Weight-loss injections explained

How do weight-loss injections work?

Weight-loss injections like Mounjaro (tirzepatide) and Wegovy (semaglutide) have gained widespread attention over the past few years as an effective new treatment for weight loss for those living with excess weight.

They’re known as GLP-1 agonists because they mimic a hormone your body releases after eating called GLP-1 (glucagon-like peptide-1).

GLP-1 agonists were originally designed to treat type 2 diabetes by triggering the release of another hormone called insulin into the bloodstream, which lowers blood sugar levels.

But GLP-1 also slows down your digestion, lowering your appetite and making you feel fuller after eating.

This means people taking them feel less hungry, so they eat less and lose weight.

Being a healthy weight is important because living with excess weight and obesity increases your risk of many health problems, including heart and circulatory diseases like heart attack, stroke and coronary heart disease that are cardiovascular diseases.

Now research is suggesting that weight-loss injections not only aid weight loss for people with diabetes, but also reduce the risk of having a heart attack or stroke or dying from heart disease in people with excess weight, even if they do not have diabetes.

But weight-loss injections are not a “quick fix” to losing weight, says the UK's Medicines and Healthcare products Regulatory Agency (MHRA).

And the National Institute for Health and Care Excellence (NICE) recommends eating a reduced-calorie diet and taking more physical activity while using weight-loss injections.

So, you'll need to make healthy lifestyle changes while taking them. This includes: 

• Eating a balanced diet – if you're less hungry and eating less food, you need to make sure your diet still has enough of the protein, vitamins and minerals your body needs. Eat a good balance of fruits, vegetables, wholegrains, lean meat, fish, pulses, and low-fat dairy foods. Avoid processed foods high in saturated fat, sugar and salt.
• Staying physically active – research shows that weight-loss injections can lead to a loss of muscle mass as well as fat. Regular exercise and doing some resistance training can help you maintain muscle as you lose weight. 

Can you get weight-loss injections on the NHS?

If you’re thinking about trying weight-loss injections, speak to your doctor first.

There are 3 different weight-loss injections available on prescription in the UK – Mounjaro (tirzepatide), Wegovy (semaglutide) and Saxenda (liraglutide). 

All 3 are designed to be self injected – see below for more details on these medicines. 

When it comes to getting them on the NHS, there are different rules for each injection and access varies between the UK's devolved nations. 

Wegovy and Saxenda are only available on NHS prescription in England, Scotland and Wales from specialist weight-loss clinics, which your GP can refer you to. 

There are specific criteria you'll need to meet before being prescribed them, and at the moment you can only be prescribed them for up to 2 years. 

Mounjaro is also available on the NHS from specialist weight-loss clinics, and at present there’s no time limit on prescriptions.

In England, Mounjaro is also being rolled out in GP surgeries and community clinics. But it's only available for people who have a number of certain conditions caused by obesity.

So, if you meet the criteria, your GP may be able to prescribe it alongside services to help you to eat a healthy diet and increase your physical activity.

Weight-loss injections are not available in Northern Ireland yet as there are no specialist weight-loss services there, although the first is planned to open in 2026.

Do weight-loss injections have side effects?

Not everyone will have side effects from weight-loss injections. But it is very common for people who take them to experience digestive problems, such as feeling sick, diarrhoea, constipation and stomach pain.

These side effects affect 1 in 10 people, according to the patient leaflets that come with Mounjaro and Wegovy.

The injections can also cause tiredness or dizziness, hair loss, low blood sugar and a fast heart rate in some people.

Speak to your doctor if you’re experiencing any side effects, as you may need to take a lower dose or stop taking them.

You may have also seen lots of media coverage about people taking weight-loss injections being at a higher risk of an inflamed pancreas (acute pancreatitis).

It’s estimated that up to 1 in 100 people taking either drug may develop the condition, according to the patient leaflets.

Speak to a GP or call 111 if you notice any symptoms, such as a sudden severe pain in the middle of your abdomen.

If you’re eating less food while taking these drugs, you may be at risk of poor nutrition. And rapid weight loss may lead to reduced muscle and bone mass too.

That’s why eating a healthy balanced diet is still important. Small, regular meals containing enough lean protein, calcium and vitamin D, and regular exercise can help to protect your muscle and bone health.

MHRA has also warned that Mounjaro may lower the effectiveness of the contraceptive pill.

It recommends using another non-oral form of contraception, like a condom, instead.

The MHRA also says you should not use weight-loss injections if you are pregnant, trying to conceive or breastfeeding.

What happens when you stop weight-loss injections?

Weight-loss injections can be effective for weight loss. But research suggests it can be hard to keep the weight off after you stop taking them.

A study published in 2022 in the journal Diabetes, Obesity and Metabolism of 327 people with obesity found that the people given a weekly injection of semaglutide lost 17.3 per cent of their body weight after 68 weeks on average.

But a year after stopping treatment, they had regained around two-thirds of the weight they’d lost – putting 11.6 per cent of their original body weight back on.

Their blood pressure and total cholesterol levels, which had dropped while they were taking semaglutide, also returned to around the same level they’d been at the start of the study.

In comparison, those given a dummy injection lost 2 per cent of their body weight during the first 68 weeks of the study and regained 1.9 per cent after they stopped taking it.

However, it’s worth noting that twice as many people in the study took semaglutide compared to people who took a dummy injection, which could artificially inflate the results, and this is still a growing area of research.

Having a healthy diet and taking regular exercise while you’re on weight-loss injections will help you manage your weight when you stop taking them.

You can also reach out to your healthcare team if you’re finding it hard to manage your weight after coming off weight-loss injections.

NICE has provided guidance that you should be offered support for at least a year to help maintain your weight loss after you stop taking weight-loss injections.

This includes regular check-ups, practical advice on how to make healthy changes part of your routine, and sharing other places you can get support, such as online resources or local weight-loss groups.

The 3 weight-loss injections

Mounjaro (tirzepatide)

Mounjaro is injected once a week. It contains the active ingredient tirzepatide.

Research suggests that tirzepatide could be more effective for weight loss than semaglutide (Wegovy).

A study published in the New England Journal of Medicine in 2025 found that people on tirzepatide lost 20.2 per cent of their body weight over 72 weeks, while those on semaglutide lost 13.7 per cent.

This may be because, as well as GLP-1, tirzepatide mimics another hormone in the body called GIP (glucose-dependent insulinotropic polypeptide), which can also reduce your appetite.

Mounjaro is planned to be rolled out to 220,000 people in England by the end of 2027.

This is being done via specialist weight-management clinics, GP surgeries and community clinics, which must offer services to support people to reach a healthy weight and increase their physical activity too.

NHS prescriptions for Mounjaro are limited to people meeting the below criteria – with the most at need receiving them first:

• a BMI of 40 or more (or 37.5 if they’re of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean origin)
• at least 4 of the following conditions: heart or circulatory disease, high blood pressure, high cholesterol, type 2 diabetes and obstructive sleep apnoea.

Wegovy (semaglutide)

Wegovy is a weekly injection that contains the medicine semaglutide.

Semaglutide is also prescribed as a treatment for type 2 diabetes under the brand name Ozempic.

It was approved for weight loss by NICE in 2023 for people who have at least 1 weight-related health problem and a BMI of either:

• 35 or more (or 32.5 or more if you’re of Asian, Chinese, Middle Eastern, Black African or African-Caribbean origin)
• 30 to 34.9 (or 27.5 to 32.4 if you’re of Asian, Chinese, Middle Eastern, Black African or African-Caribbean origin) and meet the criteria to be referred to a specialist weight loss clinic.

There’s evidence that semaglutide can also prevent heart problems.

A 2023 study published in the New England Journal of Medicine of 17,604 people with a heart or circulatory disease found that the 8,803 people taking semaglutide were 20 per cent less likely to have a heart attack, stroke or die from coronary heart disease than the 8,801 people taking a dummy injection over nearly 3 years.

In July 2024, the MHRA acted on this research by approving the use of semaglutide for people with obesity, who already have cardiovascular disease, to reduce their risk of having a heart attack or stroke or dying of heart disease.

However, the drug is not being prescribed to reduce the risk of cardiovascular disease on the NHS yet, as NICE is still reviewing the evidence before releasing new guidelines in the future.

Saxenda (liraglutide)

Saxenda is the brand name for a weight-loss injection called liraglutide. It’s also sometimes called Nevolat.

Unlike semaglutide or tirzepatide, liraglutide is injected every day.

It was approved by NICE for weight loss for people with all of the following:

• a BMI of 35 or more, or a BMI of 32.5 or more if you’re of Asian, Chinese, Middle Eastern, Black African or African-Caribbean origin
• non-diabetic hyperglycaemia (high blood sugar levels seen in pre-diabetes)
• a high risk of cardiovascular disease due to risk factors like high blood pressure or high cholesterol.

Studies suggest that, while still effective, liraglutide leads to less weight loss than semaglutide.

Research published in 2022 in JAMA of 338 people with obesity found that those given a daily injection of liraglutide lost 6.4 per cent of their body weight after 68 weeks, while those given a weekly injection of semaglutide lost 15.8 per cent on average.

What about weight-loss pills?

At the moment, GLP-1 agonist medicines designed to aid weight loss are only available as injections. But there are several weight-loss pills in development which work in a similar way.

However, more research is needed before these pills can be approved for use in the UK.

A daily pill containing semaglutide called Rybelsus can be offered on the NHS to people with type 2 diabetes to lower their blood sugar levels, but not for weight loss.

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