Fewer than 10 percent of patients in England who are eligible for Mounjaro will initially be able to access it on the health service, after officials revealed they are prioritising people with the highest needs.

The "difficult decision", which will come as a blow to people hoping to access it sooner, was made "in order to protect other vital NHS services", the National Institute for Health and Care Excellence (Nice) said.

The NHS spending watchdog has recommended Mounjaro - also known as tirzepatide and made by Eli Lilly - for those with a body mass index (BMI) of more than 35 and at least one weight-related illness. This is estimated to account for about 3.4 million people.

To help the NHS manage demand, Nice said about 220,000 people could benefit from the drug in an initial three-year period.

After this, the situation will be reviewed by Nice, who will provide further guidance on the rollout. 

Mounjaro is a glucagon-like peptide-1 (GLP-1) agonist, a family of medications that help manage blood sugar. It was initially given the green light to help manage obesity on the NHS in June as part of draft guidance from Nice.

The once-weekly jab should be prescribed along alongside a reduced-calorie diet and exercise to help people lose weight.

Other GLP-1 agonists include semaglutide - sold under the brand names Wegovy, Ozempic and Rybelsus.

Professor Jonathan Benger, chief medical officer at Nice, said: "The world will look very different in three years which is why we've taken the unprecedented decision to review the way this medicine is delivered to patients then.

"Tirzepatide and other drugs like it, such as semaglutide, will help people living with obesity to lose weight, and as a result will reduce their risk of developing heart disease or having a stroke. But tirzepatide is not for everybody, and only those with the highest clinical need will be treated initially. This means many people will have to wait."

Prof Benger added Nice "had to make this difficult decision in order to protect other vital NHS services" as well as to "test ways of delivering" this new generation of medications.

Final draft guidance published by Nice shows NHS England requested that Mounjaro is rolled out over a period of 12 years.

The watchdog recommends prioritising patients receiving care from specialist weight management services - around 220,000 people - who will be able to access the drug within 90 days of the guidance being published on December 23.

NHS England will develop a plan outlining other groups of patients that will be offered Mounjaro in the next three years, according to Nice.

Prof Benger added: "We want to help NHS England carefully manage the roll out of tirzepatide to ensure that other services are not impacted in a disproportionate way.

"Whilst the funding variation sets a maximum of 12 years, Nice will review the situation again within three years and provide further advice on how the roll-out of this medicine can be managed using the learning gained from the initial phase. This will ensure the roll out of tirzepatide reaches everyone who is eligible in a safe and effective way." 

The latest Health Survey for England shows 64 percent of adults were overweight or obese in 2022.

However, Dr Kath McCullough, NHS England's national speciality adviser for obesity, warned that this type of medications "are not a magic bullet".

She said: "Obesity is one of the greatest public health issues facing the NHS and weight loss drugs, such as tirzepatide, are an important tool in helping people lose weight while also reducing the risk of other serious long-term conditions such as diabetes, strokes and heart attacks.

"However, on their own, weight loss drugs are not a magic bullet. They need to be prescribed by a healthcare professional alongside programmes that help people lose weight and live healthier lives by making changes to their diet and physical activity - and it's also crucial that they are prioritised for those who need them most.

"This guidance enables the NHS to implement a phased roll out of tirzepatide to patients with the highest clinical need in a safe and effective way, while also protecting access to the NHS services that all patients rely on."

It comes as a trial by Mounjaro manufacturer Eli Lilly - known as Surmount-5 - found patients on tirzepatide recorded a weight loss of 20.2 percent compared to 13.7 percent in patients on semaglutide.