GLP-1 weight loss injections have become one of the most talked-about developments in medicine over the past few years. You've probably seen them in the news, on social media, or heard someone mention names like Ozempic, Wegovy, or Mounjaro. But what are they actually, how do they work, and are they right for you?
This guide covers everything you need to know — in plain English, without trying to sell you anything.
What does GLP-1 stand for?
GLP-1 stands for glucagon-like peptide-1. It's a hormone that your body naturally produces in your gut after you eat. When GLP-1 is released, it does several things:1
- Signals to your brain that you're full — acting on appetite centres in the hypothalamus to reduce hunger
- Slows gastric emptying — food stays in your stomach longer, so you feel satisfied for longer after meals
- Stimulates insulin release — helping to lower blood sugar after eating
- Reduces glucagon — another hormone that raises blood sugar
GLP-1 medications are synthetic versions of this hormone. They're designed to last much longer in your body than the natural hormone (which breaks down within minutes), producing a sustained appetite-suppressing effect throughout the day.
How did they go from diabetes to weight loss?
GLP-1 medications were originally developed to treat Type 2 diabetes. Because they improve insulin response and lower blood sugar, they proved effective for blood sugar management. But during clinical trials, researchers noticed something significant: patients were also losing a lot of weight.
This led pharmaceutical companies to test the same drugs at higher doses specifically for weight management. The results were striking — much more weight loss than any previous medication had achieved — and dedicated weight loss products were born. Novo Nordisk developed Ozempic for diabetes, then created Wegovy (the same drug, semaglutide, at higher doses) for weight loss. Eli Lilly took a similar path with Mounjaro (tirzepatide), which is licensed for both diabetes and weight management.23
Which GLP-1 injections are available in the UK?
Three injectable weight loss medications are currently licensed for weight management in the UK:
Mounjaro (tirzepatide) — a dual GIP/GLP-1 receptor agonist made by Eli Lilly. It mimics two gut hormones rather than one, which may explain its slightly greater weight loss. Weekly injection, up to 22.5% weight loss in clinical trials. Licensed for both weight management and Type 2 diabetes.2
Wegovy (semaglutide) — a GLP-1 receptor agonist made by Novo Nordisk. Weekly injection, up to 20.7% weight loss at the higher 7.2mg dose (approved by the MHRA in January 2026). Also licensed for cardiovascular risk reduction — the SELECT trial showed it reduces heart attacks and strokes by 20%.34
Saxenda (liraglutide) — an older-generation GLP-1 agonist, also by Novo Nordisk. Daily injection (not weekly), up to 8% weight loss. Less effective than Mounjaro and Wegovy but still a valid option for some people.5
You may also see Ozempic mentioned frequently. Ozempic contains semaglutide (the same ingredient as Wegovy) but is only licensed for Type 2 diabetes in the UK, not weight loss. Using it off-label for weight loss is discouraged by the MHRA.
For a detailed comparison, see our comparison of all UK weight loss injections or use our interactive comparison tool.
How effective are they?
The clinical trial results have been remarkable by historical standards. Previous weight loss medications typically produced 5–10% body weight loss; the newer GLP-1 medications routinely achieve 15–22%.
To put that in practical terms: for someone starting at 100kg (approximately 15 stone 10), treatment with Mounjaro or Wegovy at maximum doses over about 18 months might result in losing 20–22kg (about 3 stone 6). That's a transformative amount of weight loss that can significantly reduce the risk of Type 2 diabetes, heart disease, and many other weight-related health conditions.
However, it's important to note that these are averages. Individual results vary widely — some people lose more, some less. And clinical trial participants receive structured support that may not be replicated in real-world settings.
What are the side effects?
Because GLP-1 medications slow gastric emptying and alter digestive signalling, the most common side effects are gastrointestinal: nausea (the most frequently reported), diarrhoea, constipation, vomiting, and stomach pain. For most people, these are worst during the first few weeks of treatment or when the dose is increased, and improve as the body adjusts.23
Serious but rare side effects include pancreatitis (inflammation of the pancreas, affecting roughly 1 in 100 people) and gallbladder problems. Mounjaro specifically may reduce the effectiveness of oral contraceptives.2
For a complete breakdown by medication, see our side effects viewer.
Who can use them?
GLP-1 weight loss injections are prescription-only medicines. They're generally prescribed for adults who have:6
- A BMI of 30 or above (classified as obese), or
- A BMI of 27 or above with at least one weight-related health condition (such as Type 2 diabetes, high blood pressure, high cholesterol, or sleep apnoea)
For people from South Asian, Chinese, other Asian, Middle Eastern, Black African, or African-Caribbean backgrounds, BMI thresholds are typically reduced by 2.5 points (reflecting higher health risks at lower BMI in these populations).
They are not suitable for people who are pregnant or breastfeeding, have a history of pancreatitis, have Type 1 diabetes, or have a personal or family history of medullary thyroid carcinoma (MTC).
To check whether you might be eligible, try our eligibility checker.
NHS vs private access
In the UK, GLP-1 medications are available both on the NHS and privately, but NHS access is significantly restricted. Wegovy is available through Tier 3 specialist weight management services, and Mounjaro is being rolled out in phases with strict criteria (starting with BMI 40+ and 4+ comorbidities). Wait times for NHS services can be substantial.7
Most people currently access these medications through private prescriptions, typically costing £150–£300 per month depending on the medication and dose. For a detailed cost comparison, see our cost guide.
What happens when you stop?
This is one of the most important things to understand about GLP-1 medications. Research consistently shows that most people regain weight after stopping treatment. A major Oxford University meta-analysis published in January 2026 found that people regain approximately 0.8kg per month after stopping newer GLP-1 medications, and are predicted to return to their starting weight within about 1.5 years.8
This isn't a failing of the medications — it reflects the nature of obesity as a chronic, relapsing condition, similar to how blood pressure rises again when you stop taking blood pressure medication. Many experts now view GLP-1 treatment as long-term or indefinite for people with obesity, rather than a short course.
For more detail, see our article on what happens when you stop GLP-1 medication.
Are they safe?
All GLP-1 medications licensed in the UK have been through rigorous clinical trials and approved by the MHRA (Medicines and Healthcare products Regulatory Agency). Tens of millions of people worldwide have used semaglutide and tirzepatide. They are considered safe when prescribed appropriately and used under medical supervision.23
However, they are prescription-only medicines for a reason — they're not suitable for everyone, and they require monitoring. The MHRA has also warned about counterfeit products being sold online, particularly fake Ozempic and Saxenda pens. Always obtain medication from a registered pharmacy or prescriber.
The bottom line
GLP-1 weight loss injections represent a genuine breakthrough in obesity treatment. They produce more weight loss than any previous medication class, with a side effect profile that most people can manage. But they're not magic — they work best alongside dietary changes and increased physical activity, they require a prescription and medical supervision, they're expensive if accessed privately, and most people need to continue treatment long-term to maintain results.
If you're considering treatment, the best next step is to speak to your GP or a qualified prescriber who can assess your individual situation and advise on the most appropriate option for you.
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GuideHow Do Weight Loss Injections Actually Work?
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ComparisonMounjaro vs Wegovy: Which Is Better?
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PracticalGLP-1 Dosing Explained
Titration schedules for every medication — what to expect week by week.