Weight Loss Injection Options Explained

The biggest myth: all weight loss injections work the same way

The phrase "weight loss injections" tends to get used as though it describes one thing, but if you look at what weight loss injections actually are and how they differ, it becomes clear that the two licensed options work through related but distinct mechanisms, and the difference matters clinically.

Wegovy contains semaglutide, a GLP-1 receptor agonist. It mimics a gut hormone that signals fullness to the brain, slows the rate at which food leaves the stomach, and reduces appetite. It's made by Novo Nordisk and is given once a week via a pre-filled pen. The standard maintenance dose has been 2.4mg, though the MHRA approved a higher 7.2mg maintenance dose in January 2026, with a dedicated single-dose pen cleared on 14 April 2026 for adults with a BMI of 30 or above. In the STEP 1 clinical trial, participants using 2.4mg semaglutide lost around 15% of their body weight on average over 68 weeks alongside lifestyle support, as reported in the New England Journal of Medicine STEP 1 trial paper.

Mounjaro contains tirzepatide, made by Eli Lilly. It's the only dual-agonist licensed for weight management in the UK, activating both the GIP and GLP-1 receptors. That dual action produces a meaningfully different appetite and metabolic effect. In the SURMOUNT-1 trial, average body-weight reduction at the highest dose reached around 20–21% over 72 weeks. The SURMOUNT-5 head-to-head trial published in 2025 found greater average weight loss with tirzepatide than with semaglutide 2.4mg, and our page covering the three weight loss injections available in the UK sets out how each one compares in more detail. You can read more about how the two medicines sit within the broader treatment picture on the weight management treatment overview.

Both are Black Triangle medicines, meaning the MHRA requires additional monitoring while longer-term data accumulates. Neither is a shortcut, both work best alongside a reduced-calorie diet and regular activity, and both require ongoing clinical oversight.

Who these injections are licensed for, and how the prescriber decides

Licensed eligibility under each medicine's UK Summary of Product Characteristics covers adults with a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related condition. Relevant conditions include type 2 diabetes, prediabetes, high blood pressure, high cholesterol, and obstructive sleep apnoea, among others. Lower BMI thresholds may apply for some ethnic backgrounds under UK clinical guidance.

A BMI figure alone, though, isn't the whole picture. A prescriber looks at your medical history, current medicines, any conditions that might affect suitability, and what you've tried before. Our clinical team reviews each consultation personally, a prescriber reads your answers on the same day, not an automated system. If tirzepatide or semaglutide isn't appropriate, that conversation happens before anything is dispensed.

For those considering the NHS route, NICE has published separate recommendations for both medicines (tirzepatide under NICE technology appraisal TA1026 and semaglutide under TA875) but NHS access is phased, criteria are strict, and waiting times vary considerably. People who want to understand how a GP weight loss injection referral or prescription works, and how that compares to going private, will find a full explanation of the process on that page. Private prescription is the alternative for people who don't meet the current NHS thresholds or prefer not to wait.

Neither injection is licensed for use in pregnancy, while breastfeeding, or if you're actively trying to conceive. They're also not licensed for under-18s. A prescriber will go through these details during your assessment.

Side effects, injection technique, and what to keep in mind

The side-effect profile for both options is dominated by gastrointestinal symptoms: nausea, loose stools, constipation, reflux, and sometimes fatigue or headache, particularly after starting or moving up to a higher dose. For most people these are mild and fade within the first couple of weeks at each level, though not for everyone.

Pancreatitis is a less common but more serious risk. In January 2026 the MHRA issued a Drug Safety Update reinforcing that severe, persistent stomach pain (especially pain that spreads to the back) is a reason to seek urgent medical attention, not to wait and see. You can report any suspected side effects directly through the MHRA Yellow Card scheme.

On the practical side, both injections go under the skin, abdomen, thigh, and upper arm are the common sites, rotated to avoid tissue buildup. Both pens come pre-filled and pre-dosed. Cleaning the injection site beforehand is good practice; alcohol swabs are a simple addition to your routine. Storage is in the fridge at 2–8°C; both medicines have a limited room-temperature window, and the exact period is set out in the Patient Information Leaflet that comes with your pen, always follow that rather than any general figure you've read online.

Questions about injection frequency, what to do if a dose is late, or when to contact a prescriber are all covered during onboarding at our FAQs and through our aftercare team, which is available seven days a week.

Cost, subscriptions, and what to look out for when comparing providers

The cost of these medicines through private services varies by dose, provider, and what's included in the headline figure. Since Eli Lilly raised Mounjaro's UK list price from 1 September 2025, typically reported private prices have ranged from roughly £149 to £375 per month depending on dose and provider. Those figures are a useful reference point, but they don't tell you whether the price includes a prescription fee, clinical assessment, delivery, or any aftercare support.

A fuller breakdown of what those prices typically do and don't include is on the weight loss injection costs page, which is worth reading before you compare providers. At nume, there's one price per treatment, and it covers everything: consultation, prescription, same-day dispatch once clinically approved, free next-working-day tracked delivery, and access to prescriber support afterwards. If you prefer to order only when you need to, our weight loss injections with no subscription option means you stay in control of when you reorder, with no auto-renewals and no ongoing commitment. Every repeat order gets a fresh clinical review.

A note on buying safely: Mounjaro and Wegovy are prescription-only medicines, which means any seller offering them without a prescription is operating outside UK law. The MHRA has seized large quantities of counterfeit weight-loss pens over the past two years, including fake tirzepatide manufactured illegally in the UK. Unusually low prices and discount codes from unknown sources are red flags. Any legitimate online pharmacy can be verified on the GPhC pharmacy register, it takes about 30 seconds and is the single most useful check you can do before handing over any details.

If you'd like to discuss which of the available options might suit your situation, our page on choosing the right weight loss injection walks through the key considerations, and you can check your eligibility with our prescribers, the consultation is free and reviewed the same day.