Justine Martin, Mounjaro and the Evidence Behind the Headlines

What SURMOUNT-1 actually found, and why the numbers matter

Public discussions of Mounjaro, including those prompted by high-profile accounts like Justine Martin's, often centre on dramatic weight changes. It is worth grounding those conversations in what the research shows. SURMOUNT-1, published in the New England Journal of Medicine, randomised 2,539 adults with obesity (without type 2 diabetes) across multiple doses of tirzepatide over 72 weeks. At the highest 15 mg dose, participants lost an average of around 20–21% of their body weight. That figure is a population average across thousands of participants, individual results varied above and below it, and trial participants also received structured diet and activity support throughout.

One misconception worth letting go quietly: tirzepatide is sometimes described as working simply by suppressing hunger. It is more precise than that. As a dual agonist, it acts on both GIP and GLP-1 receptors, which together influence appetite signalling, gastric emptying and blood-glucose regulation. The combination is why tirzepatide's mechanism produces outcomes that have, in head-to-head data, exceeded those seen with single-pathway GLP-1 medicines.

The SURMOUNT-5 trial, published in the New England Journal of Medicine in 2025, compared tirzepatide directly with semaglutide 2.4 mg in adults with obesity over 72 weeks: tirzepatide produced greater average weight reduction. NICE cited indirect comparisons pointing in the same direction when it recommended tirzepatide in its technology appraisal TA1026, published in December 2024.

Who the licensed eligibility criteria cover, and who decides

The UK licence for Mounjaro covers adults with a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related condition such as high blood pressure, raised cholesterol, prediabetes, type 2 diabetes or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. NICE's recommendation for NHS use carries additional criteria around specific comorbidities, but private prescribing follows the licensed indication, which is broader.

None of those numbers guarantee a prescription. A prescriber reviews the full clinical picture: current health conditions, other medicines, contraindications, and whether the likely benefits outweigh the risks for that specific person. The NHS tirzepatide information page lists conditions that require prescriber discussion before starting, including a history of pancreatitis or certain thyroid conditions. If you are considering treatment after reading about someone else's experience, the honest starting point is a clinical assessment of your own situation, not a comparison with someone else's results.

Treatment itself begins at 2.5 mg. That first pen's job is to let your body settle before any therapeutic dose is reached; nausea and other gastrointestinal effects are most common in the early weeks. Understanding what the first weeks on Mounjaro involve before you begin sets more realistic expectations than public accounts typically provide.

Side effects, stopping and what the evidence says about duration

The side-effect profile in the SURMOUNT-1 trial was dominated by gastrointestinal symptoms: nausea, diarrhoea, constipation and vomiting were the most common, typically most noticeable after a dose increase and easing over days to a couple of weeks for most participants. Serious side effects were less common but include acute pancreatitis; in January 2026 the MHRA issued a Drug Safety Update specifically highlighting this risk across GLP-1 class medicines, with guidance to seek urgent help for severe, persistent stomach pain that may spread to the back.

Questions about stopping Mounjaro are common. Trial data and clinical experience both suggest that weight tends to return when treatment is discontinued without sustainable lifestyle changes in place, which is why prescribers frame tirzepatide as one component of a longer-term approach rather than a standalone fix. Similarly, people sometimes ask whether a single month of treatment produces lasting change, that is a question for a prescriber rather than a press interview. For cost context on private treatment, the Mounjaro cost overview sets out what private prescriptions typically include and how pricing has shifted since Eli Lilly revised its UK list prices in September 2025.

Less serious but real: some people notice digestive side effects (including increased flatulence) that are worth knowing about before starting. Our guide on Mounjaro and wind covers what is normal and when to raise it with your prescriber.