Tirzepatide vs Semaglutide: Comparing Mounjaro and Wegovy for Weight Loss

Step 1: Understanding what each medicine actually does

Semaglutide works on a single hormone receptor, GLP-1, the gut signal that tells your brain you've eaten enough and slows how quickly food leaves your stomach. Wegovy, the weight-loss brand, delivers 2.4mg weekly at maintenance dose (and more recently up to 7.2mg under a newly approved format). You can read more about how Wegovy works on the Wegovy overview page.

Tirzepatide, sold as Mounjaro, does something different. It activates two receptors simultaneously, GLP-1 and GIP (glucose-dependent insulinotropic polypeptide). GIP is a second gut hormone involved in both appetite regulation and fat storage. Activating both pathways appears to produce a stronger combined effect on appetite than either pathway alone. The NHS medicines page for tirzepatide describes this dual mechanism in plain terms if you'd like the detail.

A misconception worth setting down gently: some people assume semaglutide is the "safer" option simply because it's been on the market longer. Both medicines carry the same class of risks (GI side effects, a pancreatitis caution, the same pregnancy and under-18 exclusions) and both hold UK marketing authorisations assessed by the MHRA. Age of approval is not the same as relative safety profile. What matters is individual medical history, which is why a prescriber reviews the full picture before any medicine is started.

Step 2: What the trial evidence shows

The clearest comparison comes from SURMOUNT-5, a 72-week open-label head-to-head trial published in the New England Journal of Medicine (2025), involving 751 adults with obesity and no diabetes. Tirzepatide at its highest tolerated doses produced greater average weight reduction than semaglutide 2.4mg. NICE's appraisal of tirzepatide (TA1026) also notes that indirect comparisons across the trial programmes favour tirzepatide, while acknowledging the limitations of cross-trial comparisons.

Semaglutide's own STEP 1 trial (68 weeks, 2.4mg dose) showed an average body-weight reduction of around 15%. Tirzepatide's SURMOUNT-1 trial (72 weeks, 15mg dose) showed an average reduction of around 20–21%. The gap narrows when semaglutide is used at the newer 7.2mg dose: trials at that level reported around 20.7% average weight loss over 72 weeks, bringing results much closer to tirzepatide's highest dose. Both sets of figures come from controlled clinical trials; results in real-world practice vary.

The table below pulls the headline figures together.

Step 3: Eligibility, who can access which medicine privately

Both medicines share a similar private eligibility framework drawn from their UK licences. 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, high cholesterol or type 2 diabetes, may be eligible for either. Lower BMI thresholds can apply for some ethnic backgrounds under UK clinical guidance. BMI is the starting point; a prescriber also looks at medical history, current medicines and individual circumstances before confirming suitability.

On the NHS, the picture is different for each medicine. Tirzepatide's NICE recommendation (TA1026) sets a threshold of BMI 35 or above plus at least one weight-related comorbidity for NHS access, with a phased rollout that is currently under way. Wegovy on the NHS (TA875) is restricted to specialist weight management services for a maximum of two years. Waiting lists for both NHS routes can be long. For people who don't meet the current NHS criteria or prefer not to wait, a private route through a regulated online pharmacy is the practical alternative, you can explore what that involves on the treatment options page.

It's also worth knowing that Ozempic (another semaglutide product) is licensed for type 2 diabetes, not weight loss. The semaglutide vs Ozempic page covers that distinction properly if you've seen both names and wondered.

Step 4: Side effects and practical differences

The side-effect profiles of tirzepatide and semaglutide overlap substantially. Nausea, loose stools, constipation, indigestion and fatigue are common to both, most often appearing after starting treatment or after a dose increase, and typically settling within a couple of weeks. Neither medicine is considered comfortable to begin with, and that's worth knowing in advance rather than being a surprise.

A few practical differences are worth raising with a prescriber. For women using oral contraceptives, tirzepatide specifically requires an additional non-oral method (such as condoms) for the first four weeks of treatment and for four weeks after each dose increase, because absorption of the pill may be reduced. NHS guidance doesn't flag the same requirement for semaglutide. If you're on HRT, the advice leans toward transdermal formats (patches or gels) with tirzepatide for the same absorption reason. These are clinical conversations, not things to navigate from a webpage alone.

If you'd like to see how liraglutide fits into the semaglutide family picture, this comparison page covers it. For a closer look at how Mounjaro and Wegovy sit alongside each other under their brand names specifically, the Mounjaro vs Wegovy page goes into more detail. If you're weighing up the active ingredients rather than the brand names and want to understand how the two drugs compare as compounds, this page comparing the Mounjaro drug with Wegovy breaks that down clearly. And if you're thinking about where to start with Wegovy, this page on getting Wegovy in the UK explains what a legitimate route looks like.

The right choice between these two medicines depends on your health history, your goals and what a prescriber judges clinically appropriate for you. Speak to our prescribers, your consultation is free, reviewed the same day by a real clinician, and there's no obligation.