Slimming injections Wegovy: what the trial evidence actually shows

What the STEP 1 trial found, and what it means in practice

The STEP 1 trial, published in the New England Journal of Medicine, recruited adults living with obesity or overweight plus at least one weight-related condition, and ran for 68 weeks. Those receiving 2.4 mg semaglutide weekly reduced their body weight by roughly 15% on average, against around 2.4% on placebo. That gap is clinically meaningful, and it is the figure that underpinned NICE's decision to recommend Wegovy for NHS use under specific conditions.

One misconception worth setting aside: some people assume slimming injections work primarily by suppressing willpower or acting as a stimulant. They do neither. Semaglutide acts on GLP-1 receptors in the gut and brain, slowing how quickly the stomach empties and reducing hunger signals sent to the brain. The result is that smaller portions feel satisfying and the urge to eat between meals lessens, not because of mood alteration, but because of how the hormone pathway is modified.

More recently, a dedicated single-dose 7.2 mg Wegovy pen received MHRA approval in April 2026, with trial data reporting around 20.7% average weight loss over 72 weeks at that dose. That narrows the gap with tirzepatide considerably. You can read a full overview of how Wegovy injections work if you want the mechanism explained in more depth.

Who the slimming injection is licensed for, and what titration looks like

In the UK, Wegovy injections are licensed for adults with a BMI of 30 or above, or a BMI of 27 to 29.9 alongside at least one weight-related condition such as hypertension, type 2 diabetes, high cholesterol or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK clinical guidance. BMI alone does not determine suitability; a prescriber reviews your full health picture.

Titration follows a fixed schedule: 0.25 mg weekly for the first four weeks, then 0.5 mg, 1.0 mg, 1.7 mg and finally 2.4 mg, spending roughly a month at each level. The graduated approach exists to give your digestive system time to adjust. Reaching 2.4 mg typically takes around four to five months. A prescriber decides the pace, skipping steps is not clinically sound and is not something any regulated service should facilitate.

For those who want a practical sense of what each Wegovy injection looks like and how to use it, the step-by-step guide to inserting the Wegovy pen covers injection sites, technique and what to expect after the first dose. The NHS also maintains a semaglutide medicines page with patient-level information on dosing, storage and missed injections.

Side effects to know before starting, and when to act quickly

The most common side effects of Wegovy slimming injections are gastrointestinal: nausea, loose stools, constipation, indigestion, burping and occasionally vomiting. These are most noticeable in the first days after each dose increase and usually settle within a week or two as the body adjusts. Fatigue, headache and mild dizziness also appear in trial data, more often in the early weeks. Injection-site reactions, such as redness or mild bruising at the site, are generally short-lived.

A small number of people experience more serious reactions. Acute pancreatitis is a known, though infrequent, risk; the MHRA's Yellow Card scheme is where to report suspected side effects and also where to flag any suspect sellers. Seek urgent medical attention for severe, persistent stomach pain that radiates towards the back, with or without vomiting. Gallbladder problems, signs of significant dehydration, or any allergic reaction also warrant immediate medical review.

Wegovy is not suitable during pregnancy, while breastfeeding, or for anyone trying to conceive; it is not licensed for under-18s. If you are on oral contraceptives, discuss whether an additional contraceptive method is advisable with your prescriber before starting. The full Wegovy treatment overview covers these contraindications and interactions in detail. Thinking about whether injections are the right route, or curious how costs compare across options? The Wegovy cost page sets out the pricing landscape plainly.

Getting Wegovy safely in the UK

Semaglutide slimming injections are available through two routes in the UK: the NHS, via specialist weight management services under NICE's guidance, where waiting lists are often long; and private prescription, through a GPhC-registered pharmacy following a full clinical consultation. If you go the private route, the single most important check is whether the pharmacy appears on the GPhC register, you can verify any online pharmacy at pharmacyregulation.org. Sellers offering these medicines without a clinical consultation, through social media, or at prices far below market rates are a serious warning sign; counterfeit pens have caused real harm.

At nume, every consultation is personally reviewed by a GPhC-registered Independent Prescriber (not automated software) on the same day it is submitted. Our clinical team handles ID verification, live weight confirmation and GP notification as standard, and aftercare is available seven days a week. A useful starting point if you want to explore whether Wegovy or an alternative GLP-1 is right for you is our weight-loss treatment overview. Ready to take the next step? Speak to our prescribers through a free consultation, with no subscription and no hidden fees.