Mounjaro vs Wegovy: How the Two Weight Loss Injections Compare

Mounjaro and Wegovy are the two weight loss injections most people in the UK have heard of. Both are prescription-only medicines. Both are given once a week. Both are licensed for weight management alongside a reduced-calorie diet and more physical activity. The difference is in how they act on the body, and that difference is what your prescriber weighs up.

What each medicine contains

Mounjaro contains tirzepatide and is made by Eli Lilly. Wegovy contains semaglutide and is made by Novo Nordisk. The brand name is what you see on the box. The active ingredient is what does the work, and it is the part your prescriber thinks about.

How they work differently

Wegovy acts on one appetite hormone, GLP-1. Your gut releases GLP-1 after you eat. It tells the brain you have had enough and slows how quickly the stomach empties. Semaglutide copies that signal, so you feel full sooner and stay full for longer.

Mounjaro acts on GLP-1 as well, and on a second gut hormone called GIP. Acting on two hormones rather than one is the main reason clinicians treat the two medicines as different rather than interchangeable.

In a head-to-head trial published in the New England Journal of Medicine in 2025, adults taking tirzepatide lost more weight on average than adults taking semaglutide over 72 weeks. Averages hide a lot of variation. Some people respond well to one medicine and less well to the other, and there is no way to know in advance which group you fall into.

How each one is taken

Both are injected under the skin once a week, on the same day each week, using a pre-filled device. You can inject into the stomach, thigh or upper arm, and you change the spot each time.

Both start at a low dose. Mounjaro starts at 2.5 mg and can be increased in steps to 15 mg. Wegovy starts at 0.25 mg and can be increased in steps to 2.4 mg. The steps exist to give your body time to settle, which is also why side effects are usually strongest in the first weeks and just after an increase. Your prescriber decides when to move you up, and sometimes decides to hold you at a dose that is working.

Side effects are similar

The common side effects are much the same for both. Feeling sick, being sick, constipation, diarrhoea, indigestion and stomach pain are the ones most people notice. For most, these ease within a few weeks.

Both carry the same less common but serious risks, including inflammation of the pancreas and gallbladder problems. Neither is suitable in pregnancy, while breastfeeding, or for anyone with a history of pancreatitis or certain thyroid cancers. Always read the patient information leaflet that comes with your treatment.

How your prescriber chooses

A prescriber does not pick a medicine from a league table. They look at your body mass index, your other health conditions, every other medicine you take, whether you have had a weight loss medicine before and how you responded, and what you can realistically keep up. Cost and supply matter too, because a treatment you cannot continue is not the right treatment.

This is also why a questionnaire on its own is not enough. Our regulator, the General Pharmaceutical Council, requires prescribers to independently verify your height and weight before a first supply. We do that by a short video call, or with your consent from your GP or medical records.

Can I switch between them?

Often, yes. If you are already on treatment, tell us the medicine and the dose you have reached. Your prescriber can then plan a switch safely, rather than restarting you at the lowest dose. Do not switch on your own or change a dose without advice.

What happens if I stop?

Appetite tends to return when treatment stops, and weight can come back. That is why both medicines are prescribed alongside changes to what you eat and how active you are, and why your prescriber will talk to you about what happens after treatment as well as during it.

Starting with our pharmacy

The assessment is free and takes about ten minutes. We verify your height and weight independently, a registered prescriber reviews everything, and a pharmacist checks the supply before it leaves us. A prescription is not guaranteed. If treatment is not right for you, we will say so and explain why.