What Is Mounjaro? How Tirzepatide Works, Doses and UK Rules

How Mounjaro works

After you eat, your gut releases hormones that tell your brain you are full and help your body deal with the sugar from the meal. Two of those hormones are GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide). Tirzepatide is a synthetic molecule that mimics both. Older weight-loss injections such as Wegovy and Saxenda act on GLP-1 alone.

In practice, people on Mounjaro tend to notice three things: they feel full sooner and for longer because the stomach empties more slowly, they think about food less, and their blood sugar is steadier after meals. Together, these make it easier to eat less without the constant hunger that usually undermines dieting. The medicine does not burn fat directly; it changes appetite and eating, which is why the licence requires it to be used alongside a reduced-calorie diet and more physical activity.

What the trials found

The main weight-management trial, SURMOUNT-1, followed just over 2,500 adults with obesity (or overweight plus a related condition) but without diabetes for 72 weeks. Average weight loss was about 15% on 5 mg, 19.5% on 10 mg and 20.9% on 15 mg, against 3.1% on placebo. Around nine in ten people on the top dose lost at least 5% of their starting weight, and more than half lost 20% or more.

A later head-to-head trial, SURMOUNT-5, compared tirzepatide directly with semaglutide (the active ingredient in Wegovy) in adults without diabetes. Over 72 weeks, average weight loss was about 20% with tirzepatide and about 14% with semaglutide.

Two points of caution. Trial participants had regular contact with study teams, structured lifestyle support and high adherence, so real-world results are usually somewhat lower. And averages hide a wide spread: some people lose far more, some far less, and a minority do not respond meaningfully at all.

Who can be prescribed it in the UK

Under the UK licence, Mounjaro can be prescribed for weight management to adults with a BMI of 30 or more, or 27 to 29.9 with at least one weight-related condition such as prediabetes, type 2 diabetes, high blood pressure, high cholesterol, obstructive sleep apnoea or cardiovascular disease. NICE guidance lowers those thresholds by about 2.5 for people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds, though not every pharmacy applies the adjustment.

It is not recommended in pregnancy or while breastfeeding, and the MHRA advises stopping tirzepatide at least one month before trying to conceive. Prescribers will also want to know about any history of pancreatitis, gallbladder disease, severe gut problems, diabetic eye disease or certain thyroid conditions, and about other medicines you take, particularly insulin or sulphonylureas, which raise the risk of low blood sugar. Our BMI requirements guide covers the thresholds in detail, and the BMI calculator shows where you stand.

Doses and how the pen works

Everyone starts on 2.5 mg once a week for four weeks. That first month is about letting your gut adjust, not about weight loss. After that the dose can increase in 2.5 mg steps, no faster than every four weeks: 5 mg, 7.5 mg, 10 mg, 12.5 mg and 15 mg. The licence describes 5 mg and above as maintenance doses; there is no requirement to reach 15 mg, and many people settle at a lower dose that balances results against side effects.

Each KwikPen holds four weekly doses of a single strength and is stored in the fridge, though it can be kept at room temperature for up to 30 days once in use. You inject under the skin of the stomach, thigh or upper arm, on the same day each week. If you miss a dose, it can be taken within four days; otherwise skip it and carry on with the next scheduled one. From April 2026 Lilly began replacing the pen with a redesigned version that leaves almost no liquid after the fourth dose; the medicine itself is unchanged. Our guide to the so-called golden dose explains what that means.

Side effects

The most common side effects are digestive: nausea, diarrhoea, constipation, indigestion, bloating and reduced appetite. They are usually worst in the first weeks and after each dose increase, and settle for most people. Tiredness, headache, dizziness and injection-site redness are also common. Less common but more serious risks include pancreatitis, gallstones, dehydration leading to kidney problems, and low blood sugar when combined with other diabetes medicines.

Monj keeps a symptom hub with a page for each of the common side effects, including what helps and when to seek medical help. Suspected side effects can be reported through the MHRA Yellow Card scheme.

NHS or private

NICE recommended tirzepatide for NHS use in December 2024, but NHS England is introducing it in phases over up to 12 years, starting with people at the highest BMI who also have several weight-related conditions. The second phase opened on 23 June 2026 for people with a BMI of 35 to 39.9 and at least four of five named conditions. GP practices can now prescribe it, but taking part is voluntary and depends on local funding. For most people who meet the licence criteria, that means NHS access is not currently available, which is why private prescribing exists at the scale it does. See how to get Mounjaro on the NHS for the current criteria.

Privately, Mounjaro is prescribed by GPhC-registered online pharmacies and clinics after a consultation that must independently verify your height, weight and BMI. Prices vary a great deal between providers and change often, so we do not print them here; the Monj price comparison is updated daily and ranks every verified pharmacy by lowest total cost.

How it compares with the newer options

Until mid-2026 every licensed weight-loss medicine in the UK was an injection. Two tablets have since arrived. The Wegovy tablet (oral semaglutide 25 mg) was licensed on 11 June 2026 and launched privately on 6 July; it must be taken on an empty stomach with a small amount of water and a wait before eating. Foundayo (orforglipron), a non-peptide GLP-1 from Lilly, was licensed on 10 August 2026 and launched privately on 24 August; it can be taken at any time without food or water rules. Neither is available on the NHS yet.

Mounjaro remains the only licensed dual GIP and GLP-1 medicine, and the one with the largest average weight loss in its own trials. Whether an injection or a tablet suits you better is a conversation for your prescriber. Monj runs separate price comparisons for the Wegovy injection, the Wegovy tablet and Foundayo.