Mounjaro®
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Start journey Learn moreHow semaglutide works as a GLP-1 receptor agonist, who it is for, and what the trial data actually says
What GLP-1 is and why semaglutide targets it
Glucagon-like peptide-1 is released by cells in the gut after eating. It signals the pancreas to release insulin, tells the brain that food has arrived, and slows the rate at which the stomach empties. The result is a sustained sense of fullness and a blunted appetite signal. GLP-1 itself breaks down in minutes; semaglutide was engineered to resist that rapid breakdown, giving it a half-life long enough for once-weekly dosing.
A common misconception is that semaglutide simply makes food taste worse or causes nausea as its main mechanism. Nausea does occur for some people, particularly in the early weeks, but the therapeutic effect comes from genuine receptor activation that reshapes appetite signalling, not from feeling too unwell to eat. The NHS semaglutide medicines page describes the mechanism clearly and is worth reading alongside any consultation.
Because it targets a single receptor pathway, semaglutide is classified as a GLP-1 receptor agonist. Tirzepatide, by contrast, activates both GLP-1 and GIP receptors, a distinction with clinical implications for weight loss outcomes, explored separately on our GLP semaglutide overview.
What STEP 1 and the licensed evidence actually showed
The STEP 1 trial enrolled 1,961 adults with obesity but without type 2 diabetes. After 68 weeks of weekly semaglutide 2.4mg injections alongside lifestyle changes, participants lost an average of around 15% of their starting body weight compared with roughly 2.4% in the placebo group. Around one in three participants lost more than 20%. These figures come from the trial paper published in the New England Journal of Medicine and formed the primary evidence base for NICE's appraisal of Wegovy (TA875).
It is worth being precise here: 15% is an average across the trial population, not a floor or a guarantee. Individual responses varied considerably. Factors including starting weight, adherence, dose tolerated and whether lifestyle changes were sustained all influenced outcomes. NICE's guidance notes that prescribers should review whether to continue if a patient achieves less than 5% weight loss after six months at the maintenance dose.
For context on how semaglutide's results compare with tirzepatide's across different doses, the is semaglutide a GLP-1 drug page sets out the distinction in more detail.
Who the UK licence covers, and what GLP-1 or semaglutide is not licensed for
Wegovy's UK licence covers adults with a BMI of 30 or above, or 27 or above alongside at least one weight-related condition. Lower BMI thresholds apply for some ethnic backgrounds under UK clinical guidance. The medicine is taken as a weekly injection, starting at a low dose and titrated upward by the prescriber, the starting dose is there to let your system adjust, not to produce weight loss straight away.
Oral semaglutide for weight management is now also available in the UK: Wegovy tablets received MHRA approval in June 2026, making them the first oral GLP-1 medicine licensed here for weight management. Our Wegovy overview covers both formulations.
Ozempic, though it contains the same molecule, carries a type 2 diabetes licence only. Using it for weight loss is off-label; it is not what NICE assessed or what the NHS recommends for weight management. The distinction matters practically: supply, dosing conventions and monitoring differ. Anyone looking at semaglutide for weight loss in the UK should be working from the Wegovy licence, not Ozempic's.
Rybelsus (also oral semaglutide, at 3mg, 7mg and 14mg) is similarly diabetes-licensed only. If you are trying to understand how pen volumes translate into actual doses, our page on what 2ml of semaglutide means in practice explains the relationship clearly. The strengths are entirely different, and the two are not comparable doses of the same product.
Access, cost and what to expect from a clinical assessment
On the NHS, Wegovy is available through specialist weight management services under NICE TA875, but waiting lists are long and the criteria are strict. Tirzepatide (Mounjaro) is rolling out through primary care under a phased commissioning schedule from 2025 onwards. Many people who qualify medically find themselves waiting months or years for an NHS appointment.
Private treatment through a regulated online pharmacy is the route for people who want to start sooner, provided they meet clinical criteria. For a sense of what private semaglutide treatment costs in the UK and what a transparent service includes, the Wegovy cost guide sets out the market context honestly, including what headline prices sometimes leave out.
At nume, every consultation is read by a GPhC-registered Independent Prescriber on the same day it is submitted. There are no algorithms making prescribing decisions. Identity and weight are verified before treatment is issued, and clinical support is available seven days a week. If you want to understand whether you are likely to be suitable, the treatments page is the place to start.
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