Wegovy vs Mounjaro: The Differences, and What Switching Involves

Wegovy and Mounjaro are both once-weekly injectable prescription medicines licensed in the UK for weight management in adults who meet specified criteria, alongside a reduced-calorie diet and increased physical activity. For anyone searching “Wegovy vs Mounjaro”, the key difference is that Wegovy contains semaglutide and works through the GLP-1 receptor, while Mounjaro contains tirzepatide and works through both GLP-1 and GIP receptors; they are related but not the same medicine, they are not interchangeable, and trial data suggest Mounjaro generally produces greater average weight loss, with potentially higher risks for some people.

That is why the comparison matters for adults in the UK considering prescription treatment for weight management, people thinking about switching, and the clinicians assessing them. A head-to-head trial now gives a more direct comparison than is usual in this area, but it still does not answer the whole prescribing question, because an average result across hundreds of people does not determine what suits one person or what is safest and most tolerable in practice.

This page explains how Wegovy and Mounjaro differ in composition and action, what the clinical evidence shows on effectiveness and side effects, what switching between them involves, how dose and tolerability affect decisions, and which safety precautions need to be considered before treatment or a change in treatment.

Key things to know

• Wegovy contains semaglutide, which acts on the GLP-1 receptor. Mounjaro contains tirzepatide, which acts on both the GLP-1 and GIP receptors.
• In the SURMOUNT-5 head-to-head trial, average weight loss at 72 weeks was 20.2% with tirzepatide and 13.7% with semaglutide.
• That trial was open-label, meaning participants knew which treatment they were taking.
• A meta-analysis of head-to-head studies found greater weight reduction with tirzepatide but also a higher risk of serious adverse events.
• Side effects are broadly similar in type, mostly digestive, for both.
• Switching is a clinical decision. Your dose does not carry across, and you would not start where you left off.
• Results vary between individuals, and trial averages cannot predict your own outcome.

What is the difference between Wegovy and Mounjaro?

The main differences are in the active ingredient and which receptors each medicine acts on.

Wegovy contains semaglutide, a GLP-1 receptor agonist. It imitates GLP-1, a hormone released by the gut after eating, which reduces appetite, slows stomach emptying, helps the body release insulin, and helps regulate blood sugar when blood glucose rises.

Mounjaro contains tirzepatide, which acts on two receptors rather than one: GLP-1 and glucose-dependent insulinotropic polypeptide. This dual hormone mechanism means it targets both pathways involved in appetite, insulin response, and energy balance.

These are two medications used as a weight loss treatment, but they have different active substance profiles.

The reasoning behind dual action is that engaging two arms of the same system may produce a larger effect than engaging one.The effects people may notice include reduced appetite and feeling fuller sooner, although individual experiences vary.

Wegovy was the first medication approved for weight loss in the UK.

What the head-to-head trial found

Most comparisons between weight management medicines are indirect, drawing on separate trials with different participants and designs. SURMOUNT-5 is different, because it compares the two directly.

SURMOUNT-5

In clinical trials comparing Mounjaro vs Wegovy, this phase 3b, open-label, randomised trial included 751 adults with obesity, or with overweight and at least one weight-related complication, who did not have type 2 diabetes. Eligibility was based on body mass index thresholds and related health risks, and the study population could include obesity-related conditions such as high blood pressure or high cholesterol. Participants were assigned to maximum tolerated doses of either tirzepatide or semaglutide, given weekly, alongside a reduced-calorie diet and increased physical activity, for 72 weeks.

At 72 weeks, average body weight reduction from participants’ starting weight was 20.2% with tirzepatide and 13.7% with semaglutide. In absolute terms that was 22.8 kg against 15.0 kg. Average waist circumference reduction was 18.4 cm against 13.0 cm.

More participants on tirzepatide reached the higher weight loss thresholds, although people lose weight at different rates and not everyone needs the highest dose to achieve meaningful weight loss. Around 31.6% achieved a reduction of at least 25% of body weight, against 16.1% on semaglutide.

Results also differed by sex in both groups. Women lost more than men on both treatments, with average reductions of 23.8% and 18.0% in women against 17.8% and 11.0% in men.

Overall, clinical studies and this direct trial found Mounjaro showed greater weight loss than Wegovy. The most commonly reported side effects with both treatments were mild to moderate digestive effects.

What the trial does and does not settle

It does establish that, at maximum tolerated doses over 72 weeks, tirzepatide produced greater average weight loss than semaglutide in this population. That is a direct comparison rather than an inference, which makes it stronger evidence than most claims in this area.

It was open-label. Participants and investigators knew which treatment was being given. That is a recognised limitation, particularly where expectation can influence behaviour and reported outcomes.

It compared maximum tolerated doses, not every dose either medicine is used at. Someone who settles at a lower dose because of tolerability is not in the same situation as the trial population, and not everyone needs the highest dose to achieve meaningful results.

It excluded people with type 2 diabetes, so it does not describe that group.

Averages conceal wide variation. Within both arms, some participants lost a great deal of weight and some lost very little. Results vary between individuals, and no trial average can predict an individual outcome.

Safety needs weighing too. A systematic review and meta-analysis of head-to-head studies found greater weight reduction with tirzepatide, and also a higher risk of serious adverse events, including serious side effects. Effectiveness is one half of a decision.

Is one better than the other?

The key differences matter, but we do not rank medicines, and in this case it would also be an oversimplification of the evidence.

What can be said accurately is that in direct comparison at maximum tolerated doses, tirzepatide produced greater average weight loss. What that does not tell you is which is appropriate for you, because that depends on:

• Your medical history and other conditions
• other medications, including diabetes medications where blood sugar safety may need closer review
• How you have tolerated treatment previously
• Whether you can reach and stay at a higher dose
• Pregnancy plans
• What has already been tried and what happened
• Supply and continuity, since interrupted treatment produces worse results than steady treatment

The right treatment should support your weight loss goals rather than follow a trial average alone.

A medicine with a higher average result that you cannot tolerate is not the better option for you. Wegovy and Mounjaro are effective treatments only when matched to the individual through clinical assessment, and your prescriber will determine the appropriate treatment.

Are the side effects different?

Broadly similar in type. Both most commonly cause digestive effects: nausea, vomiting, diarrhoea and constipation, along with headache and tiredness. In trials of both weight loss drugs, around 77% to 79% of users reported side effects.Gastrointestinal side effects are particularly common during dose escalation and may lessen as the body adjusts to treatment. Discontinuation rates were still relatively low, with 6% of Mounjaro users and 8% of Wegovy users stopping due to side effects.

This is unsurprising, because both slow stomach emptying as part of how they work. The side effects come from the same mechanism as the benefit.

Serious side effects also apply to both. The MHRA has strengthened its warnings about pancreatitis across this class of medicine, following rare reports of necrotising and fatal pancreatitis, and advises seeking urgent medical attention for severe and persistent abdominal pain that may spread through to the back, with or without nausea and vomiting. If pancreatitis is suspected, the MHRA advises treatment stops immediately and is not restarted if confirmed.

The meta-analysis point above is worth repeating here: greater average weight reduction with tirzepatide came alongside a higher risk of serious adverse events in the pooled head-to-head data.

Your patient information leaflet lists every reported effect for your specific medicine, with how often it occurs.

Switching from Wegovy to Mounjaro

Switching between Wegovy and Mounjaro is not a simple dose-for-dose conversion. There is limited evidence to guide switching between these medicines, so the starting dose, timing of the first dose and subsequent dose increases should be decided by the prescriber based on the individual's treatment history and tolerability.

The dose of one medicine should not be assumed to correspond to a particular dose of the other. Your prescriber will decide how to start the new treatment and whether dose escalation needs to be adjusted.

Why people consider it

• Weight loss has stalled or you are no longer continuing to lose weight as expected, so a prescriber may review whether changing treatment better fits your wider weight loss journey
• Side effects on the current treatment have not settled
• A supply interruption has forced a change
• A prescriber has recommended it following review

Why your dose does not carry across

This is the most important point on the page, and the one that causes the most trouble when people assume otherwise.

The two medicines contain different active substances with different potencies, and there is no simple equivalence between a dose of one and a dose of the other. Being established on a maintenance dose of one does not mean you would start at a comparable dose of the other, and moving from mounjaro or wegovy does not mean you begin at the same stage of the dose ladder.

The new medicine has its own starting dose and dose-escalation schedule. Your prescriber will decide where you should start rather than converting your previous dose directly. The timing of the first dose of the new medicine should be determined by your prescriber rather than assumed from the dosing schedule of the previous medicine.

We do not publish switching doses or schedules. The starting dose, the timing in relation to your last injection of the previous medicine, and the pace of any increase are decisions for your prescriber, based on your assessment. They are set out in the product information for the medicine you are given, and the right dose and any timing gap depend on prescriber review.

What to expect

• A period of adjustment may occur, and gastrointestinal side effects can occur again when starting the new medicine or increasing its dose.
• A gap before the new treatment reaches the dose at which most of the effect occurs
• Weight that may fluctuate or stall during the transition, which is not a sign of failure
• A review to check how you are tolerating it

Plan the switch for a period when you are not travelling, working long shifts or attending events, in the same way you would plan a dose increase.

What to tell your prescriber

1. Which medicine you have been taking and at what stage of treatment
2. When your last injection was
3. How well it has worked, with specifics rather than impressions, and your progress against your starting weight if known
4. What side effects you have had and whether they settled
5. Whether you have had any breaks in treatment, and why
6. all the medications you take, including anything bought without a prescription, plus any supplements or herbal remedies
7. Any change in your health since your last assessment
8. Whether you are planning a pregnancy

Never start a new treatment while still taking the previous one, and never arrange a switch yourself.

When switching may not be the answer

Before changing, it is worth checking whether the current treatment has genuinely been given a fair run.

Has it been used consistently?Interruptions in treatment can affect weight-loss progress and may require the treatment plan to be reassessed.

Have you reached a dose where most of the effect occurs? Someone still partway through the build-up has not yet seen what the medicine can do.

Has it stalled, or has it plateaued? Weight settling at a new level is expected, because a smaller body needs less energy and appetite signals push back as weight falls. That is the system finding a new balance rather than the medicine failing.

Is anything else contributing? Protein intake, resistance activity, sleep, your exercise plan and other medicines all affect results, and none of them changes by switching.

Is it working well? If the current treatment is effective and tolerated, switching on the basis of a trial average may not deliver what you expect, and it may already be supporting sustainable weight loss.

What stays the same whichever you take

• Both are prescription-only medicines requiring a clinical assessment before supply, including for eligible patients accessing them via the NHS
• Both are used for chronic weight management alongside lifestyle changes, not as stand-alone treatment
• Both are built up in steps rather than started at full dose
• Weight loss can include some loss of lean mass, so adequate protein intake and resistance exercise may help support muscle preservation during treatment.
• Both carry the same serious warnings, including those on pancreatitis and pregnancy
• Weight commonly returns after stopping either, unless the wider changes have been established
• These medicines should not be used during pregnancy. If you are planning a pregnancy, become pregnant, or are breastfeeding, speak to your prescriber before continuing treatment because the recommendations differ between medicines and treatment should be reviewed.

Red flags

Seek urgent medical attention for severe and persistent stomach pain that may spread through to your back, with or without nausea and vomiting; swelling of the face, lips, tongue or throat; difficulty breathing; or signs of significant dehydration.

Contact your prescriber for persistent vomiting or diarrhoea, pain in the upper right abdomen particularly after eating, symptoms of low blood sugar if you take diabetes medications and they have not been adjusted appropriately, or side effects that are not settling after a couple of weeks.

Before you decide

The honest summary is that both medicines are designed to support weight loss, and one produced greater average weight loss in a direct comparison, but that still does not settle the question for any individual. Tolerability, consistency, other conditions, and broader lifestyle changes alongside treatment all affect the result you get.

If you are considering a switch, the useful preparation is specifics: what you took, for how long, at what stage, what happened, and what side effects you had. A prescriber can do far more with that than with a general sense that progress has been disappointing.

Medical Disclaimer

This article is for general information only. It is not medical advice, it is not a complete list of side effects, it does not provide dosing or switching instructions, and it does not replace the patient information leaflet supplied with your medicine or an individual assessment by an appropriately qualified healthcare professional.

Wegovy and Mounjaro are prescription-only medicines. They should only be used under clinical supervision of an appropriately qualified prescriber, and are effective weight loss treatments only when prescribed after an individual clinical assessment that considers your medical history, current medicines and other conditions. Treatment suitability depends on an individual clinical assessment, and results vary between individuals. Trial results described here are averages from specific study populations and cannot predict an individual outcome.

Do not start, stop, switch or restart treatment, or alter your dose, without advice. Do not take two of these medicines together.

Seek urgent medical attention for severe and persistent stomach pain that may spread to your back, swelling of the face, lips, tongue or throat, difficulty breathing, or signs of significant dehydration. Suspected side effects can be reported through the Yellow Card scheme, which the MHRA uses to monitor the safety of medicines.