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 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 that all treatments have their pros and cons. A recent systematic review published in the BMJ in January 2026 suggests a significant one is that once you stop using the jabs you can rapidly regain the weight you lost.

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.

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.  

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. 

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). 

In the US, a pill version of Wegovy has just been launched. Pill versions are likely to make it to the UK soon (though 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, 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 

Mounjaro 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). 

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.

Zoe nutrition app: is it worth it? Get our nutrition experts verdict after she tried it out, plus, the views of hundreds of users of the app

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). 

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

You may be able to get Mounjaro via your GP (without needing a referral to the NHS specialist weight management service, or SWMS), but access is being staggered and it will only be available for those with the highest clinical need first.

Whichever route you go, you will be given dietary and lifestyle advice too. This is important to get maximum benefits and also to ensure you get enough nutrients and retain muscle mass.

NHS eligibility criteria

Mounjaro

Under a phased 12-year rollout plan which started in June 2025, 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 'wraparound' support — including diet and exercise coaching — is available for every patient. As of December 2025 this is now a mandatory participation requirement. If a patient refuses or fails to engage with the support programme, their prescription can 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.

Read our reviews of popular multivitamins to find the best multivitamin supplement

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.  However, since September 2025 prices have risen steeply. Prices for Mounjaro now typically start at around £149 for the 2.5mg dose, rising to between £300 and £375 for the 15mg maintenance dose.

Wegovy prices have remained more stable, with starting doses costing around £130 and the highest maintenance dose (2.4mg) usually capped at £295. 

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.

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

Beware dangerous counterfeit weight-loss drugs

As high demand and rising prices have squeezed the market, the Medicines and Healthcare products Regulatory Agency (MHRA) has issued several urgent alerts regarding the dangers of widespread counterfeit weight-loss drugs, with increasingly sophisticated schemes duping consumers.

In October 2025, the MHRA uncovered and dismantled an illicit manufacturing facility for weight-loss medicine in the UK. The raid uncovered tens of thousands of empty injection pens and over 2,000 unlicenced ones ready for dispatch.

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 specific red flags, as advised by the MHRA:

• 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.

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.

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.

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

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

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

Keen to get into running? See our round-up of the best running shoes

Microdosing weight loss jabs: what is it and is it recommended?

As prices for weight-loss injections have risen, a trend known as 'microdosing' has gained traction, particularly on social media. This involves taking a significantly smaller amount of the medication than the official 2.5mg (Mounjaro) or 0.25mg (Wegovy) starting doses prescribed by doctors.

There are several aims or goals behind 'microdosing'. Some people may wish to avoid side effects of the drugs by 'easing' the body into the medication to try and avoid common issues like nausea or bloating. Others may be trying to cut costs, by making a single injection pen last longer than the intended 30 days. Others may be trying to attempt to maintain their target weight by using the smallest possible amount of the drug as a maintenance dose.

However, UK medical regulators (MHRA), the General Pharmaceutical Council (GPhC), and the manufacturers themselves (Eli Lilly and Novo Nordisk) do not recommend microdosing, for several reasons.

• Unsafe pen use: Mounjaro and Wegovy injection pens are designed to click into place at specific, tested doses. To microdose, users may try to 'count clicks' or even to dismantle the pen. This can lead to inaccurate dosing and also considerably increases the risks of the medication becoming contaminated with bacteria.
• Using pens for too long: Once a Mounjaro pen is opened it must be used within 30 days, or 60 days for Wegovy. Microdosing could lead to someone keeping a pen for two or three months. At this stage the preservative in the liquid breaks down, and the drug not only loses its effectiveness but may also become unsafe to inject.
• Lack of clinical evidence: Weight loss results seen in clinical trial are all based on the standard dosage and schedule. There is currently no clinical data that proves microdosing provides lasting weight loss or heart health benefits.
• Risk of plateau: Taking smaller doses than recommended can stop the drug reaching the levels required in your bloodstream to operate effectively, silencing 'food noise'. This can often lead to a weight-loss plateau. 

Both the MHRA and the GPhC have warned against 'off label' use such as microdosing. Official advice is to stay on a lower licenced dose for a longer period rather than trying to tamper with pens to create your own doses. You should always consult your prescribing doctor or pharmacist before adjusting your dosage.

Does protein powder support your muscles? Get our expert nutritionists advice and see our top protein powder picks

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.

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. 

Semaglutide (Wegovy and Ozempic) should be stopped at least two months before pregnancy and tirzepatide (Mounjaro) at least one month before.

You need to take precautions if you are trying not to get pregnant too. If you're taking the oral contraceptive pill and taking 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. 

Alternatively, switch to an alternative contraceptive option, such as a coil or implant.

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

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.

Best multivitamin supplements reviewed - independent expert reviews of the best multivitamins you can buy

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 eligibility information last checked: January 2026