The truth about injections used to reduce weight

The biggest myth: that injections simply 'suppress appetite' like an old diet pill

Most people picture a weight-loss injection as a glorified appetite suppressant, something that flips a switch and makes food unappealing. The reality is more interesting. Mounjaro (tirzepatide) activates two separate gut-hormone receptors: GIP and GLP-1. Wegovy (semaglutide) targets GLP-1 alone. Both pathways do reduce hunger, but they also slow how quickly the stomach empties, improve how the body handles blood sugar after meals, and appear to affect the brain's reward circuitry around food. The result is that people on these medicines typically feel satisfied with less food and stop thinking about eating between meals, not because they've white-knuckled their way through hunger, but because the biological signal telling them they're full arrives sooner and stays longer.

This matters because the old diet-pill framing led people to expect a fast, dramatic change. That's not how these medicines work. The starter dose (2.5mg for Mounjaro, 0.25mg for Wegovy) is there to let your body adjust, not to produce rapid weight loss. The therapeutic work happens as the dose is increased gradually under prescriber supervision. Expecting dramatic results in week one, then concluding the injection isn't working, is one of the most common reasons people stop too early. If you want to understand how a weight loss injection actually delivers its effect in the body, that page is worth reading before you start.

Which injections are actually licensed to reduce weight in the UK right now

Only two injectable medicines hold a UK licence specifically for weight management in adults. Tirzepatide, sold as Mounjaro by Eli Lilly, comes as a once-weekly pre-filled KwikPen in six strengths from 2.5mg up to 15mg. Semaglutide, sold as Wegovy by Novo Nordisk, is also a once-weekly injection; the standard maintenance dose is 2.4mg, and a higher 7.2mg dose received MHRA approval in early 2026. Both are Black Triangle (▼) medicines, meaning the MHRA collects additional safety data on them.

You may have heard of Ozempic. It is also semaglutide, but it is licensed for type 2 diabetes, not weight management. Prescribing it for weight loss sits outside its UK licence, and the NHS medicines information for tirzepatide sets out clearly what the licensed indication covers. If you've seen other injectables advertised (liraglutide (Saxenda), for instance) these exist but are not dispensed by nume; our prescribers work with Mounjaro and Wegovy. For a broader look at the names of weight-loss injectables available in the UK, that page covers each one and its licensing status.

Who the eligibility criteria actually cover, and what a clinical assessment adds

The licensed criteria for private prescription are BMI of 30 or above, or 27 or above if you have at least one weight-related condition such as high blood pressure, high cholesterol, prediabetes, type 2 diabetes or obstructive sleep apnoea. For some ethnic backgrounds, UK guidance applies a BMI threshold roughly 2.5 kg/m² lower, your prescriber will know the relevant adjustment. BMI alone never tells the whole story: your current medicines, any history of pancreatitis or thyroid conditions, whether you're pregnant or breastfeeding, and other clinical factors all feed into the decision. Under-18s are not licensed for these treatments.

This is why a clinical assessment isn't just a box to tick. A prescriber reviewing your consultation picks up interactions and contraindications that an eligibility calculator misses. At nume, our clinical team personally reviews every consultation the same day, a real clinician reads your answers, not a queue in an automated system. If you're curious about how weight loss injections are prescribed and dispensed through a regulated UK pharmacy, that page walks through each step. One quick check worth doing before you apply: look up any online pharmacy you're considering on the GPhC register, it takes under a minute and confirms the pharmacy is legitimate.

Side effects, what to watch for, and when to act

Gastrointestinal effects (nausea, loose stools, constipation, indigestion, reflux) are the most frequently reported. They're usually most noticeable in the first days after starting or after a dose increase, and for many people they ease within a week or two as the body adapts. Keeping meals smaller, eating slowly and staying well hydrated makes a real difference in that adjustment window. Fatigue and mild headaches are also reported, particularly early on.

More serious, but less common: acute pancreatitis. In January 2026, the MHRA issued a Drug Safety Update highlighting this risk for GLP-1 medicines. Severe, persistent stomach pain (particularly if it radiates to the back, with or without vomiting) needs urgent medical assessment, not a wait-and-see. Gallbladder symptoms, signs of serious allergic reaction and significant dehydration from prolonged vomiting or diarrhoea also warrant prompt contact with a clinician. If you're on oral contraceptives and starting tirzepatide, you'll need to use an additional barrier method for the first four weeks of treatment and for four weeks after any dose increase, because tirzepatide can reduce pill absorption. For a more detailed breakdown of what to expect, guidance on taking a weight-loss injection covers practical management, and if you want to feel confident with the technique before your first dose, our page on how to inject weight loss injections walks you through each step. If you'd like to compare the options side by side, the page on which weight loss injection r suits your circumstances sets out the key differences. Side effects can be reported to the MHRA through the Yellow Card scheme, patients can use it directly, not just clinicians.

If you'd like a clinician to assess whether a weight-loss injection is appropriate for you, speak to our prescribers through a free consultation.