What the evidence says about Mounjaro weight loss injections

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

The results that circulate online often get separated from their context. SURMOUNT-1 was a 72-week randomised controlled trial involving 2,539 adults who had obesity but not type 2 diabetes. Participants received tirzepatide at 5 mg, 10 mg or 15 mg alongside a reduced-calorie diet and increased physical activity. The primary endpoint was percentage change in body weight from baseline. At 15 mg, average weight loss reached around 20–21%, with some analyses reporting up to approximately 22.5%. Those on 5 mg still lost an average of around 15%. The full SURMOUNT-1 paper, published in the New England Journal of Medicine, is freely accessible if you want to read the methodology yourself. The figures were strong enough to support NICE's recommendation of tirzepatide for eligible adults in England, NICE published that guidance (TA1026) in December 2024. The trial also contributed to the wider SURMOUNT programme, which alongside the SURPASS diabetes trials means tirzepatide has been tested in more than 10,000 participants across clinical studies combined.

What the numbers cannot tell you is how an individual will respond. Weight loss on any medicine varies with dose tolerance, starting weight, diet, activity and other health factors. A prescriber's job is to hold that nuance.

How the Mounjaro injection works once it's in your system

Tirzepatide does something no other licensed UK weight-loss injection does: it binds to both the GIP receptor and the GLP-1 receptor simultaneously. GLP-1 receptor agonists like semaglutide target one pathway; tirzepatide targets two. Both pathways are involved in appetite regulation, glucose metabolism and how quickly food leaves the stomach. The combined effect is a meaningful reduction in appetite and calorie intake, alongside slower gastric emptying, which is part of why fullness lasts longer after meals.

The injection is given once weekly into the fatty tissue of the abdomen, thigh or upper arm, rotating the site each time. Each KwikPen contains four doses. Storage is straightforward: the pen lives in the fridge (2–8 °C) until needed. For the precise window during which it can be kept at room temperature (say, when you're travelling) the Patient Information Leaflet and the Mounjaro SmPC on the eMC give the exact figure; never rely on informal sources for that detail. If injection technique or site care is something you're thinking about, alcohol swabs are available for cleaning the injection site.

One question our prescribers are asked most weeks: what does 2.5 mg actually do? The starter dose exists to let your body adjust rather than to produce significant weight loss immediately. Its job is tolerability. Dose steps follow from there, guided by the prescriber, not a fixed calendar.

Side effects: what the evidence shows and when to get help

The most common side effects in the SURMOUNT trials were gastrointestinal, nausea, vomiting, diarrhoea, constipation and indigestion featured prominently, particularly in the weeks after starting or after a dose increase. For most people these settled within a few days to a couple of weeks. Fatigue, headache and injection-site reactions also appeared. The NHS tirzepatide medicines page lists the full side-effect profile and is the most practical starting point for patients.

There are some situations that require prompt medical attention. Severe, persistent stomach pain that spreads to the back (with or without vomiting) should be assessed the same day; in January 2026 the MHRA issued a Drug Safety Update drawing prescribers' and patients' attention to acute pancreatitis as an infrequent but serious risk with GLP-1 medicines. Symptoms of an allergic reaction, signs of dehydration from prolonged vomiting or diarrhoea, and gallbladder pain also warrant urgent review. If you notice anything unexpected while on treatment, contact your prescribing team. Suspected side effects can also be reported directly at the MHRA's Yellow Card scheme.

Mounjaro is not recommended during pregnancy, while breastfeeding, or for anyone trying to conceive. It is not licensed for under-18s. Women using oral contraceptives should use an additional non-oral method for the first four weeks of treatment and for four weeks after each dose increase, because the pill's absorption may be reduced, a detail worth raising at your consultation. History of certain conditions, including medullary thyroid carcinoma or pancreatitis, requires careful prescriber discussion before starting.

Who can access the Mounjaro injection, and through which route

The licensed eligibility criteria for private treatment are a BMI of 30 or above, or 27 and above alongside at least one weight-related condition such as high blood pressure, high cholesterol, type 2 diabetes, obstructive sleep apnoea or osteoarthritis. Lower BMI thresholds can apply for some ethnic backgrounds under UK clinical guidance. BMI alone does not determine approval; the prescriber weighs the full clinical picture. If you want a rough starting point, the NHS BMI calculator is a practical first check before your consultation.

NHS access to Mounjaro exists but is phased. From around June 2025, people with a BMI of 40 or above and four or more of five specified conditions became eligible; from June 2026, that expanded to a BMI of 35–39.9 with four or more conditions. Eligibility criteria differ in Scotland, Wales and Northern Ireland. GP practices may also prescribe under the 2026/27 GP contract, though participation is not universal. Waiting lists vary significantly. For a broader view of how private and NHS routes compare, the page on weight loss injections covers the landscape clearly.

Private treatment through a regulated online pharmacy requires no referral and no waiting list, provided you meet the clinical criteria and the prescriber is satisfied. It also carries cost. If understanding what drives those costs matters to you, what weight loss injections cost in the UK sets out the honest picture. If you would like to spread that cost over time, our page on weight loss injections with Klarna explains how flexible payments work. For a balanced view of the trade-offs involved in this treatment category, the page on the pros and cons of weight loss injections is worth a read before you decide. And if you are specifically researching tirzepatide's clinical profile, our page on the Mounjaro injection goes further into mechanism and dosing context.

When the treatment is right for you, the path through a GPhC-registered pharmacy like ours is straightforward. A prescriber reviews your consultation the same day (a clinician, not software) and once approved, treatment is dispatched for free next-working-day delivery. That's the service. Check whether you're eligible and start your free consultation at our treatment page.