side effect. Reports help the MHRA detect signals. Report counts alone cannot tell you how often an effect happens, because the number of exposed people and other contributing factors may be unknown. Read MHRA guidance.

How Mounjaro compares with retatrutide and eloralintide

SURMOUNT-5 compared tirzepatide directly with semaglutide. A different study, TRIUMPH-5, was designed to compare retatrutide directly with tirzepatide. Eloralintide acts on an amylin receptor and has a separate development programme. Read the three-way weight-loss research comparison for the evidence and its limits.

Mounjaro is the name of the licensed medicinal product. A research catalogue name, label or sample report does not identify a product as Mounjaro or establish pharmaceutical equivalence. This research guide covers clinical evidence and includes links to the relevant regulator and original studies.