Semaglutide or tirzepatide: which is more effective for weight loss?

The biggest misconception: that one medicine is simply 'better' than the other

Ask the internet which is more effective and you'll usually get a confident answer. The reality is messier. SURMOUNT-5 is the only large randomised trial to test tirzepatide and semaglutide 2.4mg head-to-head, and it did show tirzepatide pulling ahead on average weight reduction at 72 weeks. But a trial average is not a prediction for any individual. A meaningful proportion of people in the semaglutide arm lost substantial weight, more than many people on the highest tirzepatide dose. Biology, tolerability, adherence and how long someone stays on treatment all shape the outcome far more than the brand on the pen.

The mechanism matters here. Tirzepatide (sold in the UK as Mounjaro) activates two receptors, GIP and GLP-1. Semaglutide (Wegovy) targets only GLP-1. That dual action appears to account for the larger average effect. If you want to dig into whether semaglutide is more effective than tirzepatide for certain people, the evidence is more nuanced than headlines suggest. For a closer look at how the two mechanisms compare day to day, our semaglutide or tirzepatide overview covers the practical differences in more detail.

What the trial numbers actually show

The table below summarises the headline figures from the key UK-relevant trials. Read the numbers alongside the study design, population, duration and comparator all affect what the figures mean.

A few things worth noting. SURMOUNT-1 and STEP 1 used placebo controls, so you cannot simply subtract one figure from the other and call it the head-to-head gap, the SURMOUNT-5 trial is the right source for that. The NICE appraisal of tirzepatide (TA1026) also noted that indirect comparisons favour tirzepatide, which is consistent with the direct trial result. And semaglutide 7.2mg, approved by the MHRA in January 2026, narrows the gap considerably, approaching tirzepatide 15mg in trial data, which raises the question of whether Wegovy is as effective as Mounjaro once patients reach the higher dose. For a side-by-side breakdown of how the two UK licensed treatments compare on cost as well as outcomes, the Wegovy vs Mounjaro cost page runs through both.

Who is eligible for each, and does that affect the choice?

Both medicines share broadly similar UK licensed criteria: adults with a BMI of 30 or above, or 27 and above with at least one weight-related condition such as high blood pressure, type 2 diabetes or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. Neither licence lets a prescriber ignore clinical contraindications to favour the 'stronger' option.

On the NHS, NICE's tirzepatide guidance (TA1026) and semaglutide guidance (TA875) set different eligibility thresholds and access routes, Wegovy on the NHS requires referral to a specialist weight management service. Privately, the clinical assessment is the gate. A prescriber who reviews your health history, current medicines and personal circumstances is in a far better position to weigh up which treatment fits than a comparison table is.

One quick check you can do right now: look up any online pharmacy on the GPhC register before handing over a prescription. It takes under a minute and confirms the pharmacy is operating legally. nume's GPhC registration number is 9012878.

If you're already on one medicine and considering switching, transferring between treatments is a clinical decision that needs prescriber input, it is not straightforward and timing matters. Our page on Wegovy to Mounjaro dosing explains what that process looks like in practice.

Side-effect profiles, tolerability and what that means for real-world effectiveness

A medicine that produces 21% weight loss on paper but that a patient stops taking after eight weeks because of persistent nausea is less effective in practice than one producing 15% that the same patient tolerates for a full year. Tolerability is part of effectiveness.

Both tirzepatide and semaglutide share a GI-led side-effect profile: nausea, loose stools, constipation, reflux and fatigue are the most reported, particularly after starting or moving up a dose. For most people these settle within a couple of weeks. The MHRA's January 2026 Drug Safety Update flagged acute pancreatitis as an infrequent but serious risk with GLP-1 medicines, severe stomach pain reaching the back warrants urgent medical attention, not a wait-and-see approach. You can report any suspected side effect at the MHRA Yellow Card scheme.

Direct tolerability comparisons between the two medicines are harder to draw from trial data alone, because trial populations differ and titration schedules vary. Your prescriber will factor in your personal medical history (including any previous experience with GLP-1 medicines) when recommending a starting point.

Which treatment suits you is a clinical decision our prescribers make with you, based on your health, your history and your goals. Start your free consultation and a GPhC-registered prescriber will review your answers the same day.