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

Weight-loss injections, such as Wegovy or Mounjaro, 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 else you should be aware of, with input from leading experts. 

Do weight-loss injections work?

Yes, weight-loss injections can work. But they won't be suitable for everyone and it's important to remember that 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 regulating 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.

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.  

There's also evidence 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

What's the process for taking weight loss jabs?

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.

You usually start on a low dose for four weeks to allow your body to adapt. After this, the dose generally increases every four weeks up to the maximum dose (if clinically appropriate) – although you can stay on a lower dose for longer if you're happy with your weight loss or are experiencing side effects. 

Once you've reached your desired weight, you can either go onto a lower maintenance dose or stop altogether.

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

Wegovy and Ozempic both contain semaglutide as the active ingredient. 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 strict criteria). 

In the US, a pill version of Wegovy has just been launched. Pill versions are likely to be available in the UK at some point in the future (although approval is still pending here) and should make it both much more affordable and convenient to take these drugs.

Ozempic

Ozempic is approved for people with type 2 diabetes to help manage blood sugar levels, although 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 licensed - and shouldn't be used - for weight loss.

Mounjaro 

Mounjaro targets two hormones instead of just one. Like Wegovy and Ozempic, 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). 

Another option is 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.

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Can you get weight-loss injections on the NHS?

Yes - some people are eligible for weight-loss jabs via the NHS, and both Wegovy and Mounjaro are available to patients who meet certain guidelines set by the National Institute for Health and Care Excellence (NICE). However, it is thought just 5% of those who use the drugs get them via the NHS, with the vast majority accessing them privately.

Eligibility criteria are slightly different for Wegovy and Mounjaro, and how you access them is different too. 

NHS eligibility criteria

Mounjaro

The NHS is currently prioritising Mounjaro for those with the highest clinical risk. In this initial stage, eligibility is limited to:

• those with a BMI of 40kg/m2 or over (may be lower for some ethnic groups who are higher risk)
• AND at least four weight-related health conditions (such as type 2 diabetes, high blood pressure or cardiovascular disease)
  

Access is being slowly expanded to ensure availability of 'wraparound' support — including diet and exercise coaching — for patients. This is a mandatory participation requirement. If a patient refuses or fails to engage with the support programme, their prescription may be stopped. 

Wegovy

To qualify for Wegovy on the NHS, patients must have at least one pre-existing, weight-related health condition, 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. Check the NHS guide to weight loss injections for full details.

Wegovy can currently only be prescribed via the NHS via an Specialist Weight Management Service, so you'll need to be referred to this by your GP, and for a maximum of two years.

If you are prescribed weight-loss jabs on the NHS,  they will cost the same as a standard NHS prescription (currently £9.90), unless you qualify for free prescriptions.

Dietary and lifestyle support is important to get maximum benefits from the drugs and also to ensure you still get enough nutrients and retain muscle mass as you lose weight.

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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. 

Prices for Mounjaro start at around £150 for the 2.5mg dose, rising to above £315 for the 15mg dose.

For Wegovy, 0.25mg starting doses costing from around £100 and the highest maintenance dose (2.4mg) is usually around £240. 

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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.  These thresholds are often 2.5kg/m2 lower for certain ethnic groups.

While this process usually begins with an online form, 2025 regulations from the General Pharmaceutical Council (GPhC) mean that a questionnaire alone is no longer sufficient. To prevent people 'duping' the system, providers must now independently verify a patient's identity and BMI. 

This is usually done via a mandatory video consultation, or by the provider accessing your NHS GP records. Any site that offers medication without these checks is likely to be operating outside of UK law. 

This is a red flag as it may mean you likely aren't getting legitimate medication, which puts your health at risk.

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Where to buy weight-loss jabs

There are numerous online pharmacies offering private prescriptions for Wegovy and Mounjaro and prices vary.

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

• Asda Online Doctor 
• Boots Online Doctor
• Lloyds Pharmacy Online Doctor
• Superdrug Online Doctor
• Well pharmacy

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.

Beware dangerous counterfeit weight-loss drugs

The Medicines and Healthcare products Regulatory Agency (MHRA) has issued several urgent alerts regarding the dangers of widespread counterfeit weight-loss drugs.

In October 2025, the MHRA uncovered and dismantled an illicit manufacturing facility for weight-loss medicine in the UK. Criminals were not just making fake versions of Mounjaro and Wegovy but also retatrutide, an experimental drug that is still in clinical trials and has not yet been approved for use anywhere in the world.

Criminals have also been found re-labelling insulin pens as Mounjaro. Injecting insulin when you have no medical need for it can cause hypoglycaemic shock, coma and death. There have been several cases of severe illness and hospitalisation following use of unlicenced drugs. 

How to spot a dodgy jab

Watch for these red flags:

• DIY kits: Any product that arrives as a powder with a separate syringe for you to mix yourself is unlicensed and highly dangerous.
• Social media sellers: Legitimate pharmacies do not sell via TikTok, Instagram, or WhatsApp. They also will never ask you to pay via bank transfer or Crypto - these are signs of a scam. 
• Unrealistically low prices:  With the wholesale cost of high-dose Mounjaro now at £247.50, any seller offering a month's supply for under £150 is likely to be selling fake product.
• Packaging errors: Look for spelling mistakes, blurred text, or odd fonts on the box. Real pens have high-quality, crisp printing.

If you suspect you have purchased or seen a fake pen, do not use it. You should report it immediately to the MHRA Yellow Card Scheme.

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

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Possible side effects of weight-loss injections

Weight-loss jabs 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. Blindness is also a very rare side effect.

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.

Weight-loss injections and pregnancy/contraception

The MHRA says weight-loss jabs shouldn't be taken during pregnancy, while trying to conceive or during breastfeeding because there isn't enough safety data to know whether they could cause harm to the baby.  If you get pregnant while using them, speak to a healthcare professional and stop them as soon as possible. 

If you're taking the oral contraceptive pill as well as Mounjaro, use an additional contraceptive method, such as condoms. This is because weight-loss jabs may reduce the effectiveness of oral contraceptives in people who are overweight or obese. 

You may want to consider switching to an alternative contraceptive option, such as a coil or implant.

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Other weight-loss medications: Orlistat, Xenical and Alli

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. It reduces the amount of fat your body absorbs by around 30%.

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

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

Although 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 in 2024 that she was seeing increasing numbers of patients in A&E with issues from 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.'

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.

Which? investigation reveals some pharmacies are still prescribing jabs inappropriately, prompting regulator concerns

How to dispose of used weight loss jabs

Don't put your used jabs in the bin. Because these pens all contain needles, they should be safely disposed of in a sharps bins or container. 

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

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

Novo Nordisk – the brand that makes the pens – has started a recycling scheme called PenCycle. You can pick up a box from your local participating pharmacy or order it online. Then fill it with pens (it holds 12) and drop it off or post it free of charge. Find out more on the Pencycle minisite.

Our weight-loss jab experts

Shefalee Loth, 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.

Sara Spary, senior researcher and writer

Sara is a highly experienced journalist who has worked at Which? since 2021 with a special focus on online marketplaces, health and wellbeing and investigative research  

She helps readers find the best opticians, debunks the health products you don't need and reveals health products you should never buy at full price.

External experts

• Professor Giles Yeo, 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 Marie Spreckley is a registered nutritionist and researcher specialising in weight management research and clinical practice from the Institute of Metabolic Sciences and the School of Clinical Medicine at the University of Cambridge. 

• 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 eligibility information last checked: May 2026