Semaglutide facts: what the clinical evidence actually shows

What does semaglutide actually do in the body?

Semaglutide belongs to a class called GLP-1 receptor agonists. GLP-1 (glucagon-like peptide-1) is a hormone released naturally after eating; it tells the brain you have had enough, prompts the pancreas to release insulin in a glucose-dependent way, and slows the rate at which the stomach empties. Semaglutide mimics and amplifies this signal, which is why appetite often falls noticeably in the first weeks of treatment.

A question our prescribers hear most weeks is whether semaglutide is the same thing as Ozempic. The molecule is identical. What differs is the licensed dose and the licensed use: Ozempic tops out at 1mg (or 2mg in some formulations) for blood-sugar control in type 2 diabetes, while Wegovy titrates up to 2.4mg weekly (and now up to 7.2mg) specifically for weight management. Prescribing one for the other's purpose would be off-label. Our semaglutide overview covers this distinction in more detail.

The mechanism also explains the most common side effects: because digestion slows, nausea, bloating, indigestion and changes in bowel habit are the effects people most often notice, particularly when a dose increases. They typically settle within days to two weeks. The NHS semaglutide medicines page sets out the full side-effect profile clearly.

How strong is the trial evidence for semaglutide's weight-loss results?

The STEP 1 trial is the headline reference: 1,961 adults with obesity or overweight and at least one weight-related condition, no diabetes, randomised to 2.4mg semaglutide or placebo over 68 weeks. The semaglutide group lost around 15% of body weight on average. Around a third lost more than 20%. Placebo participants lost roughly 2.4%.

Those are group averages, and individual results vary based on starting weight, adherence, diet, activity and biology. Clinical trials enrol people who meet strict criteria and follow structured protocols; real-world outcomes tend to cluster closer to the lower end of trial ranges. That said, the size and consistency of the STEP programme findings is why NICE recommended semaglutide (Wegovy) for weight management in Technology Appraisal TA875.

The evidence picture evolved further in 2025 and 2026. The SURMOUNT-5 trial compared tirzepatide directly with semaglutide 2.4mg and found tirzepatide produced greater average weight reduction over 72 weeks. Separately, the MHRA approved a 7.2mg Wegovy dose (initially as three 2.4mg pens weekly, then as a dedicated single-dose pen approved on 14 April 2026) with trials reporting around 20.7% average weight loss at the higher dose, narrowing the gap with tirzepatide. Our Wegovy facts page covers those approval details.

Who does UK guidance say semaglutide is suitable for?

For the Wegovy injection, the licensed criteria sit at BMI 30 or above, or 27 to 29.9 alongside at least one weight-related condition such as hypertension, dyslipidaemia, type 2 diabetes or obstructive sleep apnoea. For the Wegovy tablet (approved 11 June 2026), the same thresholds apply.

NICE's TA875 recommendation is narrower than the licence: it specifies use within a specialist weight management service with multidisciplinary support, for adults with a BMI of 35 or above and at least one weight-related comorbidity, for a maximum of two years. NHS access therefore depends on meeting those criteria and joining a specialist pathway. For many people, the waiting time is substantial.

Private prescribing follows the licensed criteria rather than NICE's NHS-specific conditions. A prescriber still assesses each person individually: BMI alone is not a green light. Lower BMI thresholds apply for some ethnic backgrounds under UK clinical guidance. You can explore how the weight-loss treatment options compare across different eligibility starting points.

One important exclusion: semaglutide is not recommended during pregnancy, while breastfeeding, or when actively trying to conceive. It is not licensed for anyone under 18.

What about the newer oral semaglutide, does it work as well?

The Wegovy tablet is the first oral GLP-1 medicine approved in the UK for weight management, licensed by the MHRA on 11 June 2026. The UK was the first country in Europe to authorise it. The active molecule is semaglutide co-formulated with an absorption enhancer called SNAC, which allows it to cross the stomach wall.

In the OASIS 4 phase 3 trial (307 adults with obesity or overweight, 64 weeks, no diabetes), participants taking the oral formulation lost around 13.6% of body weight on average compared with roughly 2.4% on placebo. Among those who adhered fully to treatment, the average was closer to 17%, though that figure applies only to the fully-adherent subgroup, not the trial population as a whole.

The tablet is taken once daily, first thing in the morning on an empty stomach, with a small amount of plain water. You wait at least 30 minutes before eating, drinking anything else or taking other oral medicines. Unlike the injection, it requires no refrigeration, which matters for travel. Our semaglutide fact sheet includes the full titration schedule, including guidance on what to expect at the semaglutide 1mg stage and information on how semaglutide can affect the face during weight loss. If you are considering starting, a free consultation with our prescribers is the place to begin, they review your full clinical picture before any treatment is recommended.