Weight-loss injections: how they work and how to buy them safely

Weight-loss injections, such as Wegovy, are available to some people on the NHS. They can also be bought via private prescription if you meet certain criteria.

In this guide we outline what you need to know about weight-loss drugs, how they compare, how you can access them and what to be aware of, with input from leading experts.

Eat well, live better and stay healthy. Sign up for our Food & Health newsletter, it's free monthly

Do weight-loss injections work?

Yes, weight-loss injections can work. But experts we spoke to warned they are not a miracle cure for obesity – despite any attention-grabbing press headlines you may have seen. They won't be suitable for everyone and it's important to remember all treatments have their pros and cons.

Weight-loss jabs work by mimicking hormones in your body that send signals to the part of your brain responsible for appetite and hunger. 

Known collectively as GLP-1 receptor agonists, they slow digestion and increase the time it takes for food to leave your stomach, altering brain signals to help you feel fuller for longer, reducing appetite and increasing insulin production to regulate blood glucose levels.

The drugs come in pre-measured doses in a single-use injector pen, administered as a weekly injection in your upper arm, stomach or thigh.

People taking Wegovy or Mounjaro typically start to lose weight within two to four weeks of starting the drug. 

In trials, weight-loss drugs have helped people lose up to 22% of their body weight and, as a result, reduce their risk of weight-related co-morbidities, including diabetes, high blood pressure, cardiovascular disease and several cancers.  

Healthy living

Use our expert advice and recommendations to live your best life every day.

Get started

There's also evidence, such as from a 2023 New England Journal of Medicine paper, that these drugs, independent of weight loss, reduce the risk of heart attacks and strokes, and reduce cravings for high-fat, high-sugar foods and alcohol. 

Patients usually start on a low dose and stay on this for four weeks to allow their bodies to adapt and get used to any side effects. 

After this, the dose generally increases every four weeks up to the maximum dose – although you can stay on a lower dose for longer if you're happy with your weight loss or are experiencing side effects. 

Need to track your blood pressure? Check our reviews to find the best blood pressure monitors

Wegovy vs Ozempic vs Mounjaro: what's the difference?

Wegovy and Ozempic both contain semaglutide as the active ingredient, but they’re used for different reasons and at different doses. Mounjaro contains tirzepatide and works slightly differently. 

Wegovy 

Wegovy is approved for weight management in people with obesity, or those who are overweight and have related health conditions. It’s given at a higher dose than Ozempic to enhance its weight loss effects. It's available on private prescription and via the NHS for weight loss in the UK (if you meet certain criteria).

Ozempic

Ozempic is approved for people with type 2 diabetes to help manage blood sugar levels, though weight loss can be a side effect. 

Ozempic and Wegovy work in the same way – by mimicking a hormone (GLP-1) that helps control appetite and blood sugar – but their purpose and dosing is different. Ozempic isn't registered for, and shouldn't be used for, weight loss.

Mounjaro 

Moujaro works in a similar way to Ozempic and Wegovy, but it targets two hormones instead of just one. Like the others, it mimics GLP-1 to help control appetite and blood sugar, but it also works on GIP, another hormone that plays a role in metabolism. 

This two-hormone approach seems to lead to even better results for both weight loss and blood sugar control. It's approved for use for both diabetes and weight loss, and is available on private prescription in the UK (it's sold under the brand name Zepbound for weight loss in other countries, but not in the UK). 

You might also have heard of Saxenda, which works in a similar way to the drugs above – using the active ingredient liraglutide. This, however, requires daily rather than weekly injections. It's been around for longer and is also available via private prescription or the NHS weight management service.

Zoe nutrition app – is it worth it? Get our nutritionist's expert review of the popular gut health app, plus insights from users on if it worked for them

NHS weight-loss injections – are they available?

Some people will be eligible for weight-loss jabs via the NHS and both Wegovy and Mounjaro are available. 

To qualify, patients must meet certain guidelines set by the National Institute for Health and Care Excellence (NICE). 

NHS eligibility criteria

Adults must have at least one pre-existing, weight-related health condition, such as high blood pressure, cardiovascular disease or osteoarthritis, and:

• a Body Mass Index (BMI) of 35kg/m2 or more, OR
• a BMI of 30kg/m2 or more and qualify for referral to a specialist weight management service (SWMS) and have unsuccessfully tried to lose weight through other methods. 

Some people, such as those from certain minority ethnic backgrounds, or who have cardiovascular disease, are eligible at a lower BMI threshold of 27.5kg/m2.

Under current guidelines, Wegovy should be prescribed at the same time as a reduced-calorie diet and physical exercise, and it can only be prescribed for a maximum of two years. It can currently only be prescribed via the NHS via an SWMS, so you'll need to be referred to this by your GP.

Mounjaro should start to become available from March 2025. However, supply of both is limited by funding and availability of support services, and will roll out to those with the greatest clinical need first. 

The cost is that of a standard NHS prescription (currently £9.65), unless you qualify for free prescriptions.

Can you buy weight-loss jabs privately and how much do they cost? 

Yes – both Wegovy and Mounjaro are available to buy through private prescription in the UK. All the major high street pharmacy chains sell them, as well as specialist online pharmacies. 

It's not cheap though. The starting dose of Wegovy (0.25mg) costs from around £130 a month. The price rises as the dose increases (roughly every four weeks up to a maximum level). The maximum dose (2.4mg) costs upwards of £269 a month.

The costs for Mounjaro are similar, starting from £130 a month for the lowest dose (2.5mg), up to £180+ a month for the highest dose (15mg).

Eligibility criteria for private weight-loss jab prescriptions

Private prescriptions for weight-loss jabs have a lower threshold than NHS ones. Patients must have a body mass index (BMI) of 30kg/m2 or more, or 27kg/m2 or more plus a weight-related medical condition. 

You'll typically need to complete an online form with questions about your weight, medical history and any current medications. New rules mean you should also have an in-person or video consultation, or supporting info from your GP or medical records, to be approved.

These rules were rolled out by the General Pharmaceutical Council (GPhC) for 2025, following reports by Which? and others of it being too easy to dupe online verification processes.

Get the lowdown on ultra-processed food: fact vs fiction

Weight-loss injections: what you need to know, from the experts

Possible side effects of weight-loss injections

Weight-loss jabs such as Wegovy and Mounjaro can have unpleasant side effects, the most common being nausea and vomiting, heartburn, constipation, diarrhoea, stomach pains and headaches, which affect around 10% of people taking them. 

More severe, but rarer side effects reported have included pancreatitis, kidney problems and thyroid tumours. If you have retinopathy – a diabetic eye disease – and use insulin, using Wegovy can make your vision worse.

Other medications can interact with weight-loss drugs, and they aren't suitable for everyone, so check with your GP first if you're considering buying weight-loss injections privately.

Get the facts behind the fad diets with our expert run-down of whether popular diets work

Where to buy weight-loss jabs

There are numerous online pharmacies offering private prescriptions for Wegovy and Mounjaro and prices vary. But be very careful about where you buy from and make sure you use legitimate providers.

It's not worth risking your health by buying from unauthorised sources. You may end up with fake products that could be harmful or that haven't had the appropriate safety checks.

For example, in October 2023, the Medicines and Healthcare Regulatory Agency (MHRA) issued a warning about fake versions of semaglutide pens available online where the contents didn't match the ingredients listed on the label. And in June 2024 the World Health Organization (WHO) put out a warning as it was reported that people had been hospitalised after buying 'fake Ozempic' online.

Big-brand pharmacies who sell weight-loss jabs include:

• Asda Online Doctor 
• Boots
• Lloyds
• Superdrug
• Well.

There are a range of smaller and online-only pharmacies too, but make sure you double-check that any pharmacy you wish to use is licensed and legitimate. 

Use the General Pharmaceutical Council (GPhC) guide to checking a pharmacy is registered and safe. Always check for a registered pharmacy logo and check the registration number shown on the GPhC website.

You should also check whether the service offers free ongoing support, such as diet and exercise management advice, access to a GP, and useful extras like free injector pen bins/collection (more on this below).

For more advice, check the Medicines and Healthcare Regulatory Authority (MHRA) guidance for accessing Wegovy and other weight-loss drugs.

If you suspect that you’ve had a side effect to semaglutide, liraglutide or any other medicine, or suspect it’s not a genuine product, you can report it to the MHRA Yellow Card scheme

Find out what works to lower your cholesterol – we compare lifestyle changes, drugs and food supplements

Other weight-loss medications

Another weight-loss medication available on prescription is Orlistat, also known by the brand names Xenical or Alli.

Orlistat is a pill that is taken at the same time as a meal and reduces the amount of fat your body absorbs from your meal by around 30%. Instead it excretes it as waste. 

There are criteria to meet before patients can be prescribed Orlistat, either on the NHS or privately, and these include:

• a BMI of 28kg/m2 or more and a related illness such as diabetes or heart disease
• a BMI of 30kg/m2 or more.

If you get Orlistat via a private prescription, a monthly dose will cost from around £58.

Orlistat isn't suitable for everyone. It interacts with some medications, such as blood thinners and treatments for rheumatoid arthritis, so it's important to check with a doctor before using it. 

There are also some unpleasant side effects associated with it, including bloating, headaches and an oily discharge from your bottom. 

The dangers of using weight-loss jabs when you don't need them

While for some people who have struggled with serious weight-related issues, weight-loss jabs can have a really positive impact, there are concerns about people using them inappropriately. This can have significant consequences for your health.

Dr Vicky Price, A&E consultant and president-elect of the Society for Acute Medicine, told us that she has treated increasing numbers of patients in A&E with issues arising from people using the jabs when they don't need them, including people with ketoacidosis and pancreatitis – conditions that can be fatal if not treated.

She told us: 'The majority of people I have seen have been of a normal weight that under no circumstances meets the required BMI for these medications. We are seeing people who have lied about their weight to online pharmacies and people who are getting these drugs from beauticians and other less reputable sources.'

The demand for these drugs has also caused shortages for people who really need them. Rules are now being tightened to try to minimise the risk of this happening. 

Be wary of weight-loss ads on social media

Weight loss is big business, and where there's high demand, there are scammers lurking.  

Exercise extreme caution with weight-loss products you see advertised online and on social media. If it looks too good to be true, it probably is. In the past, we've found dodgy and fake products online and fake celebrity diet pill endorsements.

It's not worth risking your health or your money.

What to do with used Wegovy or Ozempic pens

Because Wegovy, Mounjaro and Ozempic pens all contain needles, they are considered clinical waste and should be disposed of in a safe way, ideally in a sharps bins or container. 

You can get these from your pharmacy or online. Some services provide these as standard.

Once the bin is full you can return it via the service you are using, or ask your community pharmacy if they will accept the bin. Some councils also offer a collection service for clinical waste.

The amount of pens being used as these products become more widespread creates a massive amount of plastic waste. Novo Nordisk, the brand that makes the pens, has now started a recycling scheme called PenCycle. 

You can pick up a box from your local participating pharmacy, or order it online, fill it with pens (it holds 12) and drop it off or post it free of charge when full. Find out more on the Pencycle minisite.

Our weight-loss jab research

For our investigation into weight-loss jabs, Shefalee interviewed leading experts for their insights, and asked Which? members about their experiences. Meanwhile, our health researcher Sara Spary went undercover to see if pharmacies were doing appropriate checks.

Shefalee Loth, Which? principal researcher and nutritionist

Shefalee is a public health nutritionist with 20 years' experience, and has worked for the NHS, The London School of Hygiene & Tropical Medicine and World Cancer Research Fund

At Which? Shefalee is a principal researcher, researching and writing food and nutrition content for our magazine and website and is a media spokesperson. She is also a trustee at Sustain, the food and farming alliance.

She focuses on cutting through the hype to explain what you need to know about nutrition and healthy eating. From 'How bad is ultra-processed food?' to the 'Top gut health myths you need to know about' and 'What really works to cut cholesterol', Shefalee gives no-nonsense advice to help you make healthier choices and save money.

Additional experts for this piece:

Our health researcher, Sara Spary, is a highly experienced journalist and researcher who has worked on health and wellbeing content for Which? since 2023, including the best dentist brands, health products you don't need and our guides to the best opticians. 

For this story, she spoke to experts about the issues of inappropriate private prescriptions and illegal sales of these drugs to people who aren't obese, and went undercover to see how easy it was to access a prescription.

Giles Yeo, Professor, MBE We consulted Professor Yeo in his capacity as a geneticist interested in studying the brain's control of food intake and body weight, and how these might be dysregulated in obesity. He is a fellow of Wolfson College and Honorary President of the British Dietetic Association. 

Dr Vicky Price, President-elect at The Society for Acute Medicine Dr Price is an acute medical consultant at Liverpool University Foundation Trust. She has an interest in ambulatory care, medical education and end-of-life care. We consulted Dr Price on her experiences working in A&E, specifically the rise in acute cases linked to weight-loss injections.

Pricing and medical information last checked: 13 February 2025