By Anna Fenech, Pharmacist & Independent Prescriber
If you’re considering prescription medication to support weight loss, you may have come across two names repeatedly: Mounjaro (tirzepatide) and Wegovy (semaglutide).
Both are prescription medicines that can be used alongside a reduced-calorie diet and increased physical activity to support weight management. Both are given as injections, both work partly by reducing appetite, and both can produce clinically meaningful weight loss.
They are not, however, the same medicine. They work in slightly different ways, have different dose-escalation schedules and have some differences in their evidence base and licensing.
So, which is more suitable? There isn’t a single answer that applies to everyone. Your medical history, other medicines, treatment goals, side-effect tolerance and individual risk factors all need to be considered before prescribing.
This guide explains the main differences between Mounjaro and Wegovy to help you understand the options.
Mounjaro vs Wegovy at a glance
| Mounjaro | Wegovy | |
|---|---|---|
| Active ingredient | Tirzepatide | Semaglutide |
| How it works | Activates GLP-1 and GIP receptors | Activates GLP-1 receptors |
| Administration | Once-weekly injection | Once-weekly injection |
| Usual starting dose | 2.5 mg once weekly | 0.25 mg once weekly |
| Usual maintenance doses | 5 mg, 10 mg or 15 mg weekly | 2.4 mg weekly |
| Maximum injectable dose | 15 mg weekly | 2.4 mg weekly* |
| Common side effects | Nausea, diarrhoea, vomiting, constipation and other gastrointestinal symptoms | Nausea, diarrhoea, vomiting, constipation and other gastrointestinal symptoms |
| UK weight-management licence | Yes | Yes |
| NHS availability | Phased implementation under NICE/NHS England arrangements | Primarily through specialist weight-management services |
*The current UK Wegovy product information also allows a 7.2 mg weekly dose in certain adults with obesity who need additional treatment, following at least four weeks on 2.4 mg. This is a more recent addition to the product information and should be checked against the formulation being prescribed.
How do Mounjaro and Wegovy work?
Although both medicines influence appetite and food intake, they do this through different mechanisms.
Mounjaro: tirzepatide
Mounjaro contains tirzepatide, which is a dual GLP-1 and GIP receptor agonist.
GLP-1 and GIP are naturally occurring hormones involved in regulating appetite, digestion and blood glucose. By activating receptors for both hormones, tirzepatide can help reduce appetite and food intake and can slow the movement of food through the stomach.
The GIP component is one of the key differences between Mounjaro and Wegovy.
Wegovy: semaglutide
Wegovy contains semaglutide, a GLP-1 receptor agonist.
GLP-1 is involved in appetite regulation and satiety. Semaglutide acts on GLP-1 receptors, helping people feel fuller and reducing hunger and food intake.
Neither medicine works simply by “burning fat”. Their effect is primarily through changes in appetite, satiety and energy intake, alongside metabolic effects.
If you’d like a more detailed explanation of tirzepatide specifically, see our Mounjaro article. For a closer look at semaglutide, see our Wegovy article.
How are Mounjaro and Wegovy taken?
Both injectable medicines are administered once a week, usually on the same day each week, and are given by subcutaneous injection.
The doses are deliberately increased gradually. This is important because gastrointestinal side effects are particularly common when treatment is first started or when the dose is increased.
Mounjaro dosing
Mounjaro is normally started at 2.5 mg once weekly.
After four weeks, the dose is increased to 5 mg once weekly. If further treatment is required and the medicine is tolerated, the dose can subsequently be increased in 2.5 mg increments, with at least four weeks at each dose.
The recommended maintenance doses for adults are 5 mg, 10 mg and 15 mg once weekly, with 15 mg being the maximum dose.
Not everyone needs the highest dose. The aim is to find a dose that provides an appropriate clinical benefit while remaining tolerable.
Wegovy dosing
Injectable Wegovy is normally started at 0.25 mg once weekly.
The dose is gradually increased, generally every four weeks:
- Weeks 1–4: 0.25 mg once weekly
- Weeks 5–8: 0.5 mg once weekly
- Weeks 9–12: 1 mg once weekly
- Weeks 13–16: 1.7 mg once weekly
- From week 17: 2.4 mg once weekly
If gastrointestinal symptoms are significant, dose escalation can be delayed or the previous dose may be used temporarily, depending on the clinical circumstances.
The UK product information now also includes a 7.2 mg once-weekly option for certain adults with obesity who require additional treatment after reaching 2.4 mg.
The important point is that dose is individualised. A higher dose is not automatically the right dose for every patient.
Which produces more weight loss?
Both medicines have strong clinical evidence behind them, but it is important to interpret trial results carefully.
The original STEP 1 trial studied once-weekly semaglutide 2.4 mg in 1,961 adults with overweight or obesity without diabetes. After 68 weeks, average weight loss was approximately 14.9% of initial body weight with semaglutide, compared with 2.4% with placebo, alongside lifestyle intervention. [1]
Tirzepatide has also demonstrated substantial weight loss. In the original SURMOUNT-1 trial, adults with obesity or overweight and a weight-related complication, but without diabetes, received tirzepatide or placebo for 72 weeks. Average weight loss was approximately 15%, 19.5% and 20.9% with the 5 mg, 10 mg and 15 mg doses respectively. [2]
There is now also an important head-to-head study.
In SURMOUNT-5, 751 adults with obesity but without type 2 diabetes were randomised to maximum tolerated doses of tirzepatide or semaglutide for 72 weeks. Average weight reduction at 72 weeks was 20.2% with tirzepatide and 13.7% with semaglutide. [3]
That study provides useful evidence because it directly compared the two medicines in the same trial population.
However, these figures are averages, not guarantees. Individual responses vary considerably. Some people lose substantially more or less than the trial average, and tolerability can affect the dose that someone is able to maintain.
It is also worth remembering that clinical trials involve structured follow-up and lifestyle support. Medication works best as part of a broader approach to weight management rather than as a replacement for nutrition, physical activity and behavioural support.
What are the side effects?
The side-effect profiles of Mounjaro and Wegovy are broadly similar.
The most common problems involve the gastrointestinal system and include:
- Nausea
- Diarrhoea
- Vomiting
- Constipation
- Abdominal discomfort or pain
- Indigestion or reflux
These effects are often more noticeable when treatment is started or following a dose increase. For many people, they become less troublesome as the body adjusts.
Eating smaller meals, avoiding very large or high-fat meals and maintaining adequate fluid intake can help some people manage gastrointestinal symptoms. However, persistent or severe symptoms should always be discussed with the prescriber.
More serious risks
Although uncommon, both medicines can be associated with more serious adverse effects.
Acute pancreatitis is an important potential risk. The MHRA strengthened warnings about pancreatitis across GLP-1 receptor agonists and dual GLP-1/GIP receptor agonists in January 2026, following reports of rare severe cases. Severe, persistent abdominal pain, particularly if it radiates through to the back and is accompanied by nausea or vomiting, requires urgent medical assessment. [4]
Gallbladder problems can also occur.
There are additional medicine-specific considerations. For example, the MHRA has issued a warning concerning a very rare risk of non-arteritic anterior ischaemic optic neuropathy (NAION) with semaglutide, which can affect vision. Anyone taking semaglutide who develops sudden visual loss or a rapid deterioration in eyesight should seek urgent medical advice. [5]
Both medicines also require careful consideration in pregnancy. They should not be used during pregnancy, and anyone who could become pregnant should discuss contraception and pregnancy planning with their prescriber.
Mounjaro also has specific considerations for people using oral contraception because tirzepatide can affect contraceptive reliability around treatment initiation and dose increases. Your prescriber should advise you on appropriate additional contraception where necessary.
This is one reason why an assessment by an appropriately qualified prescriber is important before starting treatment.
What about NHS availability?
NHS access is different from private prescribing.
NICE recommends both semaglutide and tirzepatide for specific groups, but the NHS eligibility criteria and implementation arrangements are not identical.
For semaglutide, NICE recommends Wegovy as part of a specialist weight-management service, alongside a reduced-calorie diet and increased physical activity. [7]
Tirzepatide has also been recommended by NICE, with implementation being phased under NHS England arrangements. NICE’s current guidance sets out a staged approach based on clinical need, BMI and weight-related health conditions. [8]
Therefore, being medically eligible for a medicine does not necessarily mean that it will be immediately available through an NHS pathway in every area.
Who might Mounjaro or Wegovy be suitable for?
There is no simple rule that says one medicine is right for everyone.
Both medicines are licensed for weight management in adults who meet specific criteria, generally involving obesity or overweight accompanied by at least one weight-related health condition. A prescriber will also consider medical history, current medicines, previous weight-management attempts and any reasons why a particular treatment may not be appropriate.
Mounjaro may be considered when:
- A patient meets the prescribing criteria for tirzepatide.
- A dual GLP-1/GIP mechanism is considered appropriate.
- Significant weight reduction is an important treatment goal.
- The person can tolerate the medicine and dose escalation.
- There are no contraindications or significant medicine interactions that would make treatment unsuitable.
Wegovy may be considered when:
- A patient meets the prescribing criteria for semaglutide.
- A GLP-1 receptor agonist is clinically appropriate.
- The person’s cardiovascular risk profile is an important consideration. Wegovy has also been authorised for reducing the risk of major cardiovascular events in certain adults with established cardiovascular disease and overweight or obesity.
- The person can tolerate semaglutide and its dose-escalation schedule.
- There are no contraindications or other clinical reasons to choose an alternative.
It is also possible for a person to tolerate one medicine better than another. Side effects, response and practical considerations can therefore be just as relevant as the headline trial averages.
Mounjaro or Wegovy: which should I choose?
The most useful question is not necessarily “Which drug is strongest?”
A better question is which treatment is appropriate and sustainable for you?
Current evidence shows that both tirzepatide and semaglutide can produce substantial weight loss. The SURMOUNT-5 trial provides direct evidence of greater average weight loss with tirzepatide than semaglutide over 72 weeks in adults with obesity without diabetes. [3]
But prescribing decisions should not be based on percentage weight loss alone.
A good clinical assessment also considers your existing health conditions, other medicines, previous experience with weight-management treatment, potential side effects, treatment goals and what level of ongoing support you need.
Weight management is usually a long-term process. The medication is one part of that process, rather than a quick fix.
If you are considering Mounjaro or Wegovy, speak to a qualified prescriber who can assess whether treatment is appropriate and explain the potential benefits, risks and alternatives for your individual circumstances.
References
- Wilding JPH, Batterham RL, Calanna S, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity. New England Journal of Medicine. 2021;384:989–1002. DOI: 10.1056/NEJMoa2032183.
- Jastreboff AM, Aronne LJ, Ahmad NN, et al. Tirzepatide Once Weekly for the Treatment of Obesity. New England Journal of Medicine. 2022;387:205–216. DOI: 10.1056/NEJMoa2206038.
- Aronne LJ, Horn DB, le Roux CW, et al. Tirzepatide as Compared with Semaglutide for the Treatment of Obesity. New England Journal of Medicine. 2025;393:26–36. DOI: 10.1056/NEJMoa2416394.
- Medicines and Healthcare products Regulatory Agency (MHRA). GLP-1 receptor agonists and dual GLP-1/GIP receptor agonists: strengthened warnings on acute pancreatitis, including necrotising and fatal cases. Drug Safety Update, 29 January 2026.
- Medicines and Healthcare products Regulatory Agency (MHRA). Semaglutide (Wegovy, Ozempic and Rybelsus): risk of non-arteritic anterior ischaemic optic neuropathy (NAION). Current MHRA safety information.
- Boots Online Doctor. Wegovy and Mounjaro treatment and pricing information. Current private prices accessed September 2026. Prices are subject to change and should be rechecked before publication.
- NICE. Semaglutide for managing overweight and obesity (TA875). National Institute for Health and Care Excellence.
- NICE. Tirzepatide for managing overweight and obesity (TA1026). National Institute for Health and Care Excellence. Updated implementation arrangements should be checked before publication.
- Electronic Medicines Compendium (emc). Mounjaro (tirzepatide) Summary of Product Characteristics. Current UK product information.
- Electronic Medicines Compendium (emc). Wegovy (semaglutide) Summary of Product Characteristics. Current UK product information.