The different kinds of weight loss injections available in the UK

How the two licensed injections differ in the way they act

Semaglutide, sold as Wegovy, is a GLP-1 receptor agonist. It mimics a gut hormone called GLP-1 that is released after eating, slowing how quickly the stomach empties and signalling to the brain that the body is full. One injection is given once a week under the skin, starting at 0.25mg and working up, under prescriber guidance, to a 2.4mg maintenance dose. A higher 7.2mg dose was approved by the MHRA in early 2026 for adults with a BMI of 30 or above.

Tirzepatide, sold as Mounjaro, takes a different approach. It activates both the GLP-1 receptor and a second receptor (GIP) that is also involved in appetite regulation and blood-sugar balance. This dual action is the reason it is sometimes described as a new class rather than simply another GLP-1 medicine. Treatment starts at 2.5mg weekly, progressing through six strengths up to a 15mg maintenance dose. The difference between these weight loss injections is therefore not just branding; it reflects genuinely different pharmacology.

Both are once-weekly subcutaneous injections using a pre-filled pen. Both are Black Triangle (▼) medicines, meaning the MHRA collects additional safety data while they are in use. Each pen contains four weekly doses, refrigerated until you open the pack. If you are still weighing up your options, our guide to the different types of weight loss injections explains how each one works and where it sits within the current treatment landscape. A question our prescribers hear fairly often is what the pen itself looks like when it arrives: it comes in plain, unbranded packaging via a tracked DPD delivery, and the pen is a small, discreet auto-injector, far less clinical-looking than most people expect.

What the trial results show for each injection

The published evidence is the most useful place to start when comparing outcomes. In SURMOUNT-1 (a 72-week randomised trial involving 2,539 adults with obesity and no diabetes) tirzepatide 15mg produced an average body-weight reduction of around 20–21% compared with placebo. That trial was published in the New England Journal of Medicine and formed part of the evidence base NICE used when recommending tirzepatide in December 2024.

For semaglutide 2.4mg, the STEP 1 trial (68 weeks, published in the same journal) showed an average weight reduction of around 15% against placebo. The more recently approved 7.2mg dose reported approximately 20.7% average loss over 72 weeks in its own trial, which brings it closer to tirzepatide's results.

A head-to-head trial, SURMOUNT-5, published in the New England Journal of Medicine in 2025, compared the two medicines directly in adults with obesity and no diabetes over 72 weeks. Tirzepatide produced greater average weight loss than semaglutide 2.4mg. NICE's appraisal committee, in its review of tirzepatide (TA1026), noted that indirect comparisons also favoured tirzepatide, while acknowledging that head-to-head evidence was limited at the time.

These averages matter, but they are population figures. Individual results vary considerably depending on starting weight, adherence, diet and activity, and biological response to each medicine.

Who each injection is licensed for in the UK

The licensed eligibility criteria for both injections are broadly similar. Tirzepatide is licensed for adults with a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related condition, examples include type 2 diabetes, high blood pressure, high cholesterol, or obstructive sleep apnoea. Semaglutide (Wegovy) carries the same threshold structure.

Private eligibility follows the licence. A prescriber looks at the whole picture: BMI, medical history, current medicines, and anything that might make one option a better fit than the other. BMI thresholds can be 2.5 kg/m² lower for some ethnic backgrounds under UK guidance. For a detailed look at the current weight loss injections available and how eligibility works in practice, our treatment overview covers this clearly.

On the NHS, access is more restricted. Tirzepatide became available through GP practices from April 2026 under a new contract, but only for patients meeting stricter phased criteria, currently a BMI of 40 or above plus four or more qualifying conditions. Wegovy is available via specialist weight management services under NICE TA875, though waiting lists are commonly long. For people who do not meet NHS criteria or prefer not to wait, a private prescription through a regulated pharmacy is the alternative. You can explore how weight loss injections work in the private setting and what the process involves.

Side effects and practical considerations across both injections

The side-effect profiles for both medicines are broadly similar, dominated by gastrointestinal effects: nausea, loose stools, constipation, reflux and indigestion. These are most noticeable in the early weeks of treatment or after a dose increase, and they typically settle as the body adjusts. They are also the main reason the titration schedules begin at low doses, the starting dose is designed to let the digestive system adapt before the therapeutic dose is reached.

In January 2026 the MHRA issued a Drug Safety Update specifically covering GLP-1 medicines and acute pancreatitis, noting it as an infrequent but potentially serious risk. The advice is to seek urgent medical help for severe, persistent stomach pain that spreads to the back, with or without vomiting. Any suspected side effects can be reported through the MHRA's Yellow Card scheme.

Both medicines are not recommended during pregnancy, while breastfeeding, or when actively trying to conceive. Neither is licensed for anyone under 18. Women taking oral contraceptives who start tirzepatide should use an additional non-oral method of contraception for the first four weeks of treatment and for four weeks after each dose increase, because absorption of the pill may be reduced, a detail the NHS highlights specifically for tirzepatide and not for semaglutide. For a fuller look at what a weight loss injection involves day to day, including storage and injection technique, that page covers the practicalities.

Choosing between the two injections is not a decision to make from a comparison article. It is a clinical one, reached through a proper conversation about your health, your history, and what matters most to you. If you are ready to have that conversation, starting a free consultation with a GPhC-registered prescriber at nume is a straightforward next step.