Mounjaro®
Starting from £179.99/mo
Start journey Learn moreFrom the trial evidence to the pen in your fridge: how weight loss injections actually work
What the clinical trials actually found
The headline numbers come from large, published trials, so they're worth reading properly rather than through a social-media filter. In SURMOUNT-1, which randomised 2,539 adults, tirzepatide produced an average body-weight reduction of roughly 20–21% at the highest dose over 72 weeks, with some analyses reaching about 22.5%. That figure sits alongside diet and increased activity, not instead of them.
Semaglutide's STEP 1 trial ran over 68 weeks at the 2.4mg dose and reported around 15% average reduction. A more recent higher 7.2mg maintenance dose has reported roughly 20.7% over 72 weeks, narrowing the distance between the two medicines. And in the head-to-head SURMOUNT-5 trial, published in the New England Journal of Medicine in 2025, tirzepatide produced greater average weight loss than semaglutide 2.4mg across 751 adults with obesity.
Averages are not promises. Individual results vary widely, and the trials measured groups, not people. You can read the patient-level detail on the NHS medicines page for tirzepatide. What the evidence tells you is direction and scale, which is exactly what a prescriber weighs against your own health picture.
Why an injection changes how hungry you feel
These medicines are gut-hormone agonists. Semaglutide activates the GLP-1 receptor; tirzepatide activates both GIP and GLP-1, which is why it's called a dual agonist and the only one of its kind licensed here. Both slow the rate at which your stomach empties, so a normal-sized meal leaves you full for longer and the background hum of hunger quietens.
That's the honest mechanism, and it's worth understanding because it explains the side effects too. If food is moving more slowly through your system, nausea or reflux early on makes sense. Our guide to how weight loss injections work goes deeper into the biology, but the short version is this: appetite is largely driven by hormones, not willpower, and these medicines nudge those hormones.
Because the effect builds over weeks, doses are increased gradually. Treatment starts low so your body can settle before the amount goes up. That first pen isn't there to shift the scales fast; its job is to let your system adjust with fewer symptoms. Any changes to the amount you take are a prescriber's decision, guided by how you're getting on.
What names you'll actually see, and what to ignore
The tabloid phrase "skinny jab" gets thrown around loosely, and it muddies real safety information. In the UK, only two injections are licensed for weight management: Mounjaro and Wegovy. If you're comparing the names behind weight loss injections, that distinction matters.
Two others cause frequent confusion. Ozempic is semaglutide, the same molecule as Wegovy, but it's licensed for type 2 diabetes, not weight loss, and shouldn't be presented as a weight-loss product. Saxenda (liraglutide) is UK-licensed for weight management but is a daily rather than weekly injection, and nume doesn't dispense it. There's also a newer oral route now: Wegovy is available as a tablet, which suits people who'd rather avoid needles entirely.
If you'd like the full comparison laid out, our page on choosing between weight loss injections weighs them side by side. The plain point is that "the best weight loss injection" isn't a single answer. Tirzepatide leads the trial data, but the right medicine depends on your health, your history and how your body responds, which is a clinical judgement rather than a league table.
Who these injections are and aren't licensed for
The licensed thresholds are the starting point, not the whole decision. In broad terms, these medicines are for adults with a BMI of 30 or above, or 27 and above with a weight-related condition such as high blood pressure, high cholesterol, prediabetes, type 2 diabetes or obstructive sleep apnoea. For people from South Asian, Chinese, Middle Eastern, Black African or African-Caribbean backgrounds, UK guidance sets those BMI thresholds around 2.5 kg/m² lower, reflecting differences in health risk.
They aren't suitable for everyone who meets a number. They're not recommended in pregnancy, while breastfeeding, or if you're trying to conceive, and they aren't licensed for anyone under 18. A history of medullary thyroid cancer, pancreatitis or certain gut conditions needs a proper conversation with a prescriber before anything is offered.
A one-minute habit worth building: know your own BMI before you enquire. The NHS BMI calculator takes your height and weight and gives you a figure in seconds, which saves guesswork later. Meeting the threshold doesn't guarantee approval, though. A prescriber assesses your medical history, current medicines and the full picture, and BMI alone never decides it.
The side effects, and the ones that mean stop and call someone
Most side effects are digestive and settle. Feeling sick, being sick, loose or hard stools, indigestion, burping, tiredness, headache and reactions where you inject are the common ones, usually most noticeable in the first weeks or after a dose goes up, often easing within days to a couple of weeks. They're the trade-off for a medicine that deliberately slows your gut.
Some symptoms are not to be waited out. In January 2026 the MHRA issued a Drug Safety Update flagging acute pancreatitis, an uncommon but serious reaction: severe, persistent stomach pain that can spread through to your back, with or without vomiting, needs urgent medical help. Signs of dehydration from prolonged sickness, gallbladder problems, allergic reactions or sudden changes in vision also warrant prompt advice.
If you'd like the fuller run-through, we've written a dedicated page on the side effects of weight loss injections, and it's worth knowing you can report any suspected reaction through the MHRA Yellow Card scheme. Reading the patient information leaflet in the box is not optional busywork; it lists the exact warning signs for your specific pen. If something feels wrong, trust that instinct and get advice rather than pushing through.
Getting them on the NHS versus a private route
NHS access exists but is tightly phased. Under NICE guidance, tirzepatide is being rolled out in stages: at present it's for adults with a BMI of 40 or more plus four or more of a defined set of conditions, with the criteria widening gradually over the coming years. From April 2026 some GP practices can prescribe under the new contract, though participation is optional, so availability genuinely varies by area. Wegovy on the NHS runs through specialist weight management services, where waiting lists are often long.
Everyone treated on the NHS also takes part in wrap-around diet and activity support, and meeting the criteria doesn't guarantee you'll get treatment. You can read the current position on NHS England's weight management injections page. Criteria differ in Scotland, Wales and Northern Ireland.
The private route through a regulated online pharmacy is the alternative for people who don't meet the NHS thresholds or don't want to wait. There's no referral and no waiting list, though it's still subject to clinical suitability rather than being handed out on request. If you want to see what's involved, our overview of how to get weight loss injections walks through it. At nume, your consultation is read the same day by a GPhC-registered Independent Prescriber, a real clinician rather than an automated form.
What a private course actually costs, honestly
Private weight loss injections in the UK are priced per pen or per month, and the figure shifts with the dose and the provider. After Eli Lilly raised Mounjaro's UK list price from September 2025, typical private prices have reported roughly in the £149–£375 a month range depending on dose. Headline prices can look tempting until you notice what they leave out.
Cheap listings often strip away the consultation fee, the prescription, delivery and any aftercare, then add them back later. For a prescription medicine, the lowest sticker price is the wrong thing to optimise for; genuine supply, real clinical oversight and support when a side effect worries you are what actually protect you. A small saving is no comfort if there's nobody to call.
We're upfront that nume isn't the cheapest, and that's on purpose. One transparent price covers the free consultation, the prescription, the treatment, free next-working-day tracked delivery and priority aftercare seven days a week, with no subscription tying you in. Current treatment pricing sits on the weight loss treatments page. If cost context is your main question, our page on weight loss by injection covers it in more depth.
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