Eli Lilly's weight loss injection: the honest guide

What makes Lilly's injection different from other weight loss jabs?

Most weight loss injections currently on the UK market work on a single pathway, they mimic GLP-1, a gut hormone that signals fullness and slows the rate at which your stomach empties. Mounjaro does that too, but it also activates the GIP receptor alongside it. GIP is a second gut hormone involved in appetite regulation and how the body handles blood sugar. Hitting both receptors at once appears to produce a stronger appetite-suppressing effect than either pathway alone, which is reflected in the trial data.

In the SURMOUNT-1 study (a 72-week trial involving around 2,500 adults with obesity) participants using the 15 mg dose lost an average of roughly 20–21% of body weight. The full SURMOUNT-1 paper is published in the New England Journal of Medicine for anyone who wants to read the methodology. More recently, the SURMOUNT-5 head-to-head trial (also published in the NEJM, 2025) compared tirzepatide directly with semaglutide 2.4 mg and found tirzepatide produced greater average weight loss over 72 weeks in adults with obesity who did not have diabetes.

Tirzepatide also slows gastric emptying, which is part of why people feel full on smaller portions. This effect tends to be most noticeable in the early weeks. It's worth knowing that the 2.5 mg starting dose is specifically chosen for tolerability (it gives your digestive system time to adjust) rather than being the dose that drives meaningful weight loss. Our overview of weight loss injections in the UK sets this in the broader context of available options.

Who can actually use Mounjaro, and what does the prescriber check?

The licensed criteria in the UK are set out in the medicine's Summary of Product Characteristics. Broadly, adults with a BMI of 30 or above qualify, as do adults with a BMI from 27 upwards if they have at least one weight-related health condition, things like high blood pressure, high cholesterol, type 2 diabetes, prediabetes, or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance.

A BMI that meets the threshold is not by itself a guarantee of suitability. A prescriber looks at the whole picture: other medicines you take, your medical history, any conditions that would make tirzepatide unsuitable, and whether the potential benefit outweighs any risk for you specifically. Contraindications include a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia type 2, and active pancreatitis. Pregnancy, breastfeeding, and trying to conceive are further reasons the medicine is not recommended; the same applies to anyone under 18.

At nume, a GPhC-registered Independent Prescriber reads your consultation personally, a real clinician reviews it the same day, not software working through a checklist. Our clinical team page gives more detail about who is overseeing that process. If you have questions before starting, our FAQs cover the most common ones.

What does using the injection look like week to week?

Mounjaro comes in a pre-filled KwikPen. Each pen holds four weekly doses, and you inject subcutaneously (into the abdomen, thigh, or upper arm) rotating the site each week to avoid irritating the same spot. If you are considering using the belly as your injection site for weight loss, the abdomen is one of the most commonly chosen areas and is suitable for most people. The pen itself is designed to be used at home without any clinical training; most people find the process straightforward after the first use.

Storage matters. The pen should be kept refrigerated between 2–8°C. Because the exact room-temperature window allowed before the pen must be used or discarded varies, always check the Patient Information Leaflet that comes with your supply rather than relying on a figure quoted online. A sharps container is the right way to dispose of used pens safely, most councils offer a free collection service for household sharps.

One practical thing worth flagging: if your first order lands just before a bank holiday, factor in that same-day dispatch relies on working days. Ordering by midday Monday to Friday gets you next-working-day delivery, something to keep in mind if payday falls on a Friday before a long weekend. Our pharmacy page explains the dispatch and delivery process in full.

What side effects come with Lilly's weight loss injection, and when should you contact a doctor?

The most commonly reported side effects are gastrointestinal, nausea, loose stools, constipation, indigestion, burping, and occasionally vomiting. They tend to peak in the days after a new dose or a dose increase, and most people find they ease within a couple of weeks as the body adapts. Fatigue, headache, and dizziness are also reported, particularly early on.

The NHS medicines page for tirzepatide lists side effects by frequency and is a reliable first reference if you are unsure whether something you are experiencing is expected. Serious effects are less common but need prompt medical attention. Severe, persistent stomach pain that spreads to the back (with or without vomiting) can be a sign of acute pancreatitis and should not be waited out. Signs of an allergic reaction, gallbladder symptoms, or significant dehydration after prolonged vomiting or diarrhoea are also reasons to seek help quickly rather than managing at home.

Any suspected side effect can be reported through the MHRA's Yellow Card scheme. Mounjaro holds Black Triangle (▼) status, which means reports from real-world use actively contribute to ongoing safety monitoring. If you are already on treatment through another provider and thinking about switching, if you have seen cheaper weight loss injections advertised elsewhere and want to understand what to look out for, or if you simply want to talk through your situation with a prescriber, our free consultation is the right place to start. You can also read more about the three weight loss injections currently available in the UK, or find out more about how a weight loss injection works before committing to a course of treatment.