The short version
Both are weekly injections that reduce appetite, and both are used alongside changes to diet and activity rather than instead of them. The difference is what they act on: Wegovy works on one appetite hormone pathway, Mounjaro on two.
| Mounjaro | Wegovy | |
|---|---|---|
| Active ingredient | Tirzepatide | Semaglutide |
| How it works | Acts on two gut hormone receptors, GLP-1 and GIP | Acts on the GLP-1 receptor |
| How it is taken | One injection a week, self-administered | One injection a week, self-administered |
| Dose steps | 2.5mg · 5mg · 7.5mg · 10mg · 12.5mg · 15mg | 0.25mg · 0.5mg · 1mg · 1.7mg · 2.4mg |
| Price range | £250 – £500 by dose | £200 – £400 by dose |
What the difference in mechanism means
Semaglutide mimics GLP-1, a hormone released after eating that signals fullness and slows how quickly the stomach empties. Tirzepatide does the same and additionally acts on GIP, a second hormone involved in how the body handles food and stores fat.
Acting on two pathways is not automatically better for a given person. It changes the profile of the medicine, and which profile suits you depends on your health, what you have taken before, and how you tolerate it.
How the doses step up
Neither is started at its full dose. Both begin low and increase in stages, usually every four weeks, so that the body adjusts and side effects stay manageable. Mounjaro has six steps and Wegovy five, and not everyone reaches the top of either — the right dose is the one that works with side effects you can live with.
Side effects
The two have broadly similar side effects, most of them digestive: nausea, diarrhoea, constipation, indigestion. They are usually worst after starting and after each dose increase. The full list, and what needs urgent attention.
How a prescriber chooses
A prescriber weighs up:
- Your BMI and any weight-related conditions you have
- Other medicines you take, and possible interactions
- Whether you have used a weight-loss medication before, and how you tolerated it
- Your medical history — thyroid, pancreas and gallbladder history in particular
- Which is available to dispense at the time
You can say which you would prefer, and that preference is taken seriously. It is not the deciding factor: if a treatment is not appropriate for you, it will not be prescribed.
Neither is a standalone answer
Both work by making it easier to eat less. Weight tends to return when treatment stops unless the habits built during it stay — which is why treatment is reviewed as you go rather than simply repeated.
Read more
Mounjaro · Wegovy · Who treatment is for
To find out which is appropriate for you, complete a consultation — a prescriber reads it and tells you either way.