Semaglutide Explained: the Science, the Evidence and What It Means for You

What the STEP 1 trial actually found — and why the headline figure needs context

The STEP 1 trial, published in the New England Journal of Medicine and cited by NICE in its appraisal of semaglutide, randomised nearly 2,000 adults with obesity or overweight and at least one weight-related condition to either 2.4mg semaglutide weekly or placebo, alongside lifestyle support. After 68 weeks, the semaglutide group lost an average of around 15% of their starting weight, against roughly 2.4% in the placebo group. Those are group averages, individual results varied considerably, which is worth knowing before comparing your progress to a headline number.

What the trial also showed is that most weight loss occurred steadily over the first year, with results tailing off as participants approached their new equilibrium. That pattern matters because it sets realistic expectations: semaglutide is not a fast-acting medicine, and the dose titration schedule (starting at 0.25mg and climbing over roughly five months toward 2.4mg) is designed to let the body adjust before reaching the therapeutic dose. You can read through the broader evidence picture on our Wegovy information page.

One figure that is sometimes quoted without its context: NICE's appraisal notes that treatment should be stopped and reviewed if weight loss is less than 5% after six months at the maintenance dose. That is not a punishment clause, it is a clinical safety check to ensure patients who are not responding do not continue indefinitely on a medicine that is not helping them.

The biology behind semaglutide: one receptor, several effects

GLP-1 (glucagon-like peptide-1) is a hormone released by the gut after eating. It signals satiety to the brain, prompts the pancreas to release insulin in response to food, and slows the rate at which the stomach empties. Semaglutide is a synthetic analogue of GLP-1, modified to resist the enzyme that breaks the natural hormone down quickly. That modification extends its half-life from a few minutes to roughly a week, which is why a single weekly injection sustains the effect.

The practical result for most people is a noticeable reduction in appetite and, for many, a different relationship with hunger between meals. Food cravings often reduce. Portion sizes that once felt inadequate can feel sufficient. The NHS medicines page for semaglutide explains the mechanism in accessible terms and lists the information every patient should read before starting.

It is also why protein intake, hydration and fibre become more important on treatment, not less, eating less overall can make nutritional gaps appear. A prescriber or dietitian can advise on this in the context of your individual plan. The semaglutide overview at nume covers what to expect from the medicine itself, and if you are partway through your programme you may also find our pages on semaglutide 1 and semaglutide 2 useful for understanding what each stage of the dose journey involves.

Eligibility in practice: what the licence and the NICE criteria actually say

Semaglutide 2.4mg (Wegovy) carries a UK licence for adults with a BMI of 30 or above, or 27 to 29.9 with at least one weight-related condition such as high blood pressure, high cholesterol, type 2 diabetes or obstructive sleep apnoea. Lower BMI thresholds apply for people from South Asian, Chinese, Middle Eastern, Black African or African-Caribbean backgrounds under UK clinical guidance, typically 2.5 kg/m² lower at each threshold.

The NICE recommendation for NHS access, set out in NICE technology appraisal TA875, is more restrictive: it applies within specialist weight management services, covers a maximum treatment duration of two years, and requires a BMI of at least 35 with one or more weight-related comorbidities. Private prescribing follows the licensed criteria rather than the NICE commissioning criteria, so more people are eligible through a private clinical route. If you want to understand cost in that context, the treatment and pricing page sets out what is included in a private prescription.

BMI is a starting point, not the whole picture. A prescriber also considers medical history, current medicines, and whether any contraindications apply, including pregnancy, breastfeeding, or trying to conceive, where semaglutide is not recommended. People under 18 are outside the licensed indication. The frequently asked questions page covers the most common eligibility queries we receive.

Storage, side effects and what to do if something feels wrong

Wegovy injections are stored in the fridge, many people keep the current pen in the fridge door, which makes the weekly routine easier to remember. The exact room-temperature window, if you are travelling or the pen has been out of the fridge, is in the Patient Information Leaflet rather than summarised here, because the detail matters and should not be paraphrased. A practical guide to checking whether a Wegovy pen has been frozen is worth reading before your first pen, accidental freezing is one of the more common storage questions we see.

Side effects are predominantly gastrointestinal. Nausea is the most frequently reported; it tends to peak in the days after a dose increase and ease as the body adjusts. Constipation, loose stools, reflux and fatigue are also common. The titration schedule exists partly to reduce the chance of severe GI symptoms by giving the digestive system time to adapt at each level before moving higher.

Pancreatitis is a less common but serious risk: in January 2026 the MHRA issued a Drug Safety Update for GLP-1 medicines specifically flagging acute pancreatitis. Severe, persistent stomach pain that spreads to the back (with or without vomiting) needs urgent medical attention, not watchful waiting. Anyone experiencing this should contact their GP or call 111 immediately. Suspected side effects can also be reported through the MHRA's Yellow Card scheme. If you have concerns about how your treatment is going, the aftercare support team is available seven days a week. Occasionally, people find weight returns after stopping, if that resonates, there is specific guidance on weight regain after Wegovy that is worth reading.