A new weight loss jab will be available in the UK within weeks after it was approved by the medicines regulator.

Mounjaro, also known as tirzepatide, can be used to treat obesity or type 2 diabetes, the Medicines and Healthcare products Regulatory Agency (MHRA) said.

The drug was developed by the US pharmaceutical firm Eli Lilly and trials show those on the highest dose lost more than three stone on average.

It is injected under the skin of the stomach, thigh or upper arm.

The drug itself was approved for use in November last year but has not been available in the UK due to huge global demand.

After slimming down by changing their lifestyle then using the jab, users lost an average of 24.3 per cent of their starting body weight on average

According to the latest data digestive problems were the most commonly reported side effects of tirzepatide, the active ingredient of Mounjaro. These included about one in five participants suffering from nausea and diarrhoea, and about one in 10 reporting vomiting or diarrhoea

But the MHRA is now the first regulator in the world to approve a new device for administering the drug, which Eli Lilly said will allow them to supply the UK ‘within weeks’.

The new jab will be supplied in a four-dose pen branded as KwikPen, which provides a month’s treatment when used once a week.

It was previously only available in single doses.

Laura Steele, president and general manager of UK and Northern Europe at Lilly, said: ‘The MHRA is the first major regulator in the world to issue marketing authorisation for the alternative KwikPen presentation for tirzepatide, demonstrating their clear focus on speeding innovation to help patients.

‘This will enable Lilly to begin supply to the UK within weeks.’

Mounjaro KwikPen has been approved to treat adults with type 2 diabetes and for weight management in obese adults, as well as overweight adult patients who have weight-related health problems like prediabetes, high blood pressure, high cholesterol, or heart problems.

Mounjaro will provide an alternative to the weight-loss drug Wegovy — also known as semaglutide — which has also been in short supply due to overwhelming demand.

Julian Beach, interim executive director of healthcare quality and access at the MHRA, said: ‘The public health importance of safe and effective treatments to help manage diabetes and obesity, which can have a significant impact on people’s health, is clear.

‘This approval enables access to the approved Mounjaro pen in a more convenient presentation of a month’s treatment, of one dose per week.’

The drugs watchdog NThe jab was given the green light for NHS use in September last year by The National Institute for Health and Care Excellence (Nice) – the drugs watchdog – approved the jab for use on the NHS in September but only for patients with type 2 diabetes who do not have the disease under control.

Some Americans are already using it ‘off label’.  One of these is Matthew Barlow, a 48-year-old health technology executive living in California, who said he has lost more than 100 pounds since November 2022 by using Mounjaro and changing his diet

It is yet to be given the green light for use on the NHS to treat obesity but can be purchased privately.

The active ingredient in the drug helps to reduce sugar levels in people with type 2 diabetes when their levels are high, and works as a weight management drug by making a patient feel full and less hungry, and making them experience fewer food cravings.

The MHRA’s newest authorisation is based on the results of a bridging study which showed the efficacy and safety of the multidose Mounjaro KwikPen are expected to be the same as those for the single-dose pen.

Douglas Twenefour, head of care at Diabetes UK, said: ‘We hope the MHRA’s approval of this device will help people living with type 2 diabetes, who are eligible, to access this effective treatment.

‘Supporting people with type 2 diabetes to lose weight and manage their blood sugar levels is key to reducing the risk of diabetes-related complications, and tirzepatide (Mounjaro) expands the range of treatment options available to help people achieve this.’

The MHRA warned the drug may affect how well the contraceptive pill works in obese or overweight female patients.

It also listed potential side effects of the medicine, including nausea, diarrhoea, vomiting – which usually goes away over time – and constipation.

Low blood sugar is also ‘very common’ in patients with diabetes, the agency added.

The MHRA said it will keep the safety and effectiveness of Mounjaro under close review.