The two injections licensed for weight loss in the UK

Step 1, understand what each injection is and how it works in the body

Mounjaro contains tirzepatide, a molecule made by Eli Lilly that binds to both the GIP and GLP-1 receptors in the gut and brain. Activating these two pathways together slows the rate at which your stomach empties, signals the brain to reduce hunger, and helps regulate blood sugar. It arrives as a pre-filled KwikPen; you inject once a week into the abdomen, thigh, or upper arm, rotating sites each time.

Wegovy contains semaglutide, made by Novo Nordisk, and targets the GLP-1 receptor alone. The effect on appetite and gastric emptying is similar in character, though mediated through a single pathway rather than two. It also comes as a pre-filled weekly pen. A newer oral version of semaglutide for weight loss (Wegovy tablets) was approved by the MHRA in June 2026, though the injections remain the longer-established route.

Both are weight-loss injections classified as GLP-1 receptor agonists; Mounjaro is additionally a GIP agonist. That dual action is the core pharmacological distinction, and if you want a plain-language overview of what the weight loss injections are and how they work, that page covers the mechanism in accessible terms. The NHS medicines page for tirzepatide and the equivalent page for semaglutide explain the mechanism in patient-friendly language if you want to read further.

Step 2, see who each injection is licensed for and how the dose builds

The licensed eligibility for both overlaps considerably. Adults with a BMI of 30 or above qualify, as do adults with a BMI of 27 or above who have at least one weight-related condition such as high blood pressure, type 2 diabetes, high cholesterol, or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance.

The dose structure differs. Mounjaro starts at 2.5 mg (a strength chosen to let your body adjust, not to produce weight loss from week one) then steps up through 5, 7.5, 10, 12.5, and 15 mg, typically in four-week intervals guided by your prescriber. Wegovy begins at 0.25 mg and titrates through 0.5, 1.0, 1.7, and 2.4 mg over around 16 weeks; a 7.2 mg dose was approved by the MHRA in early 2026 for adults with a BMI of 30 or above. Your prescriber decides the pace, not a protocol you follow alone.

A quick thing worth doing before you go further: search both medicine names on the GPhC pharmacy register to confirm any pharmacy you're considering is registered and authorised to dispense them. It takes under a minute and is one of the clearest safety checks available. You can also read more about the weight thresholds that apply in the UK.

Step 3, look at the clinical evidence side by side

The results from the pivotal trials are the most direct way to compare the two injections. In the SURMOUNT-1 trial, tirzepatide at 15 mg produced an average body-weight reduction of around 20–21% over 72 weeks in adults with obesity but without type 2 diabetes, among thousands of participants. The STEP 1 trial for semaglutide 2.4 mg reported an average reduction of around 15% over 68 weeks. These are population averages from controlled settings, individual results vary depending on diet, activity, dose reached, and how long treatment continues.

A head-to-head comparison followed. The SURMOUNT-5 trial, published in the New England Journal of Medicine in 2025, ran both medicines against each other in adults with obesity and without diabetes over 72 weeks. Tirzepatide produced greater average weight reduction than semaglutide 2.4 mg. The gap narrowed with the newer 7.2 mg semaglutide dose, though direct comparative data at that higher dose is still accumulating. If you want a clearer breakdown of the specific differences between the two options, the page covering what the 2 weight loss injections are sets them out side by side. For a fuller look at how the two medicines stack up across several measures, the guide comparing the injectable options goes into more detail.

NICE recommends tirzepatide under TA1026 and semaglutide under TA875 on the NHS, both with specific eligibility criteria. Private treatment through a regulated pharmacy follows the licensed criteria in the SmPC rather than the NHS commissioning thresholds, which are stricter. Cost is a real consideration too; you can explore what weight-loss injections typically cost privately in the UK before making any decisions.

Step 4, know what to expect in terms of side effects and monitoring

The side-effect profiles of the two injections are broadly similar, because both slow gastric emptying and act on gut receptors. Nausea is the most commonly reported effect, particularly after starting or after a dose increase. Vomiting, diarrhoea, constipation, indigestion, and fatigue also appear in the trial data. These effects tend to be most noticeable in the first one to two weeks at a new dose, then settle for many people.

Serious but less common effects include gallbladder problems and, importantly, acute pancreatitis. The MHRA issued a Drug Safety Update in January 2026 highlighting pancreatitis as a known risk with GLP-1 medicines. Severe stomach pain that spreads to the back, with or without vomiting, warrants urgent medical attention. Both medicines carry a black triangle (▼) symbol, indicating that they are subject to additional monitoring by the MHRA, patients are encouraged to report any suspected side effects via the Yellow Card scheme.

Neither injection is recommended during pregnancy, while breastfeeding, or for anyone trying to conceive. Women taking oral contraceptives starting Mounjaro should add a non-oral method of contraception for the first four weeks of treatment and for four weeks after each dose increase, as absorption of the pill may be affected; current evidence does not show the same effect with semaglutide. Both are not licensed for use in under-18s. A prescriber reviews your full medication list and medical history at consultation, that review is part of why the prescription process exists, not a formality to work around.

If you would like to speak to a prescriber about which of the two injections might suit your circumstances, you can start your free consultation with the nume clinical team. Every consultation at nume is read the same day by a GPhC-registered Independent Prescriber, not reviewed by automated software.