Mounjaro injection guide: step by step, from fridge to done

What the SmPC tells us about injecting tirzepatide correctly

The Mounjaro SmPC (the definitive reference document published on the eMC) sets out a consistent process that the pen is engineered around. Starting there, rather than trial and error, matters. Tirzepatide is delivered into subcutaneous fat, not muscle, so the technique is deliberately low-skill: a short needle, a gentle pinch if you have less subcutaneous tissue, and a slow press of the button. Where you choose to inject affects how reliably the medicine is absorbed, which is why the SmPC specifies only abdomen, thigh or upper arm, not forearm, not calf.

The SURMOUNT-1 trial, published in the New England Journal of Medicine, produced the 20–21% average weight-loss figures that most people know about; those results depended on participants injecting consistently, once a week, at the right dose. Technique isn't a footnote, it's part of why the trial worked. If you want a full walkthrough of how the process works in practice, our guide to the Mounjaro injection covers everything from preparation through to aftercare. The injection itself is one small thing, but doing it the same way each time makes the clinical results reproducible in real life too.

One practical note: if your delivery arrives on a Friday and you'd planned to start on a Monday, that's fine. The key is picking a day that you can repeat reliably week after week. A lot of patients choose a weekday morning precisely because it's routine, less easy to forget than a weekend when plans shift. What matters most is consistency, not which day you choose.

The actual steps: from fridge to done

Take the pen out of the fridge about 30 minutes before you plan to inject. Cold medicine from the fridge isn't dangerous, but allowing it to approach room temperature makes the injection more comfortable, this is a practical detail that's easy to miss the first time. While you're waiting, wash your hands thoroughly with soap and water. Lay out everything: the pen, a fresh needle from the pack, and an alcohol swab to clean the skin.

Once the skin is clean and dry (dry matters, because alcohol on wet skin can sting) pinch a small fold of skin at your chosen site if needed, or simply press the pen flat against the skin for the abdomen. Remove the inner needle cap just before you inject (not before). Press the pen firmly against the skin, then press and hold the button. You'll hear a click when the dose starts and a second click when it's done. Hold the pen in place for a full ten seconds after that second click; releasing early is the most common reason for an incomplete dose.

Remove the pen, then apply gentle pressure with a cotton pad or clean finger if there's any spotting. Put the used needle (capped immediately) into a dedicated sharps container. For a closer look at the pen itself and how each dose is selected, our overview of the Mounjaro KwikPen covers the mechanics in more detail. Full technique guidance and storage rules are in the Patient Information Leaflet inside your pack, and on the NHS tirzepatide patient information page.

Side effects you might notice after injecting, and when to get medical help

Most people experience some gastrointestinal effects, particularly around starting treatment or moving to a higher dose: nausea, looser stools, a feeling of early fullness, or occasional burping. These are the most commonly reported effects in the SURMOUNT trials and are generally at their worst in the first week or two after a change, then settle. Staying hydrated and eating smaller meals helps considerably; your prescriber can advise if symptoms are severe.

A small number of people notice redness, itching or a small lump at the injection site. Rotating sites each week (keeping a mental note or a simple written log of where you last injected) prevents this from accumulating. Detailed guidance on site rotation and technique is worth reading before your first pen.

There are rarer but more serious effects that require prompt attention. In January 2026 the MHRA issued a Drug Safety Update highlighting acute pancreatitis as an infrequent but potentially serious effect of GLP-1 medicines: if you develop severe stomach pain that spreads to your back, particularly with vomiting, seek medical help the same day, don't wait for your next scheduled check-in. Gallbladder symptoms, signs of dehydration after persistent vomiting or diarrhoea, or any allergic reaction should also prompt a call to your GP or 111. You can report any suspected side effects directly through the MHRA Yellow Card scheme. If you have questions about an effect that doesn't feel urgent, our aftercare team is available seven days a week.

Starting treatment: what comes before the first injection

Mounjaro is a prescription-only medicine, which means the steps above only apply once a prescriber has assessed you and confirmed the treatment is appropriate. At nume, that assessment is carried out by a GPhC-registered Independent Prescriber (a real clinician reading your consultation answers, not automated software) and it happens the same day you submit. Private eligibility follows the licensed criteria: broadly, adults with a BMI of 30 or above, or 27 or above alongside a weight-related health condition such as high blood pressure or type 2 diabetes. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. The prescriber assesses the full picture, not the number alone.

For context on what private treatment involves financially, our Mounjaro cost comparison page sets out what a legitimate private prescription typically includes. Once approved, the pen is dispatched the same day for free next-working-day delivery via DPD, so you're not waiting long before that first injection day arrives. The broader Mounjaro treatment overview covers how the medicine works, what clinical trials found, and what to expect over the course of treatment.