What Are Mounjaro Injections and How Do They Work?

The trial results that led to UK approval

The clearest evidence for Mounjaro comes from the SURMOUNT-1 study, which followed 2,539 adults with obesity over 72 weeks. Participants taking the 15 mg dose lost an average of around 20–21% of their body weight, with some analyses placing the figure slightly higher. That scale of reduction had not previously been seen in a licensed weight-management medicine, which is why NICE recommended tirzepatide in its appraisal TA1026 and why the treatment attracted considerable clinical attention on publication.

What the headline figures do not tell you is that results varied across participants. Weight loss of this magnitude is an average, not a floor or a ceiling. Factors including starting weight, adherence to the recommended lifestyle changes, and how well the dose is tolerated all influence individual outcomes. Mounjaro is not assessed or licensed as a quick fix, and the MHRA is explicit that GLP-1 medicines are not intended for cosmetic weight loss.

In a subsequent head-to-head study (SURMOUNT-5, published in 2025) tirzepatide produced greater average weight reduction than semaglutide 2.4 mg over 72 weeks, the first direct comparison of the two most widely used licensed options. That result informed ongoing clinical discussions, though which medicine suits a given person remains a clinical decision shaped by health history, tolerability and preference.

What actually arrives and what goes into it

A Mounjaro prescription is dispensed as a pre-filled device called a KwikPen. Inside each pen are four cartridges of tirzepatide solution (one for each weekly injection) so a single pen covers a full month of treatment. The pen has a hidden needle that deploys automatically when you press it against the skin; you do not load or handle the needle directly. Most people find the injection straightforward after the first use, though it is worth reading the patient information leaflet before you start.

When you order through a regulated UK pharmacy, the pen arrives refrigerated (cold packs included) in plain, unbranded packaging, with a DPD tracking notification so you are not waiting in all day. It needs to go into your fridge door when it arrives. If you are curious about what the device looks like before ordering, the guide on what a Mounjaro pen looks like covers it in detail. For safe disposal after use, a sharps container is the correct way to handle used pens, never the household bin.

The NHS medicines information for tirzepatide, available at nhs.uk/medicines/tirzepatide, provides a reliable and plain-English breakdown of storage, common reactions and what to do if you have concerns.

Who Mounjaro injections are licensed for in the UK

The UK licence covers adults with a BMI of 30 or above, or a BMI of 27 or above when at least one weight-related condition is present, conditions such as type 2 diabetes, high blood pressure, raised cholesterol, or obstructive sleep apnoea qualify. For people from South Asian, Chinese, Middle Eastern, Black African or African-Caribbean backgrounds, NICE guidance applies BMI thresholds that are 2.5 kg/m² lower, reflecting differences in cardiometabolic risk at equivalent BMI values.

BMI alone does not determine suitability. A prescriber also considers your medical history, current medicines, and whether any contraindications apply. If you are still building up a picture of what the Mounjaro injection is and how it works, that background can help you have a more informed conversation with your prescriber. Mounjaro is not currently licensed for people under 18, and it is not recommended during pregnancy, while breastfeeding, or for those actively trying to conceive. A full overview of Mounjaro injections on this site covers eligibility in more depth if you want to explore the criteria before speaking to a prescriber.

It is worth knowing that NICE's NHS recommendation has stricter criteria than the licensed indication. NHS access is being phased in by BMI and number of weight-related conditions; many people who are eligible for private treatment do not yet qualify under NHS thresholds. The weight-loss treatment overview sets out both routes plainly.

Safety profile: what to expect and when to seek help

The most common side effects are gastrointestinal, nausea, loose stools, constipation, indigestion and fatigue feature most often, particularly in the weeks after starting or after a dose increase. For most people these effects are mild and settle within a couple of weeks as the body adjusts. They are also one reason the titration schedule begins at 2.5 mg: the first four weeks are about letting your system find its footing, not about therapeutic weight loss. Thinking about what you can eat on Mounjaro injections during this period is a sensible way to support your body as it adjusts, and the guidance there pairs well with the advice on managing side effects covered on the page about Mounjaro side effects.

More serious but less common effects include symptoms of pancreatitis, severe, persistent stomach pain that radiates towards the back, with or without vomiting. This warrants urgent medical attention. The MHRA highlighted acute pancreatitis as a known risk in a Drug Safety Update issued in January 2026, underscoring the importance of not dismissing severe abdominal symptoms as ordinary nausea. If you experience them, contact a healthcare professional promptly rather than waiting for your next check-in.

Questions about safety in specific situations (interactions with other medicines, what to do around surgery, or how the treatment sits alongside contraception) are the kind of thing our prescribers handle regularly. The page on Mounjaro injection safety addresses the most common concerns in detail, and you can always get in touch with our team directly.