Which weight loss injection is best for me: Mounjaro or Wegovy?

What the head-to-head data actually shows

The clearest evidence we have comes from the SURMOUNT programme, published in the New England Journal of Medicine. SURMOUNT-1 ran for 72 weeks in adults with obesity and found average body-weight reductions of around 20–21% at the highest tirzepatide dose, figures that shifted the goalposts for what weight-loss medicines could achieve. SURMOUNT-5 then put tirzepatide directly against semaglutide 2.4 mg in an open-label head-to-head trial involving 751 adults with obesity and no diabetes, again over 72 weeks. Tirzepatide produced greater average weight loss. That is not a marketing claim; it is the trial result.

Wegovy's own evidence base is substantial. The STEP 1 trial, also 68 weeks, showed an average reduction of around 15% at the 2.4 mg maintenance dose, which was itself a significant advance over earlier medicines. A higher 7.2 mg Wegovy dose has since received MHRA approval, with trial data reporting approximately 20.7% average loss over 72 weeks, narrowing the gap with tirzepatide considerably. Worth knowing: the 7.2 mg dose is approved for adults with a BMI of 30 or above, and availability of specific dose formats should be confirmed at consultation rather than assumed.

So the honest summary is this: on population averages, tirzepatide currently edges ahead. On individual results, the picture is more varied. Both medicines have patients who respond exceptionally well, and both have patients who find tolerability harder than others. A number on a trial graph tells you what happened across a group. Your prescriber's job is to think about you.

How the two mechanisms translate into real differences

Mounjaro activates both the GIP receptor and the GLP-1 receptor. Wegovy activates GLP-1 alone. Both slow gastric emptying and reduce appetite, which is why the side-effect profiles are similar. If you are still building a picture of what injection is used for weight loss and how these mechanisms differ in practice, that is a good place to start before your consultation. For most people, the practical difference on a day-to-day basis is modest: both involve a weekly injection, both require titration starting at a low dose, and both work best alongside genuine changes to eating habits and activity.

One difference that does matter practically is the oral alternative. If injections are a concern, Wegovy tablets (oral semaglutide approved by the MHRA in June 2026) are now an option for eligible adults. That is a separate clinical conversation, but it is worth raising.

For people on oral contraceptives, there is a specific point to flag. MHRA guidance advises that women taking the pill should add a non-oral method of contraception for the first four weeks of tirzepatide treatment and for four weeks after each dose increase, because gastric-emptying changes can affect absorption. The same recommendation does not currently apply to semaglutide. If that distinction is relevant to you, bring it to your prescriber. You can check this detail against NHS England's guidance on weight-management injections, which covers contraception and HRT alongside both medicines.

Who each injection is licensed for, and where your BMI fits

The licensed eligibility for both medicines overlaps significantly. Under UK licensing, either may be prescribed for adults with a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related condition such as high blood pressure, type 2 diabetes, dyslipidaemia or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance, a detail worth discussing if it applies to you.

NICE's appraisal of tirzepatide (TA1026, published in December 2024) recommends it for NHS use in adults with a BMI of 35 or above plus at least one weight-related comorbidity, with phased rollout criteria tied to the number of conditions present. Private eligibility, assessed by a prescriber against the licensed criteria, is broader. If you are not sure where your BMI sits, the NHS BMI calculator takes about forty seconds to use and gives you a number to bring to your consultation.

A prescriber also considers your medical history, any medicines you are already taking, previous weight-loss attempts and how you feel about injections. Understanding the weight thresholds for these medicines is a useful starting point, but it is one input among several. BMI alone does not decide the outcome of a clinical assessment.

Making a practical decision about which to try first

If the trial data points one way and your prescriber agrees you are eligible for both, the decision often comes down to tolerability history, any relevant interactions (contraception being the main one noted above), and occasionally supply considerations. Neither medicine should be chosen on price alone. A quick, useful habit: before any online pharmacy consultation, check the pharmacy's registration on the GPhC online register, the search takes under a minute and confirms the pharmacy is authorised to dispense prescription medicines. Any seller offering these medicines without a prescription, or advertising discount codes, is a red flag worth acting on.

For a fuller comparison of how these injections differ in practice, our guide to what the evidence says about which injection works best goes deeper into the results data. If you are at an earlier stage and want to think through which weight loss injection is best for you given your circumstances, that page walks through the key questions to consider before booking a consultation. And if you are weighing up the logistics of self-injecting at home, our page on home injection technique and storage covers what to expect day to day.

At nume, every consultation is read personally by a GPhC-registered Independent Prescriber on the day it arrives, a real clinician applies clinical judgment to your answers, not an automated filter. If you have questions before starting, our FAQs address the most common ones. When you are ready to talk through which injection fits your situation, speak to our prescribers through a free consultation.