Weight Loss Injections in the UK: How GLP-1 Medicines Work

If you have searched for weight loss injections in the UK, you have probably run into two extremes: breathless headlines about "skinny jabs" and dense drug leaflets written for clinicians. Neither tells you the thing you actually want to know, which is how these medicines work, whether you can have one, and what to realistically expect.

Here is a plain-English overview. The injections people mean are GLP-1 medicines, most commonly Mounjaro (tirzepatide) and Wegovy (semaglutide). They are prescription-only. They work mainly by reducing appetite and slowing how quickly your stomach empties, so you feel full sooner and stay full for longer. In UK trials they have produced meaningful average weight loss, but the numbers vary a lot from person to person, and the medicines are supplied only after an online consultation and a clinical assessment by a prescriber. This guide walks through all of that in plain English.

What "weight loss injections" actually are

The phrase is a bit of a catch-all. Almost always it refers to a class of medicines called GLP-1 receptor agonists. GLP-1 (glucagon-like peptide-1) is a hormone your gut releases after you eat. It tells your brain you have had enough, helps regulate blood sugar, and slows the passage of food out of your stomach. The natural hormone breaks down within minutes. These medicines are engineered versions that stay active for about a week, which is why they are injected once a week rather than daily.

Two are widely used for weight management in the UK:

• Wegovy contains semaglutide, which acts on the GLP-1 receptor.
• Mounjaro contains tirzepatide, which acts on two receptors, GLP-1 and GIP. GIP is a second gut hormone involved in how the body handles sugar and fat.

Both come as a pre-filled pen you inject into the stomach, thigh, or upper arm. They are not fat burners, they do not raise your metabolism by revving up your heart rate, and they are not supplements. They are licensed medicines, and in the UK they are prescription-only for good reason: they need a prescriber to check that they are appropriate and safe for you.

How GLP-1 injections work in the body

It helps to separate the two things these medicines do.

They turn down appetite. GLP-1 acts on the appetite centres of the brain, including the hypothalamus. Many people describe the effect as the "food noise" going quiet, the constant low-level thinking about the next snack or meal. When that quietens, eating less stops feeling like a daily act of willpower and starts feeling like something you simply do not want to do as often.

They slow gastric emptying. Food stays in the stomach longer, so the physical feeling of fullness lasts longer after a meal. That is a large part of why portions naturally shrink.

Tirzepatide adds the GIP mechanism on top of the GLP-1 one, which is thought to contribute to its larger average weight loss in head-to-head and trial data. Both medicines also improve blood sugar control, which is why semaglutide and tirzepatide were originally developed and are still used for type 2 diabetes. In people who do not have diabetes, they lower appetite and support weight loss without pushing blood sugar dangerously low.

Two honest caveats. First, these are appetite-regulating medicines, not a switch that dissolves fat; the weight comes off because you eat less, and they make eating less easier. Second, they work best alongside a reduced-calorie diet and more activity, which is exactly how they were studied and how they are licensed to be used. Results vary, and no reputable clinic can promise you a specific number.

How much weight can you actually lose?

This is where a named trial matters more than a headline. The figures below are averages from the pivotal studies, achieved alongside diet and lifestyle changes, and your own result may be higher or lower.

In the SURMOUNT-1 trial, published in the New England Journal of Medicine, adults with obesity taking the highest 15 mg dose of tirzepatide lost an average of about 22.5% of their body weight over 72 weeks, compared with about 2.4% on placebo. In the STEP-1 trial, adults taking 2.4 mg semaglutide lost an average of about 14.9% over 68 weeks, versus 2.4% on placebo.

A few things worth holding onto. These are averages across large groups, so a spread sits behind each figure. The results were achieved at the full maintenance dose, reached slowly over months, not on day one. And they were achieved with lifestyle support, not instead of it. Treat these numbers as a realistic ceiling for a typical trial participant, not a guarantee, and remember that individual results vary.

Who is eligible for weight loss injections in the UK?

Eligibility is a safety boundary, not a test of how hard you have tried. Under the medicines' UK licence, granted by the Medicines and Healthcare products Regulatory Agency (MHRA), Mounjaro is authorised for adults with a BMI of 30 or above, or a BMI of 27 or above where you also have a weight-related health condition such as high blood pressure, high cholesterol, prediabetes, or obstructive sleep apnoea. Wegovy carries a similar licensed threshold. The MHRA confirmed this indication when it authorised tirzepatide for weight management.

A note on the numbers you may have seen elsewhere. The NHS funds these medicines through much narrower criteria than the licence allows, which is why an NHS route often requires a far higher BMI and several conditions. Those are funding rules, not the licensed eligibility limits. A private prescriber works to the licensed criteria and their own clinical judgement.

BMI is an imperfect measure, and it is applied with more nuance for some groups. For people of South Asian, Chinese, Black African, or African-Caribbean family background, weight-related health risk can appear at a lower BMI, so a prescriber may consider treatment at a lower threshold. A consultation is where that judgement is made, not a calculator alone.

If you would like a first indication of whether you fall within these thresholds, you can check your eligibility with a short assessment before committing to anything. For a deeper look at the criteria, our guide on who qualifies for Mounjaro goes further.

Mounjaro or Wegovy: how to think about the choice

Neither is a "better" drug in the abstract; they suit different people. Mounjaro tends to produce larger average weight loss in trials, driven by its dual GLP-1 and GIP action. Wegovy has a longer real-world track record and extensive cardiovascular outcome data. Both require the same slow dose build-up and the same commitment to lifestyle change, and the right choice depends on your health history, your goals, and how you tolerate the medicine.

Cost is often part of the decision too. At The Weight Clinic, Mounjaro starts from £125 a month, and Wegovy's first month starts from £80 (£115 standard, with £35 off your first order using code NEWME). There is no subscription and the eligibility check is free.

We compare the two properly in Mounjaro vs Wegovy, and each has its own page: Mounjaro and Wegovy. If needles are the sticking point, an oral form of semaglutide is emerging, which we cover in our Wegovy Tablet guide and on the Wegovy Tablet page.

How doses are built up over time

You do not start at the strength that produced those trial results, and you should be wary of anyone who lets you. Both medicines use a slow titration schedule so your body adapts and side effects stay manageable.

Mounjaro starts at 2.5 mg once weekly. The dose increases by 2.5 mg roughly every four weeks or more, moving through 5 mg, 7.5 mg, 10 mg, 12.5 mg, up to a maximum of 15 mg, guided by how you respond and tolerate it. Wegovy follows the same principle, stepping up from a low starting dose over several months toward its maintenance dose. Rushing this is the single most common way people make themselves miserable with side effects. Our Mounjaro dosage and titration guide sets out the full schedule.

Side effects and safety

Most side effects are gastrointestinal and are most noticeable in the first weeks of each new dose. The common ones are nausea, reflux, constipation or diarrhoea, and reduced appetite. For most people they are mild to moderate and settle as the body adjusts; a slower dose increase usually helps.

There are rarer but serious risks you should know about. These medicines have been associated with pancreatitis (inflammation of the pancreas, which can cause severe, persistent stomach pain that may spread to the back) and gallbladder disease, including gallstones, particularly with rapid weight loss. Severe, ongoing abdominal pain is a reason to stop and seek medical help promptly.

Some people should not take these medicines at all. Key contraindications include pregnancy or breastfeeding, and a personal or family history of medullary thyroid cancer or Multiple Endocrine Neoplasia syndrome type 2 (MEN 2). A previous episode of pancreatitis also needs careful review before treatment. There is a practical point specific to tirzepatide too: it can reduce the effectiveness of oral contraceptives around the time of starting and dose increases, so a barrier method may be advised for a few weeks. This is exactly the sort of detail a proper consultation is designed to catch.

If you experience any side effect, you can report it directly to the MHRA through the Yellow Card scheme, which monitors the safety of medicines in the UK. Reporting helps build the picture of how these medicines behave in real use. We go into more detail in our guides on Mounjaro side effects and Wegovy side effects.

What happens if you stop?

It is worth being clear-eyed about this. These medicines manage appetite while you take them; they do not permanently reset it. In the STEP trials, people who stopped semaglutide tended to regain a substantial share of the weight over the following year. That is not a failure of the drug, it is what you would expect when an appetite-regulating medicine leaves the system. It is why many clinicians now frame GLP-1 treatment as long-term management of a chronic condition, and why the lifestyle habits you build during treatment matter so much for holding onto your results.

How it works at The Weight Clinic

The Weight Clinic is a UK online pharmacy registered with the General Pharmaceutical Council (GPhC). Every request is reviewed by a prescriber, and medicines are dispensed by a registered pharmacy, not sold over the counter and never "without a prescription."

The process is straightforward. You complete a short online consultation covering your weight, height, medical history, and medications. A prescriber reviews it against the eligibility criteria and safety checks described above. If treatment is appropriate, your medicine is dispensed and sent to you in temperature-controlled packaging, because both medicines are biologics that need to stay cold to work. If it is not appropriate, you will not be prescribed it, and we will tell you why.

A word of caution about the wider market. If a website offers Mounjaro or Wegovy without any medical questions, ID check, or prescriber review, that is a warning sign, not a bargain. Look for the GPhC registration, and if it is absent, walk away.

Curious whether you qualify? You can check your eligibility in a few minutes. It is free, there is no subscription, and there is no obligation to go ahead. If you would prefer to read more first, our overviews of weight loss injections and the full range of treatments are a good next step.

The bottom line

Weight loss injections are not a shortcut or a gimmick. They are prescription GLP-1 medicines that reduce appetite and slow digestion, with average trial results of around 22.5% for Mounjaro (SURMOUNT-1, 15 mg, 72 weeks) and around 14.9% for Wegovy (STEP-1, 2.4 mg, 68 weeks), achieved alongside lifestyle change and with results that vary between individuals. They are licensed for a BMI of 30 or above, or 27 or above with a weight-related condition, and they are supplied only after a proper clinical assessment. Used the right way, through a regulated pharmacy, they are one of the most effective tools we have for managing obesity as the medical condition it is.