How do drugs like Wegovy and Mounjaro work?

The NHS will provide millions of people with weight loss drugs to help them manage their health.

One of the vaccines, Wegovy, is already available to some people, and a second, Mounjaro, will be available soon – although it may take more than a decade for a full rollout.

NHS experts have warned that services are being stretched to meet unprecedented demand from patients who want to try weight-loss drugs.

There are two drugs on the market: semaglutide (trade name Wegovy) and tezepatide (trade name Mounjaro). Semaglutide is also used in the treatment of type 2 diabetes Ozempic.

Both Wegovy and Mounjaro are administered once a week via a prefilled pen that can be self-injected 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 gut hormone is released after eating and often makes people feel fuller.

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

Patients usually start at a low dose and gradually increase until they reach a higher maintenance dose.

People who take these medications usually start losing weight within a few weeks.

Clinical trials show that combined with changes in diet, exercise and behavioral support, Wegovy users can lose more than 10% of their body weight after one year of treatment.

Other trials suggest Mounjaro users could lose even more – as much as 20%. But it's difficult to compare the two treatments, and the bottom line is that users of either treatment can regain weight once they stop treatment

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

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

To be eligible, adults must be obese (BMI at least 35) and have at least one weight-related health condition, such as high blood pressure.

People with cardiovascular disease who are overweight but not completely obese can also take the drug.

According to NHS guidelines, patients should follow a diet and physical activity plan and the drug should only be taken for a maximum of two years.

The National Institute for Health and Care Excellence (NICE), the body that assesses the use of all medicines, recommends: Mounjaro will also be available to UK NHS patients from March 2025. This will be provided along with advice on diet and exercise.

Like Wegovy, it will initially only be available to obese patients with weight-related health conditions who receive specialist care.

However, starting in June, the drug will become more widely available. It is unclear how the situation will be managed or who will provide ongoing monitoring and support.

Detailed NHS guidance in England is expected to be published in early 2025.

About 3.4 million people may eventually qualify for Mounjaro.

To help manage demand, NICE said it could take more than a decade for the NHS to roll out the drug. It is thought that 300,000 people will benefit in the first three years.

Separately, the government has announced plans to offer a weight-loss vaccine to unemployed people in England suffering from obesity.

Prime Minister Sir Keir Starmer said the move May be 'very important' to the economy and personal health.

But weight loss experts have warned that NHS obesity services are already struggling to keep up with demand for the drugs and that the injections should only be given as part of a wider treatment plan.

Both Wegovy and Mounjaro are available for purchase privately, including in some supermarkets, pharmacies and high street clinics.

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

These medications should not be purchased from non-regulated sources.

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

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

The most common side effects are feeling unwell, vomiting, bloating, constipation and diarrhea. Some people report hair loss.

For many people, side effects are manageable and tend to go away over time. Others said they stopped taking their medications as a result.

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

Experts warn that complications can be more serious if these drugs are misused, such as as a quick fix for weight loss or purchased from unregulated online sellers.

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

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

Research shows People do regain most of their weight within a year of stopping the drug because their food cravings return.

That's why experts recommend patients improve their diet and increase exercise during treatment.

Health Secretary Wes Streeting warned of the dangers of developing a “culture of dependence” on these drugs.

There have been reports of shortages of Wegovy in the past, although manufacturer Novo Nordisk said NHS supplies were protected.

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

In the UK, it is estimated that around a quarter of adults are obese.

The standard way to classify people's weight is to calculate their body mass index (BMI), which means dividing an adult's weight in kilograms by their height in meters squared.

For most adults, if your body mass index is:

• Below 18.5 – You are in the underweight range
• 18.5 to 24.9 – You are within a healthy weight range
• 25 to 29.9 – You are in the overweight range
• 30 to 39.9 – You are in the obesity range
• 40 or above – you are in the severe obesity range

People with Asian, Chinese, Middle Eastern, Black African, or African-Caribbean backgrounds need lower BMI scores to measure overweight and obesity:

• 23 to 27.4 – Overweight
• 27.5 or above – Obesity

BMI has some limitations because it measures whether a person has excess weight, not excess fat.

For example, a muscular person can have a higher body mass index but not carry as much fat.

But for most people, it's a useful indicator of whether their weight is healthy.