Are weight loss injections safe? An expert guide

Weight loss injections have become one of the most talked about treatments in the UK thanks to celebrity mentions, endless before-and-after pictures online, and a rise in NHS and private prescriptions. 

On Mumsnet alone, discussions around weight loss injections have had more than 3 million views since January 2025*. 

These prescription-only medicines (POMs) - semaglutide (Wegovy) and tirzepatide (Mounjaro) - are designed to help people with obesity lose weight. Studies also suggest they may lower the risk of obesity-related health problems.

But alongside the buzz, there are warnings about fakes, unknowns around long-term safety, and the reality of unpleasant (and sometimes serious) side effects.

So, we’ve asked medical experts Dr Daniel of Treated and multi-award-winning pharmacist Jane Msumba to explain the real facts: who weight loss injections are suitable for, the risks, and how to use them safely.

As one Mumsnet user puts it: “I think you have to just do your research and make an informed decision for you and your lifestyle.

What are weight loss injections and how do they work?

“Weight loss injections are a type of medicine that mimic a hormone your gut makes after you’ve eaten food,” explains Dr Daniel. “In short, this hormone (GLP-1) tells your brain you’re full after eating. 

“Weight loss injections also cause food to pass through your gut at a slower rate, again helping you to feel fuller and lower your appetite.

“By working in this way, using these injections will generally mean you eat fewer calories, helping you to lose weight.”

In short, GLP-1 hormones:

• Reduce appetite

• Make you feel full

• Slow digestion

• Trigger the pancreas to release insulin, which can help to regulate blood sugar levels

As Jane Msumba, pharmacist and founder of Inner Glow Clinic, shares, “While all GLP-1 receptor agonists work by mimicking the natural hormone that helps regulate appetite and blood sugar, there are small differences between brands - for example, in how often they’re taken and how they’re tolerated. 

“Some are daily injections, like Saxenda, while others, like Wegovy or Mounjaro, are weekly. Your prescriber will consider both medical and lifestyle factors when choosing the most suitable option for you.”  

NHS-approved options for weight management include: 

• Orlistat (capsule, works differently by reducing fat absorption)

• Liraglutide (Saxenda, daily injection)

• Semaglutide (Wegovy, weekly injection)

• Tirzepatide (Mounjaro, weekly injection, licensed for weight loss in 2023)

All are prescription-only in the UK, and should be used alongside a healthy diet and regular exercise.

Consult with your GP if you have any concerns about your weight loss treatment

What Mumsnet users say

“Literally zero regrets. 2 stone down and one to go, it’s changed my life and I feel so much more confident in myself.” Startrekobsessed

“I wouldn't say I regret it, but I am finding it very very difficult to keep the weight off. If I have any more than 1000 calories a day then I gain weight. I'm 4 months off the jabs and gaining 2 pounds a month. I'm sooooo hungry. Feeling sick and tummy pain hungry. It seems most people have to stay on them for life, I don't feel like I can do that because a) I can't afford it b) I don't want to live with the exhaustion and c) I want to be able to go out for dinner and not insult the chef with the amount of food I waste on the jabs.” Howdoidoit100

“My concern is how people come off the jabs without putting the weight back on. Those I know have had initial success with weight loss with the jabs but they have not embraced healthy eating or become more active. If you are solely reliant on the jab to reduce your appetite and you do not change your diet and exercise habits then surely the weight just pile back on as soon as you come off it.” SilverGlitterBaubles

“I tried Mounjaro for 2 months and stopped as the horrendous constipation just wasn’t resolving no matter how much fluid and laxatives I took to help. Couldn’t wait for it to get out of my system. Maintaining a 25% weight loss with calorie counting and weighing weekly. That said, I've spoken to people who had no side effects at all and who say it’s changed their life.” WilmaFlintstone1

“There are some very common side effects eg diarrhoea and some very rare side effects eg pancreatitis. You have no way of knowing what, if any, side effects you will have before starting the medication. That's why it's very important to weigh up the risks and side effects of obesity first and to get it prescribed by a reputable provider - and only if you meet the criteria.” TheInvisibleLife

Who are weight loss injections for and who should avoid them?

Not everyone is eligible for weight loss injections. NHS criteria vary slightly depending on the drug: .

• Liraglutide (Saxenda): BMI of 35+ (or 32.5+ for people of certain ethnicities), or with high cardiovascular risk.

• Semaglutide (Wegovy): BMI of 35+ (32.5+ for certain ethnicities), or BMI 30-34.9 (27.5-32.4 for certain ethnicities) with other risk factors.

• Tirzepatide (Mounjaro): BMI of 40+ plus at least four weight-related conditions, though thresholds are lower for some ethnic groups.

Criteria can differ if you’re accessing the medication through private providers, though. “These types of treatments are suitable for people who are considered obese, which means their body mass index (BMI) is 30 or over,” says Dr Daniel. “You can also use them if your BMI is 27 or over, and you have a weight-related health condition like diabetes.

“As for who should avoid them, this would be anyone with a condition that is considered a risk factor for GLP-1s, such as kidney failure. But prescribers of weight loss treatments should always check individual suitability before recommending them.”

They’re not suitable if you are:

• Pregnant, trying to conceive or breastfeeding

• Living with severe liver or kidney disease

• Have a history of pancreatitis

• Have certain thyroid cancers (see below)

• Outside the recommended age range (eg Saxenda isn’t advised if you’re over 75)

Note on thyroid cancers: In the US, people with a history of medullary thyroid carcinoma (MTC) or MEN2 are advised not to take these medicines. In the UK this isn’t a listed contraindication, but prescribers may still be cautious.

Jane Msumba points out that numbers don’t tell the full story:

“Medical safety is essential, but so is being in the right [frame of mind] to use this tool effectively. If you see yourself as someone who struggles with food, avoids movement or puts self-care last, it’s easy to slide back into old habits when the treatment ends.

“That’s why I focus on guiding [my patients] to shift their self-image, so they start thinking and acting like the healthiest version of themselves from the inside out. The injection then becomes a springboard, not a crutch.”

GLP-1s licensed by the MHRA in the UK

*Wegovy can be used to reduce the risk of cardiovascular events (like heart attack and stroke) in overweight and obese individuals. Find out more about Wegovy here.

Medical conditions or medications that make weight loss injections unsafe

Before prescribing weight loss injections, a doctor will check for conditions and drug interactions that could make these medicines risky.

“GLP-1s might not be the best weight loss treatment for you if you have pancreatitis or severe gastrointestinal issues, such as certain inflammatory bowel conditions,” says Dr Daniel. “But again, this is why it’s important to get as much information as possible from a patient before weight loss injections are prescribed – it’s a way of ensuring their safety.”

You’re considered high risk if you have:

• A history of pancreatitis

• Certain thyroid cancers

• Severe gastrointestinal disorders (eg gastroparesis)

• Advanced heart, liver or kidney failure

What’s more - while you may not have a history of any of the above, there are also medication interactions you should be aware of and your clinician will screen you for these, including:

• Contraceptive pill: Tirzepatide can reduce absorption of the oral contraceptive pill, especially after dose increases, so the MHRA  advises using extra precautions for four weeks after each dose escalation.

• HRT: Patches or gels may be better if you’re on tirzepatide.

• Insulin and sulfonylureas: May increase the risk of low blood sugar, so doses often need adjusting.

Jane Msumba explains: “Certain diabetes medicines, warfarin and some antidepressants may require monitoring or dose adjustments. If you take the oral contraceptive pill, take it at the same time each day and be aware that vomiting or diarrhoea can affect absorption, so extra precautions may be needed.”

She adds: “For HRT, there are usually no direct issues, but it’s important to review the whole medication list to check for interactions and optimise safety.”

Dr Daniel also notes: “Weight loss injections aren’t suitable for people with a history of eating disorders. In this case, consulting with a GP is recommended.”

Always share your full medical history - including over-the-counter and herbal supplements - with your GP before starting any weight loss treatment.

Weight loss injections should complement a healthy diet and exercise

Medical conditions that weight loss treatment might help

“Heart disease, diabetes, high cholesterol, high blood pressure or other weight-related illnesses such as sleep apnoea or joint problems are examples of conditions where a clinician might be more likely to recommend weight loss treatment,” says Dr Daniel. “Because it’ll probably have wider benefits on that person's wellbeing and address more than one health issue.

“Age is another consideration - someone who’s over 40 is more prone to metabolic disease and, from a risk standpoint, they may stand to benefit more from weight loss injections than a younger person.”

TheInvisibleLife

Common side effects of weight loss injections

Like all medications, weight loss injections carry risks. 

“Common weight loss injection side effects include nausea, vomiting and diarrhoea,” says Dr Daniel. “They’re usually not serious, but they potentially could be if they continued throughout your treatment. Excessive vomiting and diarrhoea can lead to dehydration, so it’s important to stop treatment and speak to your clinician if these side effects don’t settle down.”

In short, the most common side effects include:

• Nausea

• Diarrhoea

• Vomiting

• Constipation

• Heartburn

• Headache

• Fatigue

• Irritation at the injection site

They’re usually mild to moderate and often improve after a few weeks.

Jane Msumba warns: “In the first few weeks, watch for persistent nausea, vomiting, constipation, diarrhoea or signs of dehydration. These are usually manageable but should be monitored.”

More serious side effects that need urgent medical attention include:

• Severe, persistent upper-abdominal pain (possible pancreatitis)

• Sudden pain under the right rib cage (possible gallstones)

• Swelling of the face, throat or tongue, or difficulty breathing (possible anaphylaxis)

Jane Msumba continues: “Seek urgent medical attention if you have severe upper abdominal pain, persistent vomiting, yellowing of the eyes, unexplained fever or sudden changes in bowel habits accompanied by pain.

“These could be signs of pancreatitis, gallbladder disease or other complications that require immediate care.” 

You can report side effects, as well as suspected counterfeit products, via the MHRA Yellow Card Scheme. 

Should weight-loss injections be used long-term?

The long-term safety of newer weight loss drugs like tirzepatide and semaglutide in patients is still being studied. Early reports suggest they’re effective and relatively safe compared to older GLP-1 drugs.

“Although they’re considered safe for most people to use, these treatments are relatively new, so we don’t know much yet about their long-term use,” says Dr Daniel. “Research is still ongoing about the health risks involved when using weight loss injections long-term. For the time being, injections like Wegovy and Mounjaro can only be prescribed for a maximum of two years.”

On the NHS, Wegovy and Mounjaro are commissioned for up to two years within specialist weight management services, with regular reviews to decide whether to continue. This is not a hard safety cut-off but an NHS commissioning rule. Research into longer-term use is ongoing.

That said, older GLP-1 agonists have been used for type 2 diabetes for more than a decade. They’re generally safe, but rare risks like pancreatitis and gallbladder disease remain.

The National Institute for Health and Care Excellence (NICE) reviews new guidance every three years, and the NHS rollout of Wegovy has been phased to monitor safety. More recently, NICE has advised services to offer follow-up support for at least a year after treatment ends.

How effective are weight loss injections compared to other weight loss methods? 

Clinical trials show substantial weight loss when GLP-1 injections are combined with lifestyle changes like diet and physical activity. According to the Electronic Medicines Compendium (EMC), the results are as follows: 

• Mounjaro: Up to 22.5% average weight loss over 72 weeks at maximum dose (15mg)

• Wegovy: Up to 15% average weight loss over 15 months

• Saxenda: At least 5% weight loss over 12 weeks for most on maximum dose 

By comparison, lifestyle-only interventions typically result in 3-5% loss over a year.

But these injections aren’t a magic fix. Without changes to diet and exercise, weight regain after stopping is highly likely.

Adopting healthy habits and embracing an active lifestyle is key to maintaining weight loss

What Mumsnet users think about weight loss injections

Many Mumsnetters see a clear benefit of weight loss medications when used appropriately and correctly. 

CakeIsNotAvailable shared: “GLP-1 agonists have been licensed for the treatment of diabetes since, iirc, 2009. They do seem to be pretty safe. I'm a GP and less than 12 months ago I was morbidly obese. Thanks to GLP-1s I now have a BMI of 22.5. 

“I think that the benefits of losing almost 50kg are likely to outweigh the risks in my case. I accept that the risk/benefit equation may be different for someone starting from a lower BMI.” 

GameOfJones adds how injections are helping her adopt healthier lifestyle habits and diet: “[...] It doesn't magically make you burn fat, it acts as an appetite suppressant so I don't feel hungry. 

“That has made it much, much easier to stick to a healthy diet because I'm not that bothered about what I eat so it is easy to choose healthy options. And that's what is making me lose weight.

“I'm trying to use it as a support tool to help me retrain myself into enjoying nutritious food and reducing my portion sizes so that when I come off the injections I don't just put it all back on [...] I just need a tool to support me to make sensible choices for as long as it takes the habit to stick [..]” 

Buttons0522 concurs: “It’s an appetite suppressant but if you stop eating sugar because your appetite is suppressed, you’ll break your sugar addiction. If you’re an emotional/binge eater, that suggests you don’t have a great relationship with food. 

“A big part of this journey is retraining your mind/body to have a better relationship with food: making better choices, knowing when you’re full, portion control etc. Seeing food as a fuel and having only what we need [...]”

Is it safe to use weight loss injections without medical supervision?

In short: no.

Taking these injections without a prescription is dangerous. Risks include incorrect dosing, missed side effects and counterfeit products.

NHS guidance is clear for this reason: “Never take medicine for weight management if it has not been prescribed for you. These types of medicines may not be safe for you and could cause serious side effects.”

“Weight loss injections are prescription-strength medications, so they’re always prescribed by a licensed medical professional,” says Dr Daniel. 

“It’s not recommended or safe to use them without this supervision. You’ll generally need to see a demonstration of how to inject them before using them for the first time, and the optimum way to increase the dose can be quite individual - some people may benefit from staying on one dose for longer, rather than cycling up each month. 

“Medical supervision helps you to manage these situations safely and effectively.”

Risks of buying online or using unregulated products

The dangers of weight loss injections are amplified hugely when bought from unregulated supplies. Do not do this. 

When purchasing them online, always check that the online pharmacy is registered with the General Pharmaceutical Council (GPhC) and that the doctor and/or prescriber is listed with the General Medical Council (GMC) and GPhC. 

“When buying weight loss injections online, it’s important to get them from a trusted source,” says Dr Daniel. “Regulated providers will follow strict procedures to ensure your safety before supplying this medication, and that it’s delivered in a safe way.”

“The biggest risks come from unregulated or counterfeit products, incorrect dosing and lack of medical supervision”, Jane Msumba explains.

Risks of purchasing weight loss injections from an unregulated supplier include:

• Receiving counterfeit or contaminated products

• Incorrect concentrations or inactive ingredients

• No screening for medical suitability 

• No follow-up or monitoring

And it does happen - in fact, the MHRA have seized hundreds of fake Saxenda and Ozempic pens in recent years. 

“If you are getting treatment from online suppliers, look for signs of credibility and legitimacy so you know these pharmacies are legitimate, such as being registered with the Care Quality Commission (CQC) or General Pharmaceutical Council (GPhC),” says Dr Daniel. 

The role of lifestyle changes when trying to lose weight

“A healthier lifestyle should always be the first port of call when trying to lose weight,” says Dr Daniel. “Even if someone does end up using [weight loss] treatment, they’ll need to eat a healthy diet and be physically active for the treatment to be effective.”

The NHS requires weight management medicines to be prescribed alongside:

• A reduced-calorie diet

• At least 150 minutes of moderate activity a week

• Behavioural support and monitoring

As Jane Msumba puts it, “The injection can help you take that first step, but it’s your daily habits that keep you feeling healthy and confident long after.”

Useful links for more information

• MHRA Yellow Card Scheme – report suspected side effects

• NHS: Obesity treatment

• The NHS Digital Weight Management Programme 

• NICE: Semaglutide for managing overweight and obesity

About the experts

• Dr Daniel Atkinson is the Clinical Lead at Treated.com, an online healthcare service company.

• Jane Msumba is a multi-award-winning pharmacist, certified mental imagery coach, and founder of Inner Glow Clinic, a mindset-first health coaching service helping people break free from unhealthy habits and sustain long-term wellbeing. 

About the author

Our resident Leeds lass, Rebecca Roberts is a Senior Content Editor here at Mumsnet. On a daily basis, she commissions, writes and edits content designed to help parents navigate real life - from product reviews to practical features on everything from sex and relationships to fashion to skincare.

Rebecca - aka Beccy - brings unique personal insight to weight loss content here at Mumsnet, having undergone her own journey with medically supervised injections. Her firsthand experience with GLP-1 treatments provides authentic expertise that resonates with parents exploring similar options. 

Beyond her role as an editor, she can be found balancing life as a working mum of two toddlers and when she’s not at her desk, you’ll likely find her at a PTA meeting, in a nearby coffee shop, or walking her two dogs up and down country lanes.

*Mumsnet Keyword Searcher, 1 January 2025 - 13 August 2025