Tirzepatide — what it's for, what the evidence says, and who it's licensed to treat

What SURMOUNT-1 and NICE's appraisal actually found

NICE's recommendation of tirzepatide in December 2024 (published as technology appraisal TA1026) leaned heavily on SURMOUNT-1, a 72-week trial of 2,539 adults with obesity but without type 2 diabetes. Participants on the 15 mg dose lost, on average, around 20–21% of their body weight, a figure that had not been reached before by any single medicine in a head-to-head trial of this scale. The committee also noted indirect comparisons favouring tirzepatide over semaglutide 2.4 mg, a finding later supported more directly by the SURMOUNT-5 open-label trial published in the New England Journal of Medicine in 2025, where tirzepatide produced greater average weight reduction than semaglutide over 72 weeks in adults with obesity and no diabetes.

For the NHS, NICE's recommendation applies to adults with a BMI of 35 or above plus at least one weight-related comorbidity. Lower thresholds apply for people from South Asian, Chinese, Middle Eastern, Black African or African-Caribbean backgrounds, where the guidance reduces the qualifying BMI by 2.5 kg/m². If less than 5% of body weight is lost after six months at the highest tolerated dose, continuing treatment is subject to clinical review.

These figures matter because they shifted how UK clinicians and commissioners think about medical weight management. Weight loss of this scale had previously required bariatric surgery.

How tirzepatide works in the body, and why the dual mechanism is significant

Most GLP-1 medicines target a single receptor. Tirzepatide targets two: the glucagon-like peptide-1 (GLP-1) receptor and the glucose-dependent insulinotropic polypeptide (GIP) receptor. Both are activated after eating. Together they slow gastric emptying, increase the feeling of fullness, and reduce appetite signals reaching the brain. The GIP pathway also plays a role in fat metabolism and may contribute to the tolerability profile at higher doses.

The result is a slower, more sustained reduction in food intake that most people on treatment describe as a quietening of hunger rather than a forced restriction. The NHS medicines page on tirzepatide explains the mechanism in plain terms alongside a full list of possible side effects, which are worth reading before you begin. The most common are gastrointestinal (nausea, constipation, loose stools, indigestion) typically at their most noticeable after a dose starts or increases, and settling for most people within days to a couple of weeks.

If you want to explore what tirzepatide contains and how its formulation differs from single-pathway GLP-1 medicines, that page goes deeper into the pharmacology.

Who tirzepatide is licensed to treat in the UK

If you are new to this treatment and wondering what tirzepatide is and how it differs from other weight management medicines, that page gives a clear grounding before getting into the specifics below. The Mounjaro UK licence covers two distinct populations. For weight management, eligible adults have a BMI of 30 or above, or 27 or above if they also have at least one weight-related condition. Conditions that can qualify include prediabetes, type 2 diabetes, high blood pressure, dyslipidaemia (raised cholesterol or triglycerides), and obstructive sleep apnoea, among others. Tirzepatide is also licensed for glycaemic control in type 2 diabetes, where it has its own separate evidence base from the SURPASS programme, and if you are unsure whether tirzepatide and Mounjaro refer to the same thing, that page explains the relationship clearly.

BMI alone does not determine whether treatment is right for you. A prescriber looks at the whole clinical picture, including other medicines you take, any history of pancreatitis or thyroid conditions, and whether you are pregnant, breastfeeding, or planning a pregnancy (none of these situations are suitable for tirzepatide). It is not licensed for anyone under 18.

The six available strengths run from 2.5 mg to 15 mg. The first pen's job is to let your system settle, not to produce immediate results. Dose increases happen at intervals set by your prescriber, and the right maintenance dose varies between individuals. Questions about lower or modified doses are worth raising directly in a clinical consultation rather than adjusting independently.

One practical note: if you are thinking about starting treatment, orders placed with a clinical service like nume by noon on a working day can go out for next-day delivery once a prescriber has reviewed and approved them, useful if you are planning around a Monday start or a busy period ahead.

NHS access versus private prescription, where tirzepatide sits right now

NICE's recommendation triggered a phased NHS rollout. From June 2025, adults with a BMI of 40 or above alongside four or more of the five qualifying conditions became eligible. A second cohort (BMI 35–39.9 with four or more conditions) was added from June 2026. Further phases are expected through to 2027. From April 2026, GP practices can prescribe under a new contract arrangement, though participation varies by practice and area, and NHS patients must engage in structured diet and activity support alongside the medicine.

For people who do not yet meet the NHS criteria, or who would rather not wait for a referral, a private prescription via a regulated online pharmacy is the established alternative. What that costs and what's included is worth understanding clearly before you commit, the price difference between services often comes down to what is or is not bundled in. For context, Eli Lilly raised Mounjaro's UK list price significantly from September 2025, with the highest-dose pen rising from around £122 to around £330; private market prices have typically ranged between roughly £149 and £375 per month depending on dose and provider.

If you are still working out which treatment might suit you, the treatment overview page sets out the options clearly. And if you have broader questions about what Mounjaro is and how it is used in practice, that page covers the medicine from a patient perspective in one place. Our clinical team at nume reviews every consultation personally, a prescriber, not an automated system, reads your answers and makes the decision.