How do weight-loss drugs like Mounjaro and Wegovy work?

Some NHS patients in England can now get the weight-loss drug Mounjaro from their GP.

Mounjaro and Wegovy, another weight-loss jab, were already available through specialist NHS clinics and can be bought privately.

The drugs will be offered to millions more patients in the coming years, but experts warn services are struggling with unprecedented demand.

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 type 2 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 is an intestinal hormone which 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 level dose which is gradually increased until they reach a higher maintenance dose.

The UK's medicines regulator, the MHRA, has warned women that the lack of information about how the drugs could affect unborn children means they must not be taken:

• during pregnancy
• while trying to get pregnant
• during breastfeeding

The drugs can also stop the contraceptive pill being absorbed properly, so the MHRA also recommends that some women taking the jabs use additional or alternative contraception.

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

The first head-to-head trial of the two drugs suggested that Mounjaro was more effective.

Mounjaro's 20% weight reduction, after 72 weeks of treatment, exceeded the 14% from Wegovy.

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

Previous clinical studies suggested 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 have suggested Mounjaro users might lose up to a fifth of their weight.

Crucially, users of either drug can put weight back on once they stop treatment.

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Wegovy is currently available on the NHS in England, Wales and Scotland.

It must be prescribed by a weight management specialist.

To qualify, adults must be obese (with a BMI of at least 35) 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.

Mounjaro has been available from specialist NHS weight-loss clinics in England and Wales since March 2025.

In June, some GPs in England started offering it to NHS patients who meet strict criteria:

• a BMI of 40 or over (or 37.5 if from a minority ethnic background)
• and four out of five of the following conditions: type 2 diabetes, high blood pressure, heart and vascular disease, high cholesterol and obstructive sleep apnoea

Patients who are prescribed Mounjaro will also get regular check-ups, exercise support and advice on healthy eating.

But the drug is not available from all GPs. People are urged not to approach their local surgery unless they are eligible.

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

But the body which assesses how the NHS uses drugs, the National Institute for Health and Care Excellence (NICE), has warned that it could take more than a decade to roll out access to everyone who could benefit.

Separately, the government has said it also 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.

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Wegovy and Mounjaro can both be bought privately, in 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.

Under guidance introduced in February 2025, online pharmacies must employ stricter checks to stop people who are already a healthy weight or have a history of eating disorders buying weight-loss jabs.

The BMI of patients who have completed online questionnaires or sent photos must be verified through in-person or video consultations, and checked with information from their GP or medical records.

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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.

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.

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

There have also been hundreds of reports of people suffering problems with their pancreas - although none are confirmed as being caused by the medicines.

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.

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.

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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 weight, not fat. Someone who is very muscular can have a high BMI without having excess fat.

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