Comparing Weight Loss Injections: Mounjaro and Wegovy Side by Side

You've narrowed it down to two injections. Here's where they actually differ.

Say you've spent a weekend reading about weight loss injections and arrived at the same two names everyone arrives at: Mounjaro and Wegovy. Both are weekly injections, both come as pre-filled pens you use yourself at home, and both work by slowing how quickly your stomach empties and dialling down appetite signals in the brain. So far, so similar.

The real difference is in the receptor biology. Wegovy contains semaglutide, a GLP-1 receptor agonist, it mimics one gut hormone involved in appetite and blood-sugar regulation. Mounjaro contains tirzepatide, which is a dual GIP and GLP-1 receptor agonist, meaning it activates two pathways simultaneously. It is the only dual-agonist weight-loss medicine currently licensed in the UK. Whether that translates into meaningfully different outcomes for any given person is the question the trials were designed to answer, and now largely have. You can read a detailed breakdown of how each medicine works on the weight loss injection overview page.

Both start at a low tolerability dose (the first pen exists to let your system adjust, not to produce dramatic results immediately) and the prescriber titrates the dose upward over several months. Titration timing and the right dose for you are decisions made with your clinical team, not something to self-manage.

What the clinical trial evidence shows when you put the numbers side by side

For years the comparison rested on two separate trials run against placebo: SURMOUNT-1 for tirzepatide (around 20–21% average body-weight reduction at the 15 mg dose over 72 weeks) and STEP 1 for semaglutide 2.4 mg (around 15% average over 68 weeks). Both figures are from the SURMOUNT-1 paper in the New England Journal of Medicine and the equivalent STEP 1 publication. Comparing across separate trials is imperfect (different populations, different run-in periods) but the gap was consistent enough that NICE's committee noted indirect comparisons favour tirzepatide when it published TA1026.

Then SURMOUNT-5 settled things more directly. In that open-label head-to-head trial, 751 adults with obesity and no diabetes were randomised to tirzepatide or semaglutide 2.4 mg for 72 weeks. Tirzepatide produced greater average weight reduction. That result is now cited in NICE's appraisal of tirzepatide (TA1026) and informs how clinicians weigh the options.

One caveat worth stating plainly: a newer, higher-dose semaglutide pen (7.2 mg, approved by the MHRA in April 2026) narrows that gap. Average weight loss at 7.2 mg reached around 20.7% in trials. So the picture is still moving. If you want to see how these medicines stack up across every variable that matters, our page covering weight loss injections compared walks through the key differences in one place. If you want a closer look at how costs track alongside these results, the weight loss injections prices page puts the numbers in context.

Eligibility, side effects and the factors that tip the clinical decision

Both medicines share the same licensed eligibility criteria in broad terms: adults with a BMI of 30 or above, or 27 or above where at least one weight-related condition (such as high blood pressure, type 2 diabetes, high cholesterol or obstructive sleep apnoea) is present. Lower BMI thresholds apply for some ethnic backgrounds under UK clinical guidance. Neither prescription is automatic, the prescriber weighs your full health picture, your current medicines and your history before deciding which, if either, is appropriate.

Side-effect profiles are similar too. Both produce a GI-led pattern: nausea, loose stools, constipation, reflux and indigestion are the most commonly reported effects, typically peaking around a dose increase and settling over days to a couple of weeks. There are differences in the specific contraindications and interactions, tirzepatide affects absorption of oral medicines including the contraceptive pill during the first four weeks of treatment and after each dose increase, which affects how some women need to manage contraception. That is the sort of detail a prescriber covers before issuing a prescription, not something to navigate alone.

One practical consideration: oral contraceptive users starting Mounjaro should add a non-oral method for the first four weeks and for four weeks after each dose increase, as NHS guidance confirms. No equivalent interaction evidence exists for semaglutide. If you want a clearer picture of which companies provide these injections privately and how they handle clinical oversight, the guide to finding a reputable provider covers the key questions to ask.

What this comparison doesn't settle, and why a prescriber has the last word

A side-by-side table can show you trial averages and mechanisms, and if you find that format useful our weight loss injection comparison resource goes into further detail on the specific figures. It cannot tell you which medicine your system will tolerate better, whether a past medical history rules one out, or whether the dose trajectory that suits you matches what the higher-averaging option requires. Some people do better on semaglutide. Some cannot tolerate tirzepatide's GI effects at higher doses. Some find the reverse. Trial averages are population numbers; your result is one data point.

Payday orders, Monday dispatches, pre-holiday stock checks, whatever the timing, the clinical assessment has to come first regardless of when you want to start. A prescription for a weight loss injection cannot be issued without it, legally or safely. That is not a bureaucratic inconvenience; it is the step that makes everything that follows appropriate for you specifically.

For a fuller exploration of what private treatment involves and what to look for in a provider, the weight loss injections guide covers the process end to end. Our clinical team's approach to prescribing is outlined on the clinical team page if you want to understand who reviews consultations. When you're ready to find out which option fits your situation, check your eligibility with a free consultation, a GPhC-registered prescriber, not software, reads every submission the same day.