The main 72-week trial in context
SURMOUNT-1 enrolled adults with obesity, or with overweight and a weight-related complication, who did not have diabetes. Participants took tirzepatide or placebo alongside a lifestyle programme, with a gradual dose increase at the start. In the treatment-regimen analysis, average reductions at 72 weeks were 15.0%, 19.5% and 20.9% for the 5, 10 and 15 mg groups.
Each figure covers the whole course from baseline. They are separate group results, so do not add them together or read them as extra loss after each dose step. Results varied between participants, and some stopped treatment. If you take Mounjaro for type 2 diabetes, expect your result to differ from a study which excluded diabetes.
A percentage needs a starting weight
A 15% fall from 100 kg is 15 kg. The same percentage from 80 kg is 12 kg. These are arithmetic examples, with no promise of a result. Percentage change lets you compare people of different starting weights. The figure says nothing about body composition, nutrition or your own health gains.
Headlines sometimes quote different analyses of the same study. An analysis of people while they kept taking the medicine gives different numbers from one covering everyone assigned to a regimen. When you read a figure, look for which analysis produced it. A good report names the analysis and explains the number.
Early weeks do not set your pace
Weight change in the first weeks reflects appetite, water balance and bowel contents as well as body tissue. A fast early fall often slows. A modest start does not predict a poor long-term result. The starting dose introduces you to treatment and tells you little about how a maintenance dose will suit you.
Weigh yourself in similar conditions and follow the trend over weeks. Frequent weighing helps some people and upsets others. Agree a review plan with your prescriber so normal day-to-day swings do not trigger a dose change each time.
Health gains go beyond the scales
Useful outcomes include blood glucose, blood pressure, mobility, sleep-related symptoms and quality of life. Good nutrition and keeping your strength matter too. Weight lost through repeated vomiting or dehydration is a problem to report, and no sign of success.
A trial result in one condition does not create a new licensed use. A plausible mechanism does not prove every claimed benefit either. When you read claims about the heart, kidneys or other organs, check the population studied, the endpoint measured and the regulator's wording.
When the response counts as too small
The UK label and NICE say to review the balance of benefit and risk if you have lost less than 5% of your starting weight after six months on the highest dose you tolerate. The six months start once you reach that dose, and your highest tolerated dose may be lower than 15 mg.
Your clinician looks at how regularly you took the medicine, side effects, time on treatment, other health factors and your goals. The outcome may be to continue, change the plan or stop. Before-and-after photos on social media are no substitute for that review, and no fair yardstick for anyone else's treatment.
Sources
Your own medicine leaflet and prescriber come first. A source from another country does not mean the same rule applies in the UK.
- Mounjaro UK Summary of Product Characteristics, updated September 2026 medicines.org.uk
- SURMOUNT-1: tirzepatide once weekly for obesity pubmed.ncbi.nlm.nih.gov
- NICE: prescribing, reviewing and stopping tirzepatide nice.org.uk
- NICE TA1026: recommendations nice.org.uk
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