The main ingredient in Wegovy — what semaglutide is and how it works

What the STEP 1 trial actually showed about semaglutide's effects

The clearest picture of what semaglutide does comes from the STEP 1 study, a randomised, placebo-controlled trial involving 1,961 adults with obesity or overweight plus at least one weight-related condition, none of whom had type 2 diabetes. Over 68 weeks, participants receiving 2.4mg semaglutide weekly alongside lifestyle support lost an average of roughly 15% of their body weight, compared with around 2.4% in the placebo group. Those figures, published in the New England Journal of Medicine, formed the evidentiary backbone for NICE's subsequent recommendation and the UK licence. They are trial averages, not guarantees (individual responses vary considerably) but they represent the most robust evidence available for this active ingredient in a weight-management context. The trial also captured meaningful reductions in waist circumference and improvements in cardiometabolic markers, suggesting that the medicine's effects go beyond what the scales show.

If you want to read more about Wegovy as a treatment, our Wegovy overview page covers the full licensed picture in the UK.

The mechanism: why semaglutide's single receptor pathway matters

Semaglutide latches on to GLP-1 receptors, the same receptors that the body's own GLP-1 hormone targets after a meal. By activating them, semaglutide slows gastric emptying, so food sits in the stomach longer and fullness signals persist. It also acts on appetite-regulating centres in the brain, reducing the drive to eat and making it easier to respond to smaller portions. This single-receptor approach is distinct from tirzepatide, which activates both GLP-1 and GIP receptors; understanding the difference matters when a prescriber is deciding which medicine suits a particular patient. The semaglutide page on this site goes into the pharmacology in more detail if you are weighing up the mechanisms. A question our prescribers hear most weeks is whether Wegovy and Ozempic are really the same thing, the answer is that the active ingredient is the same, but the licensed indication, dosing schedule and pen format differ, something we cover directly in this comparison.

The NHS patient information for semaglutide, available at nhs.uk/medicines/semaglutide, sets out the standard clinical information in accessible language and is a good starting point alongside any prescriber conversation.

Who can access Wegovy, and what the NICE guidance says

NICE recommended semaglutide (Wegovy) for NHS use under TA875, but with specific conditions: it is indicated for adults with a BMI of 35 or above and at least one weight-related comorbidity, used for a maximum of two years, within a specialist weight management service offering multidisciplinary support. Lower BMI thresholds apply for people from South Asian, Chinese, Middle Eastern, Black African or African-Caribbean backgrounds. On the NHS, waiting lists for specialist services can be long, which is why many people explore the private route. Through a regulated private pharmacy, the licensed eligibility criteria are broader: adults with a BMI of 30 or above, or 27 and above alongside a qualifying condition such as high blood pressure, high cholesterol or obstructive sleep apnoea. A prescriber still makes the final call after reviewing the full clinical picture. For context on what private treatment currently costs in the UK, the Wegovy price comparison page walks through what typical figures include and what they often leave out.

Semaglutide's side-effect profile and when to seek help

Most side effects linked to semaglutide's active ingredient are gastrointestinal: nausea, loose stools, constipation, indigestion and occasional vomiting are the most commonly reported. They tend to peak in the days after starting or after a dose increase, and most people find they settle within a couple of weeks as the body adjusts. Fatigue and headache are also noted, particularly early on. Serious reactions are less common but warrant attention: persistent, severe stomach pain that radiates to the back could signal pancreatitis, and anyone experiencing it should seek medical help promptly rather than waiting for their next clinical check-in. The MHRA highlighted acute pancreatitis as a known though infrequent risk for this class of medicines in a Drug Safety Update in January 2026. Semaglutide is not recommended during pregnancy, while breastfeeding, or for anyone trying to conceive, and it is not licensed for under-18s. The inactive components of the pen formulation also matter for a small number of people, our page on Wegovy's inactive ingredients covers excipients in full. You can report any suspected side effects directly at yellowcard.mhra.gov.uk. If you are curious about skin reactions in particular, this page looks at whether Wegovy causes breakouts, a question that comes up more often than you might expect.

Once you have a clear picture of what semaglutide involves, the next step is a clinical conversation. Start your free consultation with our GPhC-registered prescribers, who review every case the same day.