The new weight loss injections: what they are, how they work, and who they're for

Step 1, Understanding which injections are actually licensed in the UK

It helps to be clear about what exists and what doesn't. Two injectable medicines hold a UK licence specifically for weight management in adults: tirzepatide, sold as Mounjaro by Eli Lilly, and semaglutide, sold as Wegovy by Novo Nordisk. You may have seen other names online (Ozempic, Saxenda, Zepbound) and it's worth knowing exactly where each one stands.

Ozempic is also semaglutide, but it is licensed for type 2 diabetes, not weight loss. Prescribing it for weight management would be off-licence, and the MHRA has been clear about this distinction. Zepbound is the US brand name for tirzepatide and does not exist as a UK product. When you see it mentioned on UK sites, they are describing Mounjaro. A fuller overview of all the weight loss injections covers these distinctions in detail.

Both Mounjaro and Wegovy come as pre-filled, once-weekly injection pens. Tirzepatide pens contain four doses each; you inject once a week, typically into the abdomen, thigh, or upper arm, rotating sites. Treatment starts at a low dose to help your body adjust, then the prescriber titrates upward. The NHS's guidance on tirzepatide and on semaglutide set out the full side-effect and storage detail at patient level.

Step 2, How these medicines actually change appetite and weight

The mechanism is where tirzepatide and semaglutide differ most clearly. Semaglutide is a GLP-1 receptor agonist, meaning it mimics a gut hormone released after eating. That signal tells the brain food has arrived, slows how quickly the stomach empties, and reduces hunger. Tirzepatide does all of this and adds a second receptor: GIP, another gut hormone involved in fat metabolism and insulin release. Activating both pathways together appears to produce stronger appetite suppression for many people.

Neither medicine is a stimulant or a fat-burner in the traditional sense. Weight reduces because appetite falls and calorie intake drops as a result. That is why both medicines are intended to be used alongside a reduced-calorie diet and increased physical activity, the injection changes the biology; the lifestyle change builds on that. More on how weight loss injections work in practice explains the day-to-day picture in plainer terms.

Clinical trial results give a sense of scale. In the SURMOUNT-1 trial (2,539 adults with obesity), tirzepatide at the 15mg maintenance dose produced average body-weight reductions of around 20–21% over 72 weeks. In the STEP 1 trial, semaglutide 2.4mg produced around 15% over 68 weeks. A 2025 head-to-head trial, SURMOUNT-5, found tirzepatide produced greater average loss than semaglutide 2.4mg when compared directly. These are averages from tightly controlled studies, and individual results vary, but the scale of effect is genuinely new compared with older weight-loss medicines.

Step 3, Working out whether you're likely to be eligible

Eligibility for these medicines is set by the terms of their UK licences, not by any single clinic's rules. For both Mounjaro and Wegovy, the licensed starting point is a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related health condition, things like high blood pressure, type 2 diabetes, high cholesterol, or obstructive sleep apnoea. Your prescriber will also look at your full medical history, any medicines you already take, and whether there are any contraindications.

There are some groups for whom neither medicine is currently recommended: people who are pregnant, breastfeeding, or actively trying to conceive, and anyone under 18. A personal or family history of medullary thyroid carcinoma or a condition called MEN2 requires careful prescriber discussion before any GLP-1 medicine is considered.

If you want a quick initial sense of where you stand, the NHS BMI calculator takes under a minute. BMI alone doesn't decide the outcome (the prescriber weighs everything together) but it's a useful first check. What weight you need to be for weight loss injections covers the thresholds in more depth, including the lower BMI thresholds that apply for some ethnic backgrounds under UK clinical guidance.

Step 4, Getting started safely through a regulated route

Because these are prescription-only medicines, the only legal way to obtain them in the UK is through a valid prescription following a proper clinical assessment. That assessment exists to protect you: dosing, timing of increases, interaction with existing medicines (the oral contraceptive pill, for example, may be less reliably absorbed during the first weeks of tirzepatide treatment, so a non-oral method is typically advised alongside it during that window), and monitoring for side effects all require a clinician who knows your history.

The side-effect profile for both medicines is led by gastrointestinal symptoms: nausea, loose stools, constipation, reflux. These are most noticeable when starting out or after a dose increase and tend to settle. If you are still deciding between the two options, a closer look at what the 2 weight loss injections are can help clarify which licensed medicine may suit your circumstances, while a guide to what the 3 weight loss injections are covers the broader landscape including Saxenda for anyone who has seen that name mentioned. Rarely, more serious effects can occur, the MHRA advises anyone on a GLP-1 medicine to seek urgent medical help for severe, persistent stomach pain that radiates to the back, as this can indicate pancreatitis.

If you are considering private treatment, checking any online pharmacy on the GPhC pharmacy register before placing an order is the most direct safety step available. At nume, every consultation is reviewed by a GPhC-registered Independent Prescriber (a real clinician, not automated software) the same day it's submitted. Our approach to clinical oversight sets out what that means in practice. For anyone ready to take the next step, understanding what the weight loss injections are and how to begin is a good place to start, and our weight loss treatments page explains what's available and how to begin.