The Mounjaro jag: what happens at each step

Step 1, clinical assessment before anything else

Before a single jag is given, a prescriber needs to confirm that tirzepatide is clinically suitable for you. That means reviewing your BMI, any weight-related health conditions such as hypertension or type 2 diabetes, your current medicines, and your medical history. The full Mounjaro overview covers the licensed eligibility criteria in detail, but the short version is: adults with a BMI of 30 or above, or 27 or above with at least one relevant health condition, may qualify. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance.

At nume, this assessment is completed the same day you submit your consultation. A GPhC-registered Independent Prescriber reads your answers, a real clinician, not software. If you have already been taking tirzepatide elsewhere and want to transfer, you will be asked to provide evidence of your current dose; that is standard clinical practice, not a hurdle.

It is also the point at which contraception is discussed. Women using oral contraceptives are advised to add a barrier method for the first four weeks of treatment and for four weeks after each dose increase, because the jag slows gastric emptying in a way that can reduce how reliably the pill is absorbed. Our FAQs address this and other common questions.

Step 2, what the jag actually involves

The KwikPen is designed to be used at home without any clinical training. You choose an injection site, clean the skin, press the pen against it and activate the dose. The needle is fine and short; most people find the sensation much less noticeable than they expected. If you would like a closer look at how Mounjaro jags work in practice, our dedicated guide walks through the process step by step.

Rotating between the abdomen, thigh and upper arm matters. Using the same patch of skin repeatedly can cause localised lumps or soreness. If you inject into the abdomen, stay at least two inches from the navel. The pen should be kept refrigerated between 2 °C and 8 °C; if you need to carry it somewhere, the exact room-temperature storage window is in the Patient Information Leaflet rather than here, because the prescriber and that leaflet are the right authority for that detail.

The Mounjaro jab guide goes into the physical technique in more depth, including what to do if you miss a dose. In short: always follow the advice of your prescriber and the leaflet that comes with your pen.

Step 3, what to expect in the weeks after starting

The starting dose of 2.5 mg exists to let your system adjust, not to produce dramatic weight loss immediately. The job of that first pen is largely tolerability. Most of the common side effects (nausea, loose stools, constipation, indigestion, fatigue) are at their most noticeable in the first couple of weeks at any given dose level and tend to settle as the body adapts. Tirzepatide's clinical profile, including the full side-effect picture, is worth reading before you start.

The SURMOUNT-1 trial, published in the New England Journal of Medicine, reported average body-weight reductions of around 20–21% at the 15 mg dose over 72 weeks, results that positioned tirzepatide ahead of older single-pathway medicines in head-to-head evidence. Those figures come from controlled trial conditions, and individual results vary; a prescriber will never guarantee a specific outcome.

One practical note: if you order by midday on a working day and your consultation is approved, your pen is dispatched the same afternoon and arrives the following working day by tracked DPD delivery. Knowing that can help you plan around work or the school run rather than wondering when the package will arrive. Patients who have completed their first month and are ready to move on to Mounjaro 2 will find full guidance on what that next stage involves. For a sense of what treatment costs across the dose schedule, the Mounjaro prices page gives honest market context.

Side effects worth knowing about before your first jag

Gastrointestinal effects are the most commonly reported. Nausea is the most frequent, followed by changes to bowel habits and occasional reflux or burping. These are generally mild to moderate and most settle within a fortnight of starting or increasing a dose. Eating smaller portions, avoiding fatty foods during the early weeks and staying well hydrated all help.

A small number of people experience more significant reactions. The MHRA issued guidance in early 2026 reminding patients and clinicians that acute pancreatitis, while infrequent, is a known risk with GLP-1 class medicines. Severe, persistent stomach pain that spreads to the back (with or without vomiting) needs same-day medical attention, not a wait-and-see approach. You can report any suspected side effects directly through the MHRA Yellow Card scheme.

Women who are pregnant, breastfeeding or actively trying to conceive should not use tirzepatide; the medicine is not recommended in these situations. The same applies to anyone under 18. Certain conditions (including a history of medullary thyroid carcinoma) require a direct conversation with the prescriber before treatment can proceed. The NHS tirzepatide medicine page lists the full contraindication detail in plain language.

If you have read through all of this and want to find out whether tirzepatide is right for you, the next step is a free clinical consultation with our prescribers. Check your eligibility and the process takes it from there.