The first head-to-head trial of two blockbuster weight-loss drugs has shown Mounjaro is more effective than rival Wegovy.

Both drugs led to substantial weight loss, but Mounjaro's 20% weight reduction, after 72 weeks of treatment, exceeded the 14% from Wegovy, according to the trial's findings.

Researchers who led the trial said both drugs had a role, but Mounjaro may help those with the most weight to lose.

Both drugs trick the brain into making you feel full so you eat less and instead burn fat stored in the body – but subtle differences in how they work to explain the difference in effectiveness.

Wegovy, also known as semaglutide, mimics a hormone released by the body after a meal to flip one appetite switch in the brain. Mounjaro, or tirzepatide, flips two.

The trial, which was paid for by Eli Lilly, the manufacturer of Mounjaro, involved 750 obese people, with an average weight of 113kg (nearly 18 stone).

They were asked to take the highest dose they could tolerate of one of the two drugs.

The findings, presented at the European Congress on Obesity in Malaga and in the New England Journal of Medicine, showed:

• 32% of people lost a quarter of their body weight on Mounjaro compared to 16% on Wegovy
• Those on Mounjaro lost an average of 18cm from their waistlines compared with 13cm on Wegovy.
• Those on Mounjaro had better blood pressure, blood sugar and cholesterol levels.
• Both had similar levels of side-effects.
• Women tended to lose more weight than men.

Dr Louis Aronne, who conducted the trial at the Comprehensive Weight Control Center at Weill Cornell Medicine in New York, said: "The majority of people with obesity will do just fine with semaglutide (Wegovy), those at the higher end may ultimately do better with tirzepatide (Mounjaro)."

Private tirzepatide sales 'well ahead of semaglutide'

In the UK, the two medicines are available from specialist weight-management services, but can also be bought privately.

Prof Naveed Sattar, from the University of Glasgow, said the drugs were "good options" for patients, but while "many will be satisfied with 15% weight loss… many want as much weight loss as possible".

"In the UK, tirzepatide sales privately are now well ahead of semaglutide – that's just a reality – and this paper will accelerate that I imagine," he added.

However, Wegovy is also licensed for other conditions – such as preventing heart attacks – while the equivalent trials with Mounjaro have not been completed.

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A huge amount of research into weight-loss drugs is still taking place. Higher doses of current drugs are being tested, as are new ways of taking them such as oral pills and new medicines that act on the body in different ways are being investigated.

It means the final winner in this field has yet to be determined.

Prof Sattar says the amount of research taking place means we may be approaching the point where "obesity prevention may also be possible soon", but argues "it would be far better" to make our society healthier to prevent more people developing obesity.