What are the weight loss injections available in the UK?

How do weight loss injections actually work in the body?

Mounjaro and Wegovy belong to a class of medicines called GLP-1 receptor agonists. GLP-1 is a hormone released naturally in the gut after eating; it tells the brain you've had enough, slows the rate at which food leaves the stomach, and helps regulate insulin. Wegovy activates the GLP-1 receptor only. Mounjaro goes a step further: it activates both the GLP-1 receptor and a second receptor, GIP, making it the only dual-agonist weight-loss injection licensed in the UK. The practical result of both mechanisms is a significant reduction in appetite and, for most people, a shift in their relationship with food that makes eating less feel manageable rather than gruelling.

Neither injection speeds up your metabolism or blocks fat absorption the way orlistat does. They act centrally and peripherally on appetite signals. You still eat, you just find you want to eat considerably less. Clinical teams at nume... at nume describe this routinely: patients are often surprised the medicine doesn't feel like willpower, because the biology genuinely changes. Strength activity and adequate protein intake matter alongside that change, because both medicines can reduce muscle mass if diet isn't managed thoughtfully.

Both are given as a subcutaneous injection into the abdomen, thigh or upper arm, once a week. Treatment begins at a low starting dose to allow your system to adjust, the first weeks are about tolerability, not full effect. Dose is then increased in steps by your prescriber. You can read a fuller breakdown of how weight loss injections work in practice if you want more detail on the mechanics.

What does the evidence say about how much weight people lose?

The trial results for both medicines are substantial by the standards of any non-surgical intervention. In the SURMOUNT-1 trial (2,539 adults with obesity and no diabetes, 72 weeks), participants on the highest tirzepatide dose lost an average of around 20–21% of their body weight. In the STEP 1 trial for semaglutide 2.4mg (68 weeks), the average was around 15%. A more recent head-to-head study, SURMOUNT-5, compared the two medicines directly over 72 weeks in adults with obesity; tirzepatide produced greater average weight loss than semaglutide 2.4mg across the trial period.

These are averages from clinical trials with structured lifestyle support alongside the medicine, individual results vary, and no prescriber can tell you in advance exactly what you'll lose. What the data do establish clearly is that these are not supplements or lifestyle aids with modest effects; they are regulated medicines with meaningful clinical evidence behind them. For people considering their options, the newest weight loss injections page covers how the landscape has evolved over recent years, including Wegovy's newer higher-dose formulation.

It's also worth knowing that a higher dose doesn't automatically mean a better outcome for you specifically. Some people achieve their treatment goals at a mid-range dose with fewer side effects. That titration conversation belongs with a prescriber, not a forum.

Who are these injections licensed for, and how do prescribers decide?

Both medicines are licensed for adults. Mounjaro is indicated for people with a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related health condition, such as high blood pressure, high cholesterol, type 2 diabetes, obstructive sleep apnoea or cardiovascular disease. Wegovy carries the same broad eligibility framework. For some ethnic backgrounds, UK guidance applies BMI thresholds that are 2.5 kg/m² lower, your prescriber accounts for this during assessment.

BMI is the starting point, not the whole picture. A prescriber also looks at your medical history, any medicines you're taking (particularly oral contraceptives, where an additional method is recommended during early tirzepatide treatment due to potential effects on absorption), and conditions that may make one medicine more suitable than the other. A history of pancreatitis, medullary thyroid carcinoma or certain gastrointestinal conditions, for example, would alter the clinical calculus. Pregnancy, breastfeeding or actively trying to conceive are situations where neither medicine is recommended.

If you're curious what the criteria look like across all the weight loss injections available in the UK, that page sets out the licensed indications side by side. The NHS medicines page for tirzepatide also covers who these medicines are and aren't suitable for in plain language.

What side effects should you know about before starting?

The most common side effects are gastrointestinal: nausea, diarrhoea, constipation, indigestion and burping are all reported frequently, particularly in the first few weeks of a new dose. For most people these settle as the body adjusts. Headache, fatigue and dizziness also feature in the trial data.

Less common but more serious side effects include acute pancreatitis. In January 2026 the MHRA issued a Drug Safety Update specifically highlighting this risk for GLP-1 medicines: severe, persistent stomach pain that spreads to the back, with or without vomiting, is a reason to stop the injection and seek urgent medical attention. Gallbladder problems, allergic reactions and significant dehydration from persistent vomiting or diarrhoea are also reasons to contact a healthcare professional promptly.

Side effects are one reason the titration schedule matters. Starting at the lowest dose and moving up gradually gives the body time to adapt, and if you're based locally and want to understand how weight loss injections work for people in Essex, that page covers what to expect from the process in more detail. If you're weighing up the options and cost is part of the decision, the context around weight loss injection costs in the UK is worth reading, but for a prescription medicine, the clinical pathway around you matters at least as much as the headline price.

You can report suspected side effects directly to the MHRA via the Yellow Card scheme, and that reporting helps build the long-term safety picture for medicines still accumulating real-world data. If you're still getting to grips with the landscape, a useful place to start is an overview of what the three main weight loss injections are and how they differ from one another.