Mounjaro vs Wegovy: which weight loss injection works better?

Both medicines are given as a single weekly injection, and both are licensed in the UK for weight management. The table below sets out the main facts from the product information.

	Mounjaro	Wegovy
Active ingredient	Tirzepatide	Semaglutide
Receptors targeted	GIP and GLP-1	GLP-1
Dosing	Once a week	Once a week
Device	KwikPen	Pre-filled pen
Highest licensed dose	15 mg once a week	7.2 mg once a week (obesity only, from January 2026)
Routine NHS prescribing for weight loss in NI	Not available at the time of writing	Not available at the time of writing

The Wegovy 7.2 mg dose was approved by the MHRA for adults with obesity, with a BMI of 30 or above. It is not approved for people with overweight alone. Details are in our dose schedule guide.

The most direct evidence is SURMOUNT-5, a trial published in the New England Journal of Medicine in May 2025. It randomised 751 adults to tirzepatide (10 mg or 15 mg) or semaglutide (1.7 mg or 2.4 mg), at the highest doses each person could tolerate. Participants were followed for 72 weeks.

Average weight loss was 20.2% with tirzepatide and 13.7% with semaglutide, a statistically significant difference. In absolute terms, the trial reports about 22.8 kg with tirzepatide and 15.0 kg with semaglutide. These figures come from a secondary summary of the trial, so check the published paper for exact values.

The trial has two limits you should keep in mind:

• It was open-label. Participants and the clinical team knew which medicine was being given. This can influence how people report symptoms and how doses are adjusted.
• It was funded by Lilly, the company that makes Mounjaro. Independent replication would strengthen the result.

SURMOUNT-5 is one trial, so it does not prove that every person will lose more weight on Mounjaro. It is the best head-to-head evidence available, and it points in one direction.

It is tempting to set the STEP-1 and SURMOUNT-1 results side by side, but these trials were not designed to be compared directly. They enrolled different populations, ran for different lengths of time, and used different doses.

Trial	Medicine and dose	Length	Average change	Placebo
STEP-1	Semaglutide 2.4 mg	68 weeks	-14.9%	-2.4%
SURMOUNT-1	Tirzepatide 15 mg	72 weeks	-20.9%	-3.1%
SURMOUNT-5	Tirzepatide 10 or 15 mg vs semaglutide 1.7 or 2.4 mg	72 weeks	-20.2% vs -13.7%	Not applicable

The SURMOUNT-5 row is the only one where both medicines were tested in the same study. The first two rows use placebo as the comparison. The data for STEP-1 and SURMOUNT-1 come from the published trial papers, and the SURMOUNT-5 figures are from a secondary summary.

Weight loss on these medicines is gradual. STEP-1 ran for 68 weeks and SURMOUNT-5 for 72 weeks, and much of that time is spent working up to a maintenance dose, because both medicines are increased in steps.

NICE guidance on both medicines suggests reviewing progress at around six months. According to a secondary summary of that guidance, treatment may be stopped if less than 5% of starting weight has been lost by then. Check the NICE guidance itself for the exact wording, and remember that a review is a conversation with your prescriber rather than an automatic cut-off.

If you are not losing weight as expected, speak to your prescriber before changing anything yourself. They can check your dose, your adherence and any other factors that might be affecting your progress.

Both medicines commonly cause gastrointestinal side effects. The most reported are nausea, diarrhoea, constipation and vomiting. In SURMOUNT-5, fewer people on tirzepatide stopped treatment because of gastrointestinal events than those on semaglutide, at 2.7% versus 5.6%, according to a secondary summary.

The safety profile of these medicines is more than a comparison of side effects. Both carry the same class warnings about pancreatitis, and both have rules for contraception. Our side effects and safety guide covers these in full, including what to do if you have severe stomach pain.

Do not assume one medicine is safer because it is less likely to cause nausea. Speak to your prescriber about your own history, including any past pancreatitis, gallbladder problems and other medicines you take.

Neither medicine is routinely available on the NHS for weight loss in Northern Ireland at the time of writing, so most people would pay privately. Prices differ between providers, and they change over time. Current private prices are on the price pages listed below, and you can compare providers on the comparison page.

• See Mounjaro prices in Northern Ireland for private price information.
• See Wegovy prices in Northern Ireland for private price information.
• Use the cost calculator to estimate monthly costs across doses.

The NHS Drug Tariff price is not what you would pay privately. It is the amount pharmacies are reimbursed when they dispense NHS prescriptions, so it is a different measure from private prices.

The medicine you are prescribed should match your medical history, your goals and what else you take. Bring the following points to your appointment:

• Your starting BMI and any weight-related conditions, such as high blood pressure, sleep apnoea or type 2 diabetes.
• Any medicines that lower blood sugar, such as sulfonylureas or insulin. Mounjaro carries a warning about hypoglycaemia when used with sulfonylureas.
• Contraception. Both medicines have guidance on contraception, and Mounjaro advises a non-oral or barrier method for the first 4 weeks after starting or increasing the dose.
• Any history of pancreatitis or gallbladder problems.
• Whether you have a planned operation, because the anaesthetic team must know you are taking these medicines.

There is no single right answer for everyone. Your prescriber will weigh your situation against the trial evidence and the licence.