Drug comparisons
What Trial Scans Show on Mounjaro vs Ozempic Muscle Loss in the UK
No trial has scanned Mounjaro vs Ozempic muscle loss side by side. In separate scans, a quarter of the loss on tirzepatide was lean mass, as on diet alone.
Medically reviewed by Farnia Bahari, PharmD
Published 8 min read
SURMOUNT-5, which compared tirzepatide (Mounjaro) with semaglutide directly, reported no lean-mass scans (Aronne et al., 2025). Separate scans put lean mass at 26% of the weight lost on tirzepatide (Look et al., 2025) and 39% on semaglutide 2.4 mg (Mozaffarian et al., 2025). Because tirzepatide takes off more weight, the two come out level in kilograms, about 5.3 kg of lean mass for a 100 kg adult, and roughly half of lean mass is muscle (Mozaffarian et al., 2025).
Key facts
- Wegovy is the UK’s weight-management semaglutide, and Mounjaro covers diabetes and weight management under one UK prescribing document (Wegovy SmPC, section 4.1; Mounjaro SmPC, section 4.1).
- In SURMOUNT-5, weight fell by 20.2% on tirzepatide and 13.7% on semaglutide over 72 weeks (Aronne et al., 2025).
- On MRI, tirzepatide took 0.64 L off thigh muscle volume in 52 weeks, about what UK Biobank data predicted for that weight loss (Sattar et al., 2025).
- NICE recommends Mounjaro from a BMI of 35 with at least one weight-related condition, phased in on the NHS (NICE TA1026).
Lean mass on Mounjaro and Wegovy in the trials
In the UK the weight-loss comparison is Mounjaro against Wegovy. Ozempic’s Summary of Product Characteristics (SmPC), the UK prescribing document, licenses it for type 2 diabetes only (Ozempic SmPC, section 4.1). Zepbound is the US brand of tirzepatide for weight management, so the US comparison is Zepbound against Wegovy.
Each trial scanned its own subgroup, by DXA or MRI.
| Trial and medicine | Who was scanned, and how | Result |
|---|---|---|
| SURMOUNT-1, Mounjaro (tirzepatide) | 160 adults with obesity or overweight, DXA at 72 weeks | 26% of the weight lost was lean mass (placebo 25%) |
| STEP 1, Wegovy (semaglutide 2.4 mg) | 140 adults (95 on semaglutide), DXA at 68 weeks | 5.3 kg of an average 13.6 kg lost was lean mass (5.3 ÷ 13.6 = 39%) |
| STEP UP MRI sub-study, Wegovy | 55 adults, MRI at 72 weeks | Fat fell more than lean mass; the SmPC gives no percentage |
| SURPASS-3 MRI, Mounjaro | 246 adults with type 2 diabetes, thigh MRI at 52 weeks | Thigh muscle volume fell 0.64 L, in line with the weight lost |
Across SURMOUNT-1 and STEP 1, 25 to 39% of the loss is lean mass (Look et al., 2025; Mozaffarian et al., 2025). In SURMOUNT-1 the placebo group, losing weight through diet and exercise, lost almost the same share as the tirzepatide group. The UK Mounjaro SmPC sums up that scan study in one line: "most of the total weight loss was attributable to a reduction in fat tissue, including visceral fat" (Mounjaro SmPC, section 5.1).
Why Mounjaro can cost more kilograms of lean mass
SURMOUNT-5 compared the two drugs directly in 751 adults with obesity but without diabetes (Aronne et al., 2025). At 72 weeks, weight fell by 20.2% on tirzepatide (10 or 15 mg) and 13.7% on semaglutide (1.7 or 2.4 mg) (Aronne et al., 2025). Every brand’s dose ladder shows where those doses sit. The trial’s main results report weight and waist size, and no scan of lean mass.
A bigger loss at a similar share means more kilograms of lean mass. For a 100 kg adult, the SURMOUNT-5 results mean 20.2 kg lost on tirzepatide and 13.7 kg on semaglutide. Applying the scan shares to those losses gives the lean kilograms in each case (20.2 × 0.26 = 5.3; 13.7 × 0.26 = 3.6; 13.7 × 0.39 = 5.3; 20.2 × 0.39 = 7.9).
| Scenario | Tirzepatide (lean kg) | Semaglutide (lean kg) |
|---|---|---|
| Both at SURMOUNT-1’s 26% | 5.3 | 3.6 |
| Each at its own trial’s share | 5.3 | 5.3 |
| Both at STEP 1’s 39% | 7.9 | 5.3 |
At each drug’s own share, the two come out level at 5.3 kg, because tirzepatide’s larger loss makes up for its lower share.
Lean mass is not the same as muscle
On a DXA scan, lean mass is everything that is not fat or bone, so it counts organs and body water as well as muscle (Look et al., 2025). The joint advisory on nutrition during GLP‑1 treatment estimates muscle at about half of lean mass, which puts muscle at roughly 20% of the weight lost in STEP 1 (Mozaffarian et al., 2025).
MRI measures muscle directly. In SURPASS-3 MRI, thigh muscle volume fell by 0.64 L over 52 weeks on tirzepatide, within 0.04 L of what UK Biobank data predicted for the same weight loss (Sattar et al., 2025). That difference was not statistically significant, while fat inside the muscle fell by more than predicted. In the 15 mg group, a muscle volume score adjusted for sex, height and body size also fell more than predicted (Sattar et al., 2025). The analysis was post hoc, in people with type 2 diabetes, and funded by Eli Lilly and Company.
How to protect muscle on Mounjaro or Wegovy
Strength training is what protects muscle, with protein in support. The joint advisory judges extra protein on its own "likely inadequate" without structured strength training (Mozaffarian et al., 2025). It asks for strength training on 3 or more occasions each week, with at least 150 minutes of moderate aerobic exercise alongside (Mozaffarian et al., 2025).
For protein, the GLP‑1 protein calculator works out a daily target from your weight, based on ranges well above the UK reference intake for healthy adults.
Nausea, vomiting and diarrhoea on Mounjaro are more common while the dose is being increased, and decrease over time (Mounjaro SmPC, section 4.8). On those days, protein that goes down in small portions keeps the daily total up. Marked weight loss is also a recognised trigger of hair shedding on Mounjaro (BAD patient leaflet).
What to track besides the scale
A bathroom scale gives one number for fat, muscle and water together. A waist measurement adds a second number that follows central fat. NICE’s method is to find the bottom of your ribs and the top of your hips. Wrap the tape midway between them, just above the belly button, and breathe out naturally before reading it (NICE NG246, box 1).
Divide the result by your height in the same units. NICE’s advice is a waist below half your height, which is a ratio under 0.5 (NICE NG246). At 165 cm tall, that means a waist under 82.5 cm (165 ÷ 2).
Getting Mounjaro or Wegovy on the NHS
The two reach NHS patients by different routes. NICE recommends Mounjaro for adults with a BMI of at least 35 and at least one weight-related condition, in primary care or specialist services (NICE TA1026). Wegovy is recommended only within specialist weight management services, for up to 2 years (NICE TA875). It is for adults with at least one weight-related condition and a BMI of at least 35, or 30 to 34.9 if they meet the referral criteria for those services (NICE TA875).
Mounjaro is being introduced in phases set by BMI and by how many of five conditions you have: high blood pressure, dyslipidaemia, obstructive sleep apnoea, cardiovascular disease and type 2 diabetes. The qualifying BMI is 2.5 kg/m² lower for people of Black African, African-Caribbean, South Asian, Chinese, other Asian or Middle Eastern backgrounds (NICE TA1026). These are the phases as NICE set them out on its page last updated 1 September 2025 (NICE TA1026).
| Phase on the NICE page | BMI (kg/m²) | Listed conditions |
|---|---|---|
| First group | 40 or more | 4 or more |
| From around June 2026 | 35 to 39.9 | 4 or more |
| From around March 2027 | 40 or more | 3 |
Your GP practice can tell you which phase it is prescribing to. Switching from Ozempic or Wegovy to Mounjaro needs a new prescription.
Where Mounjaro and Wegovy stand
Mounjaro (tirzepatide)
- 20.2% of body weight lost in SURMOUNT-5 (Aronne et al., 2025).
- Lean mass 26% of the loss in SURMOUNT-1, next to 25% on placebo (Look et al., 2025).
- On the NHS in primary care or specialist services, phased in (NICE TA1026).
Wegovy (semaglutide 2.4 mg)
- 13.7% of body weight lost in SURMOUNT-5 (Aronne et al., 2025).
- Lean mass 39% of the loss in STEP 1 (5.3 ÷ 13.6 kg), a separate trial (Mozaffarian et al., 2025).
- On the NHS only in specialist weight management services, for up to 2 years (NICE TA875).
Questions
Frequently asked questions
Does Mounjaro cause muscle loss?
Yes, lean mass goes down with the weight, at about the share that diet and exercise alone produced in SURMOUNT-1’s placebo group (Look et al., 2025). Thigh muscle measured by MRI in SURPASS-3 fell about as much as the weight lost predicted (Sattar et al., 2025).
Does Mounjaro cause more muscle loss than Wegovy?
We found no trial that has scanned lean mass on both drugs in the same people. Taken from separate studies, the lean share was lower on tirzepatide. Tirzepatide takes off more weight, though, so the kilograms of lean mass lost can be as large or larger (Look et al., 2025; Aronne et al., 2025).
Does the NHS check muscle or body composition on Mounjaro?
Not under NICE’s recommendations, whose review point is weight, with a decision on continuing if the loss after 6 months on the highest tolerated dose is under 5% of the starting weight (NICE TA1026, 1.2). For Wegovy, NICE says to consider stopping at the same mark after 6 months of treatment (NICE TA875, 1.2).
Is walking or cardio enough to keep muscle?
Not on its own. The joint advisory says "Aerobic activity alone has a smaller effect on preserving lean mass during rapid weight reduction", and asks for structured strength training alongside it (Mozaffarian et al., 2025).
Can I get Mounjaro on the NHS for weight loss?
Only if you meet NICE’s criteria for the phase your GP practice has reached (NICE TA1026). The licence is wider, covering adults with a BMI of 30 or more, or of 27 or more with a weight-related condition (Mounjaro SmPC, section 4.1). A private prescription can work from the licence.
Sources
- Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight. Diabetes, Obesity and Metabolism, 2025.
- Nutritional priorities to support GLP‑1 therapy for obesity: a joint advisory from the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association, and The Obesity Society. Obesity Pillars, 2025.
- Impact of semaglutide on body composition in adults with overweight or obesity: exploratory analysis of the STEP 1 study. Journal of the Endocrine Society, 2021.
- Tirzepatide as compared with semaglutide for the treatment of obesity. New England Journal of Medicine, 2025.
- Tirzepatide and muscle composition changes in people with type 2 diabetes (SURPASS-3 MRI): a post-hoc analysis of a randomised, open-label, parallel-group, phase 3 trial. The Lancet Diabetes & Endocrinology, 2025.
- Mounjaro KwikPen 15mg solution for injection in pre-filled pen, Summary of Product Characteristics. Eli Lilly and Company Limited, 2026.
- Wegovy 2.4 mg FlexTouch, Summary of Product Characteristics. Novo Nordisk, 2026.
- Ozempic 0.5 mg pre-filled pen, Summary of Product Characteristics. Novo Nordisk, 2026.
- Tirzepatide for managing overweight and obesity (TA1026). National Institute for Health and Care Excellence, 2025.
- Semaglutide for managing overweight and obesity (TA875). National Institute for Health and Care Excellence, 2023.
- Overweight and obesity management (NG246): identifying and assessing overweight, obesity and central adiposity. National Institute for Health and Care Excellence, 2026.
- Telogen effluvium, patient information leaflet. British Association of Dermatologists, 2025.
