How to Start Mounjaro: What to Decide Before Your First Injection

Is Mounjaro the right call for you right now?

This is the decision that comes before everything else. Mounjaro is licensed for weight management in adults with a BMI of 30 or above, or 27 or above if at least one weight-related condition is present, things like high blood pressure, type 2 diabetes, high cholesterol or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. But BMI alone does not close the case. A prescriber also needs to know about your medical history, any medicines you currently take, and whether there are contraindications such as a history of medullary thyroid carcinoma or pancreatitis that would make tirzepatide unsuitable.

If you are pregnant, breastfeeding, trying to conceive, or under 18, Mounjaro is not licensed for you. These are firm lines, not administrative ones.

It is also worth being honest with yourself about readiness. Mounjaro works alongside a reduced-calorie diet and increased physical activity, not instead of them. The clinical trials that produced results of around 20% average body-weight reduction at 15mg were built on that combined approach, as the SURMOUNT-1 trial published in the New England Journal of Medicine makes clear. A good prescriber will ask about your lifestyle as well as your BMI, and that is a feature, not a hurdle.

If you want to explore what treatment involves before committing, our tirzepatide overview covers how the medicine works in more detail.

Where to get a prescription and what the assessment involves

In the UK, the only legal way to start Mounjaro is via a prescription following clinical assessment. Many people come to this point having already spent time on NHS waiting lists, or having been told they don't yet meet the current NHS criteria. That is a genuinely frustrating position to be in, and a private route through a regulated online pharmacy is a legitimate alternative.

The assessment for a private prescription typically involves confirming your identity, verifying your current weight, reviewing your medical history, and checking that no contraindications apply. At nume, a GPhC-registered Independent Prescriber reads every consultation personally on the day it is submitted. There is no algorithm making the call. If you order before 12pm Monday to Friday and your consultation is approved, the treatment is dispatched the same day for free next-working-day delivery via DPD in plain, unbranded packaging.

If you are transferring from another provider or looking to increase your dose, you will need to provide evidence of your current treatment, that is standard clinical practice, not bureaucracy for its own sake. You can read more about how to get started in our practical guide to taking Mounjaro for the first time.

For context on what private treatment costs, see our treatment pricing page.

What the first few weeks on Mounjaro actually look like

The first pen you inject is 2.5mg. Its job is adjustment, not dramatic weight loss. Most people stay on that dose for four weeks before their prescriber considers the next step. Side effects, if they come at all, are most common in the early weeks and tend to centre on the gut: nausea, constipation, loose stools, reflux or indigestion. These are the body responding to a medicine that slows gastric emptying and shifts appetite signals. They are generally mild to moderate and settle with time for most people.

Eating smaller portions, staying hydrated, and avoiding very fatty or heavily processed meals during the first few weeks makes a meaningful difference to how the transition feels. If you plan to build in more activity as treatment progresses, our page on exercising on Mounjaro covers what the evidence says about timing and intensity.

If you experience severe, persistent stomach pain that radiates to the back, seek medical help promptly, this can be a sign of acute pancreatitis, which the MHRA flagged in a Drug Safety Update in January 2026 as an infrequent but serious risk with GLP-1 medicines. You can also report any suspected side effects using the MHRA Yellow Card scheme.

Questions about when to schedule your first injection, or which day of the week suits your routine, are worth raising at consultation. There is a useful breakdown on the best day to start Mounjaro if that is on your mind.

How long before Mounjaro starts working, and what realistic progress looks like

Appetite suppression often begins within the first week or two, though visible weight change takes longer to appear on the scales. Most people do not see significant weight movement until they are past the first dose or two. That can feel discouraging if you are expecting an immediate shift. It is not a sign that the treatment is not working.

Progress then tends to build through the titration phase as doses increase toward the therapeutic range. How quickly that happens depends on tolerability and your prescriber's assessment at each review. There is no fixed timeline that applies to everyone. For a clearer picture of timelines, our page on how soon Mounjaro starts to work walks through what the evidence shows week by week.

If you are weighing up whether to start tirzepatide specifically or are comparing it with other options, the weight-loss treatment overview sets out what is currently available through a UK private prescription.