Mounjaro UL: the clinical evidence behind tirzepatide for weight management

What the SURMOUNT-1 trial found, and how NICE used it

The SURMOUNT-1 trial, published in the New England Journal of Medicine, randomised 2,539 adults with obesity but without type 2 diabetes to tirzepatide (5 mg, 10 mg or 15 mg) or placebo over 72 weeks. At the highest dose, participants lost an average of around 20–21% of their body weight. That figure includes everyone who took the medicine, not just those who responded particularly well.

NICE reviewed this data as part of its technology appraisal TA1026, published in December 2024. The committee judged tirzepatide cost-effective for a defined population on the NHS, adults with a BMI of 35 or above plus at least one weight-related condition such as high blood pressure, type 2 diabetes or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. For people who do not meet NHS criteria or prefer not to wait, a private prescription route exists through a regulated pharmacy after clinical assessment.

The mechanism behind the results matters. Tirzepatide acts on two gut-hormone receptors (GIP and GLP-1) slowing gastric emptying, increasing satiety signals and improving blood-sugar regulation. No other weight-loss medicine currently licensed in the UK shares that dual pathway, which is one reason the trial outcomes sit above earlier single-agonist data. You can read a fuller account of how the medicine works, including its dual-agonist mechanism, on our Mounjaro page, and if you are considering the electronic prescribing route you may also find our e-Mounjaro page useful for understanding how that process works.

The dose schedule: why 2.5 mg is not the end of the story

Treatment starts at 2.5 mg because the body needs time to adjust. Most GI side effects (nausea, loose stools, a feeling of fullness that can tip into discomfort) are sharpest at the beginning or after a dose step up, then tend to settle over a couple of weeks. The 2.5 mg pen's job is to let your system adapt, not to drive weight loss.

From there, the prescriber typically moves in 4-week steps through 5 mg, 7.5 mg, 10 mg, 12.5 mg and up to 15 mg, based on how you tolerate each level. The 5 mg dose is often where people notice the first real change in appetite. The highest dose you tolerate well is generally where you stay. NICE's guidance is clear that if less than 5% weight loss has occurred after six months at the highest tolerated dose, continuing should be reviewed.

Each pen contains four weekly doses. A plain, unbranded box arrives via DPD with a tracking link sent to your phone; the pen itself is an auto-injection KwikPen with a covered needle that retracts after use. Storage is in the fridge at 2–8 °C; for the exact window during which the pen can be kept at room temperature, the Patient Information Leaflet is the authoritative source.

The 2.5 mg starter pen page covers the opening phase in more detail, including what to expect in the first four weeks.

Side effects the evidence records, and when to seek help

The NHS medicines page for tirzepatide summarises the most common side effects drawn from trial data: nausea, vomiting, diarrhoea, constipation, indigestion, burping, fatigue, headache and injection-site reactions. Most are gastrointestinal and are more noticeable around dose increases than during a settled period on a given strength.

Pancreatitis is less common but more serious. The MHRA issued a Drug Safety Update in January 2026 flagging acute pancreatitis across GLP-1 medicines. The symptom to act on immediately is severe, persistent stomach pain that radiates towards the back, with or without vomiting. That warrants same-day medical attention, not a wait-and-see approach.

Other reasons to contact a clinician promptly include signs of a serious allergic reaction, significant dehydration from persistent vomiting or diarrhoea, or gallbladder symptoms. Women taking oral contraceptives should add a non-oral method of contraception during the first four weeks of treatment and for four weeks after any dose increase, because tirzepatide slows gastric emptying in a way that can reduce pill absorption. This is not a theoretical concern, it is in NHS prescribing guidance. You can report any suspected side effects directly to the MHRA via the Yellow Card scheme.

Mounjaro is not licensed for people under 18, during pregnancy, or for anyone trying to conceive or breastfeeding. Anyone with a history of medullary thyroid carcinoma, MEN2 syndrome or a prior episode of pancreatitis needs a prescriber conversation before starting.

NHS access versus private prescription, the honest picture

NICE TA1026 recommended tirzepatide for NHS use in a phased rollout. As of summer 2026, the first NHS cohort covers adults with a BMI of 40 or above and four or more of a defined list of qualifying conditions. A second cohort, for people with a BMI of 35–39.9 and four or more conditions, activated in June 2026. Wider eligibility phases follow through 2027, and from April 2026 some GP practices may prescribe under the new contract, though not all practices have opted in.

If your BMI or condition list falls outside the current NHS threshold, or if the wait is not practical, a private prescription through a GPhC-registered online pharmacy is the alternative. The private prescription route page covers what that involves. Private eligibility follows the licensed indication (BMI of 30 or above, or 27 or above with a weight-related condition) and is assessed by a prescriber, not an algorithm.

For context on what private treatment currently costs and what reputable pricing typically includes, the Eli Lilly price increase page gives a factual breakdown. Private prescriptions are a clinical pathway, and the prescriber's assessment is the constant whether you go through the NHS or a regulated pharmacy. Explore your options on the weight-loss treatments page, or speak to our prescribers to see whether tirzepatide is clinically appropriate for you.