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

view original post

There are currently thought to be about 1.5 million people on weight-loss drugs in the UK.

Some people are able to get the injections – called Mounjaro and Wegovy – through the NHS but most buy them privately.

The jabs are highly effective, but experts say users should be aware of the risk of weight gain once they stop taking the drugs.

How do Wegovy and Mounjaro work?

There are two drugs on the market – semaglutide, marketed under the brand name Wegovy, and tirzepatide, sold as Mounjaro.

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

Both drugs work as an appetite suppressant by mimicking a hormone called GLP-1 which makes people feel fuller. Mounjaro also affects metabolism and helps regulate energy balance.

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

How much weight do people lose and what happens when people stop taking the drugs?

People 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% for Wegovy.

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

However, research published in the British Medical Journal suggests people put most of the weight back on quickly, as their normal food cravings return.

It found that slimmers who came off weight-loss injections regained their lost pounds four times faster than those who stopped a programme of conventional dieting and exercising.

The NHS has been urged to provide regular check-ups, support and advice to help prevent this rebound weight gain.

It recommends the jabs for people who are overweight with obesity-related health risks – not people who just want to get a bit thinner.

Doctors should also prescribe lifestyle changes that include eating healthily and getting enough exercise to help people keep the weight off.

Who can get weight-loss drugs on the NHS?

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

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 2025, 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 these 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.

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

However, 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.

What do Wegovy and Mounjaro cost to buy privately?

Wegovy and Mounjaro can both be bought privately, in supermarkets, chemists and high-street clinics. Estimates suggest nine in 10 pay for the drugs privately.

Pens typically cost between £100 and £300, depending on the dose.

The drugs should not be bought from unregulated sources.

Since February 2025, online pharmacies have been required to 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 also be verified.

What are the side-effects of weight-loss injections?

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.

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.

What are the warnings about pregnancy?

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.

The MHRA recommends that women taking the jabs consider using additional or alternative contraception.

What is obesity and what is a healthy BMI?

The term obese describes a person with 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.

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:

• 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 also have a high BMI.

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