The Wegovy weight loss jab: what clinical trials and UK regulators actually show

What the STEP 1 trial actually measured, and what the numbers mean for you

The place to start with any weight loss jab is the evidence base, because marketing language and clinical data are often two different things. STEP 1 was a 68-week, placebo-controlled trial published in the New England Journal of Medicine involving nearly 2,000 adults with obesity or overweight and at least one related condition. Participants using semaglutide 2.4mg lost roughly 15% of their body weight on average. Those on placebo lost around 2.4%.

Fifteen percent is a meaningful number. For someone weighing 100 kg at the start, that translates to roughly 15 kg over a year and a half. But it is an average, and averages hide a range: some people in STEP 1 lost considerably more, others less. The trial also required participants to make lifestyle changes alongside the jab, which the licence still reflects today, Wegovy is approved as an adjunct to a reduced-calorie diet and increased physical activity, not a replacement for them.

The newer 7.2mg dose has since been approved by the MHRA. Trial data at that higher strength reported around 20.7% average weight loss over 72 weeks, narrowing the gap with tirzepatide results seen in SURMOUNT-1. If you want to explore how the two jabs compare on the evidence, the weight loss jabs and Wegovy comparison page sets that out clearly.

How the Wegovy jab works in the body

Semaglutide mimics a hormone called GLP-1, which your gut releases naturally after eating. By activating GLP-1 receptors in the brain and digestive system, it reduces appetite, increases feelings of fullness, and slows how quickly food moves out of the stomach. The result, for most people, is that they feel satisfied on less food and experience fewer cravings, particularly in the first few months of treatment.

It is worth knowing that semaglutide is not the only GLP-1 medicine available. Semaglutide as an active ingredient also appears in Ozempic (approved for type 2 diabetes) and Rybelsus (oral diabetes treatment), but Wegovy is the only semaglutide formulation licensed specifically for weight management in the UK. Those other products are not interchangeable for weight loss purposes under UK law.

The jab is injected once a week into the abdomen, thigh or upper arm, and the pen rotates injection sites to avoid tissue build-up. Treatment starts at a low dose to give the body time to adjust, with a prescriber titrating upward at roughly monthly intervals. The specific schedule (what dose you start on, when to increase, and at what point to review progress) is always a clinical decision, not something to self-manage. More detail on how the Wegovy pen works is available if you want a practical walkthrough before your consultation.

Who the Wegovy weight loss jab is licensed for in the UK

The UK licence covers 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 some ethnic backgrounds under UK clinical guidance, 2.5 kg/m² lower, typically, for South Asian, Chinese, Middle Eastern, Black African or African-Caribbean backgrounds. The NICE appraisal of semaglutide for weight management, TA875, sets out additional NHS-specific criteria including a maximum treatment duration of two years and the requirement for specialist multidisciplinary support.

A private prescription route exists for people who do not meet NHS criteria or who prefer not to wait. On that route, the licensed eligibility thresholds still apply, a prescriber checks your BMI, health history and current medicines before approving anything. If you are considering this option, our Wegovy weight loss service covers what the process involves and how to get started safely. Wegovy is not suitable during pregnancy, breastfeeding or when actively trying to conceive, and it is not licensed for anyone under 18. A history of certain conditions, including medullary thyroid carcinoma or pancreatitis, requires specific discussion with a prescriber before treatment can begin.

If you are weighing up the private cost alongside the clinical picture, the Wegovy cost page explains what a legitimate private prescription typically includes and what to watch for in pricing that seems unusually low.

Side effects of the Wegovy jab and when to get medical help

The most common side effects of Wegovy are gastrointestinal: nausea, vomiting, diarrhoea, constipation and indigestion. These tend to be at their worst when starting treatment or after a dose increase, and they generally settle within days to a couple of weeks as the body adjusts. Keeping meals smaller, eating slowly and staying well hydrated helps most people through the early weeks. One question our prescribers hear regularly is whether the nausea ever goes away, for the majority of patients, it does ease considerably once a dose has been stable for a few weeks.

Some side effects require prompt medical attention. In January 2026 the MHRA issued a Drug Safety Update highlighting acute pancreatitis as a known but infrequent risk with GLP-1 medicines: if you experience severe, persistent stomach pain that radiates to the back, with or without vomiting, seek urgent medical help. Gallbladder problems, signs of dehydration if vomiting has been severe, and symptoms of a serious allergic reaction should also be assessed quickly. Patients and carers can report suspected side effects directly to the MHRA via the Yellow Card scheme.

For a fuller picture of semaglutide for weight loss (including how side effects tend to evolve over the treatment course) that page goes into more depth. And if you are ready to talk through your own situation with a clinician, checking your eligibility with our prescribers is a good place to start.