Why the two can disagree
A scale weighs all of you at once. Your clothes respond to your size, especially around the waist. The two are linked, but they are not the same measurement.
The trials measured both. In SURMOUNT-1, the average waist shrank by 14.0 to 18.5 cm on tirzepatide, depending on dose (SURMOUNT-1, NEJM, 2022). In STEP 1 it shrank by 13.5 cm on semaglutide (STEP 1, NEJM, 2021). In a SURMOUNT-1 body-scan substudy, about 74% of the weight lost was fat and 26% was lean mass (SURMOUNT-1 DXA substudy, 2025).
These are averages over more than a year, not proof of what is happening to you this month. But they show why a tape and photos are worth keeping alongside the scale.
Is it a plateau?
Average weight loss slows over time on these medicines. In STEP 1, average weight did not reach its lowest point until week 60 (STEP 1, NEJM, 2021). In SURMOUNT-1, the 5 mg group had reached a plateau by week 72 (SURMOUNT-1, NEJM, 2022).
So a flatter scale later in treatment is what the trial averages looked like. Earlier on, or if you are worried, ask your prescriber. More on plateaus.
Check it properly
Before you decide nothing has changed, look at more than one record:
- Waist: measure midway between your lowest rib and the top of your hip bone, after breathing out naturally (NICE NG246, 2025). Use the same tape and the same spot each time.
- Waist to height: NICE and the NHS use a waist under half your height as the healthy range (NICE NG246, 2025) (NHS, 2026).
- Photos: the same four angles, the same light, the same clothes.
- Weight: compare the average of the last four weekly weigh-ins with the four before, rather than one day with another.
When to talk to your prescriber
NICE builds in a check, and the clock differs by medicine. For tirzepatide, NICE says to consider stopping if less than 5% of your starting weight has been lost after 6 months on the highest dose you can tolerate (NICE TA1026, 2024). For semaglutide, NICE says to consider stopping if less than 5% has been lost after 6 months of treatment (NICE TA875, 2023). That decision is for you and your prescriber. Never change your dose on your own.
Before you call it a stall.
- Measure your waist the same way
- Compare matched photos
- Average four weekly weigh-ins
- Check how long you've been on this dose
- Write down your questions
- Ask your prescriber
Where these facts come from.
- Jastreboff AM et al. Tirzepatide once weekly for the treatment of obesity. N Engl J Med 2022;387:205-216.
- Look M et al. Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight. Diabetes Obes Metab 2025;27:2720-2729.
- Wilding JPH et al. Once-weekly semaglutide in adults with overweight or obesity. N Engl J Med 2021;384:989-1002.
- Overweight and obesity management. NICE guideline NG246: identifying and assessing overweight, obesity and central adiposity. Published 14 Jan 2025.
- Overweight and obesity. NHS conditions. Last reviewed 29 Apr 2026.
- Tirzepatide for managing overweight and obesity. NICE technology appraisal guidance TA1026. Published 23 Dec 2024, updated 1 Sep 2025.
- Semaglutide for managing overweight and obesity. NICE technology appraisal guidance TA875. Published 8 Mar 2023, updated 4 Sep 2023.
Checked 25 Oct 2026. Trial figures are average results in the trial, not predictions. Labels change; your prescriber and the leaflet in your pack have the current version.
Pip Body Scan is a personal progress record. It is not a medical device and does not give medical advice, diagnose conditions or recommend doses. Speak to your prescriber or another qualified healthcare professional about symptoms, side effects or changes to your treatment.