Which injection is better for weight loss: Mounjaro or Wegovy?

The myth: one injection is simply 'stronger' than the other

The most common misconception here is that comparing these two medicines is like comparing the 5mg and 10mg versions of the same product, a straight scale where more potency means better for everyone. It isn't that simple. Mounjaro and Wegovy work through related but distinct pathways. Mounjaro is a dual GIP and GLP-1 receptor agonist, the only weight-loss medicine in the UK to activate both of those appetite-regulating pathways simultaneously. Wegovy acts on GLP-1 alone. That biological difference shapes not just the average trial results but also the side-effect profiles and the clinical situations where one might suit someone better than the other.

The trial data does show a directional difference. In the SURMOUNT-5 study (a 72-week open-label randomised trial involving 751 adults living with obesity and no diabetes) participants taking tirzepatide lost more weight on average than those taking semaglutide 2.4mg. The SURMOUNT-1 trial published in the New England Journal of Medicine had already shown tirzepatide achieving around 20–21% average body-weight reduction at its highest dose over 72 weeks. These are population averages, though. Some people respond exceptionally well to semaglutide and more modestly to tirzepatide, and the prescriber's job is to think about the individual rather than the headline figure.

So the myth to correct is this: 'better' is not a fixed property of a medicine. It is a relationship between that medicine and the person taking it, assessed clinically.

What the evidence actually shows about each injection

Wegovy has a well-established evidence base. The STEP 1 trial (68 weeks, semaglutide 2.4mg) reported around 15% average weight reduction, and the medicine has been available in the UK long enough for prescribers to have considerable real-world experience with it. NICE recommends semaglutide for weight management under technology appraisal TA875, though the NHS criteria are strict and include a maximum two-year treatment period within specialist services. Privately, Wegovy's eligibility mirrors its licence: adults with a BMI of 30 or above, or 27-plus with at least one weight-related condition.

Mounjaro's licence covers the same BMI thresholds, and NICE recommended it for NHS use in December 2024 (TA1026). The higher average weight loss seen in SURMOUNT-5 has led many prescribers to consider it the stronger option when the primary goal is substantial weight reduction and both medicines are clinically suitable. A newer development worth knowing: Wegovy now has a 7.2mg dose approved by the MHRA (the dedicated single-dose pen was approved in April 2026), which narrowed the gap somewhat, around 20.7% average weight loss at 7.2mg in trials. That is still broadly comparable to, though slightly below, tirzepatide's highest-dose results. If you are trying to understand which injection is good for weight loss given these trial differences, our dedicated page walks through the evidence in more detail.

If you are weighing up the options before your consultation, our overview of how these weight-loss injections work covers the mechanics in plain terms.

The side effects are similar, but not identical

Both injections share a broadly gastrointestinal side-effect profile: nausea, diarrhoea, constipation and indigestion are the most commonly reported, particularly when starting or after a dose increase, and for most people they settle within days to a couple of weeks. The NHS medicines pages for both tirzepatide and semaglutide give detailed patient-level information on what to expect.

There are some practical differences. Because Mounjaro can slow the absorption of oral medicines, women taking the contraceptive pill are advised to add a non-oral method of contraception for the first four weeks of treatment and for four weeks after each dose increase, NHS guidance notes no equivalent evidence of this interaction for semaglutide. HRT users on tirzepatide may also be advised to consider transdermal formulations for the same reason. These are conversations to have with a prescriber, not decisions to make independently.

One safety point applies equally to both: the MHRA highlighted acute pancreatitis as a known, infrequent but potentially serious risk across GLP-1 medicines. Severe stomach pain that spreads to the back, with or without vomiting, warrants urgent medical attention. Suspected side effects from either medicine can be reported at the MHRA Yellow Card scheme.

For those considering the practical question of needle use, alcohol swabs are a straightforward accessory for keeping injection sites clean between uses.

So which injection is right for you, specifically?

The honest answer is that this question cannot be resolved by a web page. It can only be resolved in a clinical consultation. Both injections are prescription-only medicines, and the prescriber reviewing your case will consider your BMI, medical history, other medications, and your goals before recommending one over the other, or neither, if neither is suitable.

What you can do before that conversation is get informed. A number of people find it helpful to think through the broader question of which weight-loss injection might suit their situation before speaking to a clinician. Others want to understand how these options compare to the newer oral alternative, there is a thorough comparison of injections versus pills for weight loss if that question is also on your mind.

The question of cost sometimes comes up alongside effectiveness. Private treatment pricing varies by provider and dose; if you are also asking which is the best weight loss injection from a value-for-money perspective, our guide to costs sets out what legitimate pricing typically includes and what to look for before you commit anywhere. Our guide to weight-loss injection costs in the UK sets out what to look for and what legitimate pricing typically includes.

At nume, every consultation is personally reviewed the same day by a GPhC-registered Independent Prescriber, a real clinician reading your answers before anything is dispensed. Orders placed before 12pm on a weekday, once clinically approved, are dispatched the same day and arrive free the next working day in plain packaging. If you are ready to get a clinical view on which injection suits your situation, start your free consultation with our prescribers.