GLP-1 weight loss injections: what they are, how they work, and who they suit

You've heard the name. Here's what's actually happening in your body.

Picture this: you've eaten a normal meal, but something feels different. You feel full much sooner than usual, the food seems to sit longer, and the background noise of hunger that normally creeps back within a couple of hours is quieter. That's the experience most people describe in the first weeks of a GLP-1 injection, not a dramatic switch, but a gradual settling of appetite signals that were, for many people, working against them.

GLP-1 stands for glucagon-like peptide-1, a hormone released naturally after eating. It signals fullness to the brain and slows the rate at which the stomach empties. The medicines licensed in the UK for weight loss injections amplify this effect. Wegovy (semaglutide) activates the GLP-1 receptor. Mounjaro (tirzepatide) goes further, it activates both the GLP-1 and GIP receptors, making it the only dual-agonist weight-management medicine licensed in the UK. The practical differences between these GLP-1 injections matter when a prescriber is deciding which suits you.

Neither medicine acts like a stimulant or a fat-burner. The mechanism is hormonal, not metabolic acceleration. Appetite falls, portion sizes tend to shrink naturally, and weight loss follows a reduced calorie intake over months, not days. The NHS medicines page for tirzepatide explains this mechanism clearly and is worth reading before you start.

The evidence behind glp-1 injections for weight loss

The clinical programme for these medicines is extensive. In SURMOUNT-1 (a 72-week randomised trial involving 2,539 adults with obesity) tirzepatide at its highest dose produced an average body-weight reduction of around 20–21%, with some analyses approaching 22.5%. The STEP 1 trial, 68 weeks, showed semaglutide 2.4 mg averaging around 15% weight reduction. A more recent head-to-head study, SURMOUNT-5 (published in the New England Journal of Medicine, 2025), showed tirzepatide producing greater average weight loss than semaglutide 2.4 mg over 72 weeks, useful context, though NICE notes the comparison is indirect in the wider evidence base.

NICE reviewed this evidence carefully before recommending tirzepatide in December 2024 (updated September 2025) for eligible adults, and semaglutide in 2023. These are not marginal effects. They represent the most significant pharmacological advance in weight management in a generation.

It matters to say clearly that results vary. Trial averages include people who responded strongly and people who responded modestly. Biology, adherence, diet and activity all play a part. The clinical literature is honest about this, and so are our prescribers.

If you're thinking about the cost side of treatment, a plain explanation of what drives the price of weight loss injections may help set expectations before you decide.

Who these injections are licensed for, and who they're not

Both medicines share a similar eligibility framework. The licensed criteria for private prescription cover adults with a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related condition, prediabetes, type 2 diabetes, high blood pressure, high cholesterol or obstructive sleep apnoea, for example. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. A prescriber assesses your complete picture: medical history, current medicines, contraindications.

Neither medicine is licensed for use during pregnancy, breastfeeding or when actively trying to conceive. They are not licensed for anyone under 18. People with a history of medullary thyroid carcinoma or MEN2, or with certain gastrointestinal conditions, will be unable to use them. A full list of contraindications is in each medicine's Patient Information Leaflet.

Wondering whether you'd be likely to qualify? The eligibility guide for weight loss injections covers this in more detail. NHS access exists but criteria are strict and phased, our treatment overview explains where private prescription sits in relation to the NHS route.

What the injection itself looks like, and what the first weeks feel like

Treatment starts at a low dose. For tirzepatide that's 2.5 mg; the first pen's job is to let your body adjust, not to produce rapid results. Semaglutide starts at 0.25 mg. A prescriber titrates the dose upward, typically in four-week steps, based on tolerance and response. Missed doses, timing and any changes always go through your prescriber and the Patient Information Leaflet, not through a website's general guidance.

The injection itself is subcutaneous, under the skin of the abdomen, thigh or upper arm. The pen is pre-filled. Rotating the injection site each week reduces the chance of irritation. Choosing the right injection site is something our prescribers cover at the start of treatment.

When your medication arrives from a GPhC-registered pharmacy, it comes in plain, unbranded packaging via DPD with a tracking notification to your phone, the pen itself is refrigerated in transit and should go straight into the fridge at home. Side effects in the first weeks are mostly gastrointestinal: nausea, loose stools, constipation, reflux. They tend to ease as the body adapts. Severe, persistent stomach pain that spreads to the back needs urgent medical attention, it can rarely indicate pancreatitis, a risk the MHRA highlighted in a Drug Safety Update in January 2026.

Knowing what to expect before you start makes the early weeks much easier to navigate. Our guide to injectable GLP-1 weight loss treatment goes into the practical detail. If you have specific questions, our FAQs cover many of the things people ask most often, or you can get in touch directly.

When you're ready to find out whether treatment is suitable for you, start your free consultation, a GPhC-registered prescriber, not software, reads your answers and reviews your case the same day.