Slimming weight loss injections: what you actually need to know

Which injections are actually licensed for slimming and weight loss in the UK?

Two medicines hold a UK licence for weight management by injection. Tirzepatide, sold as Mounjaro by Eli Lilly, is a dual GIP and GLP-1 receptor agonist, the only medicine of its kind licensed here. Semaglutide, sold as Wegovy by Novo Nordisk, is a GLP-1 receptor agonist. Both are given as once-weekly injections into the abdomen, thigh or upper arm using a pre-filled pen.

You may have heard Ozempic mentioned in the same breath. It is also semaglutide, but its licence covers type 2 diabetes, not weight loss, these are not interchangeable, and a prescriber cannot lawfully issue it for slimming. Saxenda (liraglutide) holds a weight-management licence but is not dispensed by nume (sorry, by us at nu) by our pharmacy. The two medicines we prescribe are Mounjaro and Wegovy.

Wondering about the difference between the two? The full comparison of UK-licensed weight loss injections covers the trial data, mechanism differences and practical considerations side by side. The short version: tirzepatide's dual mechanism produced greater average weight loss in the SURMOUNT-5 head-to-head trial published in the New England Journal of Medicine in 2025, but which suits your health picture is a clinical question, not a marketing one.

Who do these injections qualify for, and what does 'eligible' actually mean in practice?

The licensed criteria, drawn from the medicines' UK SmPCs, are: adults with a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related condition such as type 2 diabetes, hypertension, high cholesterol or obstructive sleep apnoea. For some ethnic backgrounds, UK guidance sets the thresholds 2.5 kg/m² lower, so 27.5 instead of 30, for example. The weight management treatment overview has the full eligibility detail.

Meeting a BMI threshold on paper does not automatically mean treatment is right for you. A prescriber also considers your full medical history, any current medicines that could interact, and conditions that make these injections unsuitable, including a personal or family history of certain thyroid tumours, current or planned pregnancy, and a history of pancreatitis. Pregnancy, breastfeeding and trying to conceive are reasons these medicines are not recommended; the same applies for under-18s, for whom they are not licensed.

On the NHS, eligibility is considerably stricter and phased, NHS access to weight loss injections is currently limited to specific BMI and comorbidity thresholds, with waiting lists in many areas. Private treatment through a regulated pharmacy is the practical route for many people who either do not meet NHS criteria or would rather not wait.

What side effects do slimming injections typically cause?

The side-effect profile of both medicines is dominated by the gastrointestinal system. Nausea is the most commonly reported; constipation, diarrhoea, indigestion, reflux and bloating also appear regularly in trial data and in clinical practice. These effects are usually at their most noticeable when treatment starts or when the dose increases, and tend to settle within a couple of weeks as the body adjusts. The practical guide to taking slimming injections covers how to manage them day to day.

Less common but more serious: the MHRA has highlighted acute pancreatitis as a known side effect of GLP-1 medicines that warrants urgent attention. Severe, persistent stomach pain (especially pain that radiates toward the back) is a reason to seek medical help promptly, not to wait and see. Gallbladder problems, significant dehydration from prolonged vomiting or diarrhoea, and signs of an allergic reaction are also reasons to contact a doctor or call 111 without delay. You can report any suspected side effect directly via the MHRA Yellow Card scheme.

If you are taking an oral contraceptive pill alongside tirzepatide specifically, current NHS guidance recommends adding a barrier method for the first four weeks of treatment and for four weeks after each dose increase, because slowed gastric emptying may reduce pill absorption. That is a conversation worth having with your prescriber before your first injection.

How does the injection process actually work, and what arrives in the post?

Both Mounjaro and Wegovy use a pre-filled pen. Treatment starts at the lowest dose (2.5mg for tirzepatide; 0.25mg for semaglutide) and is increased in steps set by the prescriber, typically every four weeks, with the aim of reaching the dose where your weight management is effective and the side effects are manageable. The clinical picture around stopping or stepping down is also worth reading before you start, so the full arc of treatment makes sense from the outset.

At nume, at nume, when you order by midday on a weekday and your consultation has been approved by our GPhC-registered Independent Prescriber, your treatment is dispatched the same day. It arrives the next working day via DPD, in plain unbranded packaging, with tracking sent to your phone. Inside: your pen, the patient information leaflet, and everything you need to begin. Pens are stored in the fridge (2–8°C); your prescriber and the information leaflet will tell you exactly what applies to your specific medicine and dose.

Curious about the cost side of things? The guide to how much weight loss injections cost in the UK lays out what private pricing typically looks like and what a legitimate price should include. For those considering the process more broadly, the guidance on coming off weight loss injections is useful context too, understanding the full treatment journey before you start tends to produce better outcomes than discovering it midway through. If you'd like to check whether treatment is appropriate for you, start a free consultation with our clinical team and a prescriber will review your answers the same day.