What the Mounjaro Trials Tell Us About Tirzepatide in the UK

SURMOUNT-1: where the headline figures come from

Most of what you read about Mounjaro's effectiveness traces back to SURMOUNT-1, a 72-week randomised controlled trial published in the New England Journal of Medicine. It randomised 2,539 adults with obesity (BMI ≥30, or ≥27 with at least one weight-related condition) but without type 2 diabetes. Participants received tirzepatide at 5mg, 10mg, or 15mg, or a placebo, alongside a reduced-calorie diet and increased physical activity.

At 15mg, the average weight reduction was around 20–21%. That figure is a mean across all participants who reached that dose, individual results varied, as they do in any large trial. What made the finding significant was its consistency: even at 5mg, participants lost an average of around 15% of their body weight. The trial is described in detail in our SURMOUNT overview if you want the numbers broken down dose by dose.

One practical check worth doing: when you read a weight-loss claim for any medicine online, look for whether the figure comes from the highest dose tested or an average across doses. The two can differ substantially, and conflating them is a common source of confusion.

How SURMOUNT-2 extended the evidence to people with type 2 diabetes

SURMOUNT-2 examined tirzepatide in adults who had both obesity and type 2 diabetes, a population where weight loss is often harder to achieve and where the medicine's dual GIP and GLP-1 receptor activity is particularly relevant. Average weight reductions were somewhat lower than in SURMOUNT-1, which is consistent with what the biology would predict: people with type 2 diabetes tend to lose less weight on GLP-1 class medicines. That said, the results remained clinically meaningful.

The broader Mounjaro weight-loss trial programme also included SURMOUNT-3 and SURMOUNT-4, which looked at questions like intensive lifestyle intervention before starting treatment, and what happens when treatment is withdrawn. SURMOUNT-4 confirmed that participants who stopped tirzepatide regained a significant portion of the weight they had lost, a finding NICE acknowledged in its guidance and one that matters when thinking about long-term treatment planning.

You can read about the full breadth of tirzepatide's trial history, including the SURPASS diabetes programme, which covered a separate set of trials in type 2 diabetes management.

SURMOUNT-5: the head-to-head comparison with semaglutide

Until 2025, direct comparisons between tirzepatide and semaglutide relied on indirect analysis, comparing two separate trials rather than randomising participants to one or the other. SURMOUNT-5 changed that. Published in the New England Journal of Medicine in 2025, it randomised 751 adults with obesity but without diabetes to either tirzepatide or semaglutide 2.4mg (the standard Wegovy maintenance dose at the time) over 72 weeks.

Tirzepatide produced greater average weight loss. NICE's appraisal committee discussed these findings in its assessment of tirzepatide (TA1026), noting that indirect comparisons had favoured tirzepatide and that SURMOUNT-5 reinforced that picture. The committee's recommendation was positive: tirzepatide was deemed cost-effective for NHS use within the defined eligibility criteria.

It is worth being precise about what head-to-head means here. SURMOUNT-5 compared tirzepatide against semaglutide 2.4mg, not the newer 7.2mg dose that became available in 2026. The landscape continues to evolve, and the MHRA-approved Mounjaro page keeps the current licensed position up to date.

What the trial evidence means for private treatment

NICE's recommendation sets the eligibility bar for NHS access, and the phased rollout means many people who might benefit from tirzepatide will wait some time before they qualify through that route. Private treatment through a regulated online pharmacy is a separate path (no referral, no waiting list) but the same clinical standards apply. A prescriber still reviews your full picture: BMI, health conditions, medicines you already take, and whether tirzepatide is appropriate for you specifically, including any less commonly discussed effects such as ear ringing that some people on Mounjaro have reported.

The trial results are persuasive, but they describe populations, not individuals. Around 20% average weight loss at 15mg means some participants lost more and some lost less; a small number lost very little. A prescriber who understands the evidence is better placed to set realistic expectations than any headline figure can. If you are weighing up the evidence and thinking about next steps, the free consultation at nume is reviewed the same day by a GPhC-registered prescriber, a real clinician, not a questionnaire engine. The cost of private Mounjaro treatment is covered separately if that is a factor in your thinking.