The Mounjaro injection: evidence, technique and what actually happens

What the SURMOUNT-1 trial actually found about the injection

The SURMOUNT-1 trial enrolled 2,539 adults with obesity and no type 2 diabetes, following them for 72 weeks. Participants using the 15 mg weekly injection achieved an average weight reduction of around 20–21%, with some analyses placing the figure closer to 22.5% in those who remained fully adherent. Those are the figures NICE considered when appraising tirzepatide, and they sit behind NICE's recommendation (TA1026), published in December 2024, for adults with a BMI of 35 or above and at least one weight-related comorbidity.

The trial results reflect treatment alongside a reduced-calorie diet and increased physical activity. The Mounjaro injection does not operate independently of lifestyle, it works by slowing gastric emptying, signalling fullness earlier and reducing appetite, which makes it easier to sustain a calorie deficit consistently. Lower BMI thresholds apply for certain ethnic backgrounds under UK guidance; a prescriber will apply the correct criteria to your individual case.

Private licensed eligibility is somewhat broader than the NHS threshold: adults with a BMI of 30 or above, or 27 and above with at least one weight-related condition, may qualify. The full overview of Mounjaro at nume explains the eligibility picture in plain terms.

The KwikPen: what actually arrives and how to use it

Once a consultation is clinically approved and dispatched, the pen arrives via DPD the next working day, in plain, unbranded packaging with a tracking notification to your phone. Inside is the KwikPen itself: a pre-filled, multi-dose device containing four weekly Mounjaro injections at your prescribed strength. The six licensed UK strengths are 2.5, 5, 7.5, 10, 12.5 and 15 mg.

Injection sites are the abdomen, outer thigh or upper arm. Rotating the site each week reduces the chance of skin reactions at any one location. Mounjaro injection pens are designed to be used without drawing up a dose or handling a separate needle, the mechanism delivers a fixed volume when the button is pressed and held. For a careful walkthrough of each step, the injection technique guide covers preparation, site choice and safe disposal in full.

Storage matters: the pen is refrigerated at 2–8°C between uses. The precise window for temporary room-temperature storage is set out in the Patient Information Leaflet supplied with your medication; always follow that rather than any summary. After use, the needle should be disposed of in a dedicated sharps container rather than ordinary household waste.

Side effects to know before your first injection

The most commonly reported effects are gastrointestinal: nausea, vomiting, diarrhoea, constipation and indigestion. They are most noticeable at the start of treatment or in the days following a dose increase, and for most people they ease within one to two weeks as the body adjusts. Eating smaller portions, avoiding rich or fatty foods during the early weeks, and staying well hydrated all help.

In January 2026 the MHRA issued a Drug Safety Update for GLP-1 medicines, highlighting acute pancreatitis as an infrequent but potentially serious event. The symptom to act on urgently is severe, persistent abdominal pain that spreads to the back, with or without vomiting. If you experience that, seek medical help the same day rather than waiting. Any suspected side effect can also be reported through the MHRA Yellow Card scheme, which helps the regulator monitor the safety of medicines in real-world use.

Mounjaro is not recommended during pregnancy, while breastfeeding, or when actively trying to conceive. It is not licensed for anyone under 18. If you use oral contraceptives, clinical guidance recommends adding a barrier method for the first four weeks of treatment and for four weeks after each dose increase, because absorption of the pill may be reduced, a detail worth discussing with your prescriber before your first injection.

Choosing a clinically safe route to the injection in the UK

Tirzepatide is a prescription-only medicine. The only lawful route is a prescription issued following a proper clinical assessment. Any seller offering it without that step is operating outside UK law, and the MHRA has seized large quantities of counterfeit pens over the past two years, including fake tirzepatide devices that would not have undergone the same manufacturing controls as licensed products.

When choosing a private provider, check them on the GPhC pharmacy register before parting with any money. At nume, every consultation is read and decided by a GPhC-registered Independent Prescriber on the day it arrives. If you want context on private treatment costs before committing, the Mounjaro pricing page sets out what a typical private prescription includes and what the Eli Lilly list-price change in September 2025 means for consumers.

If you have questions about whether the injection suits your situation, speak to our prescribers through a free consultation. There is no obligation, and no clinical assessment is ever automated.