What the evidence says about Mounjaro — and what to expect

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

The SURMOUNT-1 trial enrolled 2,539 adults with obesity but without type 2 diabetes and ran for 72 weeks. Participants taking tirzepatide 15 mg lost around 20–21% of their body weight on average. To put that in context: earlier GLP-1 medicines had typically achieved figures in the 10–15% range. The trial was the basis for NICE's recommendation and for tirzepatide's UK licence for weight management.

A later head-to-head study, SURMOUNT-5, compared tirzepatide directly against semaglutide 2.4 mg in adults with obesity over 72 weeks. Tirzepatide produced greater average weight reduction, and our overview of tirzepatide's pros explains what those efficacy advantages mean in practical terms. That paper was published in the New England Journal of Medicine in 2025 and is the strongest direct evidence available. NICE cited indirect comparisons favouring tirzepatide in its TA1026 committee discussion, so the head-to-head trial confirmed what the committee had already expected.

Results vary between individuals. Clinical trials report averages across thousands of participants; your own outcome depends on your starting weight, dose, adherence, diet, activity and biology. No responsible prescriber promises a specific number.

How tirzepatide works in practice, including the bit about the fridge

Tirzepatide is a dual GIP and GLP-1 receptor agonist. Most people are familiar with GLP-1 medicines; tirzepatide also activates the GIP pathway, which appears to amplify the appetite-suppressing and metabolic effects. The result is slower gastric emptying, reduced hunger signals and improved blood-sugar regulation, working through two overlapping mechanisms rather than one.

The medicine comes in a pre-filled KwikPen delivering four weekly doses. It needs to be stored in the fridge at 2–8°C; many people keep their pen in the fridge door so it's easy to find on injection day. The exact room-temperature window (for travel or if refrigeration is briefly unavailable) is set out in the Patient Information Leaflet, always check that rather than relying on a general figure, since the licensed window can vary.

Injection sites are the abdomen, thigh or upper arm; rotating between sites each week reduces localised reactions. The tirzepatide overview on our site covers the practical how-to in more detail. If a specific dose isn't your main question, the pros and cons of Mounjaro page sets out a balanced picture of both the benefits and the limitations.

Who the UK licence covers, and what the NICE criteria say

The Mounjaro licence covers adults with a BMI of 30 or above for weight management, or a BMI of 27 or above alongside at least one weight-related condition such as high blood pressure, dyslipidaemia, type 2 diabetes or obstructive sleep apnoea. BMI alone is not the whole picture; the prescriber weighs up the full clinical context.

NICE's TA1026 recommendation sets a higher bar for NHS use: BMI of 35 or above with at least one qualifying comorbidity. For people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds, that threshold is reduced by 2.5 kg/m² under UK guidance. NHS access is phased and tied to specific eligibility cohorts; many people find that private treatment costs are worth exploring if they don't meet the current NHS criteria or prefer not to wait.

Under-18s are not within the licensed indication. Pregnancy, breastfeeding and active attempts to conceive are contraindications, your prescriber will discuss appropriate contraception if relevant, particularly for tirzepatide, where oral contraceptive absorption may be affected during the first four weeks of treatment and after each dose increase.

Side effects: the GI profile and when to seek help

The most commonly reported side effects are gastrointestinal: nausea, diarrhoea, constipation, indigestion and burping. These are most noticeable when starting treatment or stepping up to a higher dose and usually ease within days to two weeks as the body adjusts. Starting at 2.5 mg exists precisely because the tolerability dose gives the system time to adapt before any therapeutic dose is reached. Our page on managing GI symptoms on Mounjaro covers practical approaches, including what your prescriber may suggest.

A smaller number of people experience fatigue, headache, dizziness or injection-site reactions. On the more serious end, acute pancreatitis is a known but infrequent risk; the MHRA highlighted this in a Drug Safety Update in January 2026. Seek urgent medical attention for severe, persistent stomach pain (especially if it spreads toward the back) with or without vomiting. Gallbladder symptoms and allergic reactions also warrant prompt attention.

If you have questions about what's normal during treatment, our FAQs cover common concerns, and our aftercare team is available seven days a week. For anything urgent, contact your GP or 111.