The weight-loss injection Mounjaro (tirzepatide) leads to greater weight loss than Wegovy (semaglutide), according to UK news outlets reporting on a new study.
The research found Mounjaro reduced body weight in people with obesity by just over 20 per cent, whereas Wegovy cut it by nearly 14 per cent.
Both medicines are used in NHS specialist weight-loss clinics in combination with diet and exercise to aid weight loss.
They work by tricking the brain into thinking you’re full, so you eat less. They do this by mimicking a hormone that’s released after eating called GLP-1 (glucagon-like peptide-1).
However, Mounjaro also mimics another hormone that regulates appetite called GIP (glucose-dependent insulinotropic polypeptide) – so it works in 2 ways.
Which one is better for weight loss has been a topic of debate for years, which is why this latest study in the New England Journal of Medicine in May 2025 made headline news.
Is Mounjaro or Wegovy better for weight loss?
The head-to-head trial between tirzepatide and semaglutide aimed to find out which was more effective in people with obesity who did not have type 2 diabetes.
It involved 751 people with an average weight of 113 kgs (nearly 18 stone). Some also had obesity-related health conditions such as high blood pressure or high cholesterol.
They were asked to take the maximum amount of either tirzepatide or semlaglutide they could tolerate for around a year and a half. The drugs were given as weekly injections.
The US researchers found:
- Those on tirzepatide lost 20.2 per cent of their body weight.
- Those on semaglutide lost 13.7 per cent of their body weight.
- Those on tirzepatide saw their waist sizes shrink over 5cm (around 2 inches) more on average than those on semaglutide.
What about side effects?
The researchers found that a similar amount of people experienced side effects or adverse events on both weight-loss drugs, with between 77 per cent and 79 per cent reporting at least 1 side effect – most commonly digestive problems such as feeling sick, constipation or diarrhoea.
Overall, around 6 per cent of people taking tirzepatide and 8 per cent of those taking semaglutide stopped taking the drug before the end of the study due to side effects.
Managing the Mounjaro side effects
What do the researchers say?
The researchers say their findings, presented at the European Congress on Obesity in Malaga, Spain, show that tirzepatide is more effective for weight loss than semaglutide in people with obesity who do not have type 2 diabetes.
They say this aligns with a large study in JAMA Internal Medicine in 2024 that found tirzepatide led to almost 7 per cent more body weight being lost on average than semaglutide after 1 year among obese people with and without diabetes.
The researchers suggest that tirzepatide could be used to reduce the risk of heart and circulatory diseases that are caused by obesity by helping people lose more weight.
Currently, only semaglutide is approved in the UK to reduce the risk of serious heart problems in people with obesity.
A 2023 study of 17,604 people who already had a heart or circulatory disease in the New England Journal of Medicine found that semaglutide cut the risk of dying from coronary heart disease or having a heart attack or stroke by 20 per cent after almost 3 years compared to a dummy injection.
More research is now needed to see if tirzepatide can lower this risk more than semaglutide, say the researchers.
How good was the research?
The study was funded and designed by Eli Lilly, the company that makes Mounjaro, and was peer-reviewed and published in a reputable journal.
The study was relatively large and diverse, including 751 people without type 2 diabetes and either a BMI (body mass index) of 30 or above, or a BMI of 27 or above and at least 1 obesity-related health problem like high blood pressure or high cholesterol.
However, while it was randomly decided who got tirzepatide or semaglutide, it was not a ‘blind’ study. This means the participants knew which medicine they were taking, which could have affected the results.
How good was the media coverage?
Many UK news outlets reported on the study, including the Daily Mail, Daily Express, Daily Mirror and the BBC.
While most coverage was accurate, the Daily Mail, Daily Express and Daily Mirror’s headlines suggested Mounjaro was now the most effective weight-loss injection on the market.
But this is misleading, as the study only looked at tirzepatide and semaglutide, and did not include the weight-loss injection liraglutide, which is also available on the NHS under the brand name Saxenda.
Our Opinion
This research shows that, while both weight-loss injections can help you lose a lot of weight if you have obesity, tirzepatide (Mounjaro) may be more effective than semaglutide (Wegovy).
For people living with obesity, losing more weight may further reduce their risk of heart attack or stroke.
However, we do not know yet if tirzepatide can lower the risk of heart and circulatory diseases by as much, or more than semaglutide, and more research is needed to confirm this.
