Which weight loss injection works best right now?

Step 1: understand what each injection actually does

Wegovy contains semaglutide, a GLP-1 receptor agonist made by Novo Nordisk. It mimics the gut hormone GLP-1, slowing the passage of food from the stomach, reducing appetite, and improving the body's insulin response. Treatment begins at 0.25mg weekly and titrates up to 2.4mg — or now to 7.2mg, following the MHRA's approval of the higher-dose pen in April 2026.

Mounjaro contains tirzepatide, made by Eli Lilly. It does the same job via GLP-1 but also activates a second receptor, GIP, making it the only dual-agonist weight management medicine licensed in the UK. The addition of GIP signalling appears to amplify appetite suppression. Treatment opens at 2.5mg, a tolerability dose designed to settle your system before gradual increases toward 15mg over several months. The way the prescriber moves through that schedule matters as much as which pen you hold.

If you want a closer look at the mechanics, the guide to how weight loss injections work covers the receptor biology in plain terms, alongside the NHS patient information page for tirzepatide.

Step 2: look at what the trials actually found

A question our prescribers hear most weeks is something like: "I've seen a big number for Mounjaro (is it really that much better?" The honest answer is: on average, yes, meaningfully so) but averages describe populations, not individuals.

SURMOUNT-1, published in the New England Journal of Medicine, found average body-weight reductions of around 20–21% at the 15mg dose of tirzepatide over 72 weeks. The STEP 1 trial for semaglutide 2.4mg showed roughly 15% average reduction over 68 weeks. Then SURMOUNT-5, the 72-week head-to-head trial published in the NEJM in 2025, put both medicines in direct competition in 751 adults with obesity and no diabetes. Tirzepatide came out ahead. NICE's appraisal of tirzepatide (TA1026) acknowledges the indirect and now direct comparisons favouring it.

The newer Wegovy 7.2mg data narrows the gap (around 20.7% average weight loss at that dose over 72 weeks) so the picture continues to shift. Neither figure tells you what will happen to you specifically. You can read an overview of which weight loss injection works fastest if speed of early response matters to your decision.

Step 3: factor in the things the trials don't tell you

Raw trial figures are only part of the clinical picture. Several practical factors often swing the prescriber's recommendation one way or the other.

Existing conditions. Both injections are licensed for weight management and type 2 diabetes, but if you have cardiovascular disease, your prescriber will consider the broader evidence base for each. Tirzepatide holds a Black Triangle (▼) status with the MHRA, meaning additional monitoring applies as post-marketing data accumulates, not a reason to avoid it, but a fact worth knowing.

Medicines you already take. Because tirzepatide slows gastric emptying, oral contraceptive pill absorption may be reduced during the first four weeks of treatment and for four weeks after each dose increase. Adding a barrier method during those windows is the standard clinical advice. Semaglutide doesn't carry the same documented effect on oral contraception. If you're on several oral medicines, that conversation with your prescriber matters.

Tolerance. Both injections carry a gastrointestinal side-effect profile (nausea, loose stools, constipation, reflux) most pronounced at the start or after a dose step. Some people find one easier to tolerate than the other; prescribers can't predict this in advance, but they can adjust the titration pace. The guide on picking the injection that suits your situation goes into this in more detail.

Cost is a real consideration too. Private treatment prices differ by medicine and dose, and there's useful context on what injection costs actually cover, including what's often left out of headline prices.

Step 4: how a clinical assessment turns this into a recommendation

Choosing between Mounjaro and Wegovy isn't a consumer decision made by comparing packaging. It's a clinical one. A GPhC-registered prescriber looks at your BMI, any weight-related conditions, your current medicines, your medical history and your preferences, then recommends accordingly, sometimes one medicine clearly fits; sometimes either would be suitable and the prescriber explains the trade-offs.

NHS access to both medicines is available but restricted and phased. The weight loss injection overview covers current NHS thresholds if you want to understand that route. For private treatment through a regulated pharmacy, the process is faster: a consultation, a same-day clinical review, and a decision that day. The companion page on which injection tends to perform best looks at this from a slightly different angle if you'd like a second read before deciding.

When you're ready to find out which one your prescriber would recommend for you, check your eligibility with our team.