Weight loss injections: How do drugs like Wegovy and Mounjaro work?

Weight-loss drugs will be offered to millions of people on the NHS, to help them manage their health.

One jab, Wegovy, is already available to some people and a second, Mounjaro, will soon be available – although it could be more than a decade before it is fully rolled-out.

NHS experts have warned that services are overstretched trying to deal with unprecedented demand from patients wanting to try weight-loss drugs.

There are two drugs on the market – semaglutide, marketed under the brand name Wegovy, and tirzepatide, sold as Mounjaro. Semaglutide is also used in the diabetes treatment Ozempic.

Both Wegovy and Mounjaro are given as weekly injections via pre-filled pens that can be self-administered into the upper arm, thigh or stomach.

They work as an appetite suppressant by mimicking a hormone called glucagon-like peptide-1 (GLP-1).

This intestinal hormone is released after eating, and typically makes people feel fuller.

Mounjaro also affects another hormone, glucose-dependent insulinotropic polypeptide (GIP), which affects metabolism and helps regulate energy balance.

Patients generally start on a low dose which is gradually increased until they reach a higher maintenance dose.

People taking these drugs typically start to lose weight within a few weeks.

Clinical trials suggest that – when combined with changes to their diet, exercise and behavioural support – Wegovy users could lose more than 10% of their body weight after a year on the treatment.

Other trials suggest Mounjaro users might lose even more. But it is difficult to compare the two treatments and, crucially, users of either can put weight back on once they stop treatment

At the moment, only Wegovy is available on the NHS in England, Wales and Scotland.

It must be prescribed by a weight management specialist, and access to these is restricted.

To qualify, adults must be obese and have at least one pre-existing weight-related health condition such as high blood pressure.

People with cardiovascular disease who are classed as overweight, but not quite obese, can also be given the drug.

Patients should follow a diet and physical activity programme, and the drug should be only prescribed for a maximum of two years, according to NHS guidelines.

The National Institute for Health and Care Excellence (NICE) has recommended that Mounjaro should also be offered to NHS patients in England from March 2025. This would be alongside advice on diet and exercise.

As with Wegovy, it will initially only be available to obese patients who have a weight-related health condition and are under the care of a specialist.

However, from June, the drug will be offered more widely. It is not clear how this will be managed, or who will provide ongoing monitoring and support.

Detailed NHS England guidance is expected in early 2025.

It is possible that around 3.4 million people could eventually qualify for Mounjaro.

To help manage demand, Nice has said the NHS can take more than a decade to roll out access to the drug. It is thought 300,000 could benefit in the first three years.

Separately the government has already announced plans to offer weight loss jabs to unemployed people in England living with obesity.

Prime Minister Sir Keir Starmer said the move could be “very important” for the economy and individuals’ health.

But weight-loss specialists have warned that NHS obesity services are already struggling to deal with demand for these drugs, and that the injections should only be used as part of a wider treatment plan.

Wegovy and Mounjaro can both be bought privately, including at some supermarkets, chemists and high-street clinics.

Pens typically cost between £200 and £300, depending on the dose.

The drugs should not be bought from non-regulated sources.

A BBC investigation in November 2023 found online sellers offering semaglutide as a medicine, without a prescription.

It also found the drug being offered in beauty salons in Manchester and Liverpool.

The most common side-effects are feeling sick, vomiting, bloating, constipation and diarrhoea. Some people report hair loss.

For many the side-effects are manageable and tend to go away in time; others say they stopped taking the drugs because of them.

In rare cases, serious side-effects can include gallbladder and kidney problems and depression.

Experts warn the complications can be worse if the drugs are abused, for example, taken as a quick fix to lose a few pounds or bought from unregulated online sellers.

Doctors say they have seen dangerous, life-threatening complications such as inflammation of the pancreas in such cases.

That is why experts say they should only be used under medical supervision.

Research suggests people do put most of the weight back on within a year of stopping the drugs, as their normal food cravings return.

That is why experts recommend that patients improve their diet and increase the amount of exercise they take while on the treatment.

Health Secretary Wes Streeting has warned about the danger of people developing a “dependency culture” on these drugs.

In the past there have been reports of Wegovy shortages, although the manufacturer Novo Nordisk says the NHS has a protected supply.

The term obese describes a person who has excess body fat.

In the UK it’s estimated that around one in every four adults is obese.

The standard way of categorising people’s weight is by calculating their body mass index, or BMI, which means dividing an adult’s weight in kilograms by the square of their height in metres.

For most adults, if your BMI is:

• below 18.5 – you’re in the underweight range
• 18.5 to 24.9 – you’re in the healthy weight range
• 25 to 29.9 – you’re in the overweight range
• 30 to 39.9 – you’re in the obese range
• 40 or above – you’re in the severely obese range

Those with an Asian, Chinese, Middle Eastern, Black African or African-Caribbean family background need to use a lower BMI score to measure overweight and obesity:

• 23 to 27.4 – overweight
• 27.5 or above – obese

BMI has some limitations because it measures whether a person is carrying too much weight but not too much fat.

For example, people who are very muscular can have a high BMI without much fat.

But for most people, it is a useful indication of whether they are a healthy weight.