What is in Wegovy that causes weight loss — and how does the science actually work?

Semaglutide is built to mimic a hormone you already have

GLP-1 (glucagon-like peptide-1) is released naturally from cells in your small intestine when you eat. It tells your pancreas to release insulin, signals to your brain that you are full, and slows the movement of food from the stomach into the intestine. The effect is short-lived because an enzyme called DPP-4 breaks natural GLP-1 down within minutes.

Semaglutide was engineered to resist that breakdown. A small structural change to the natural GLP-1 molecule, plus a fatty acid chain that anchors it to albumin in the blood, extends its half-life to roughly one week, which is exactly why Wegovy is injected once weekly rather than daily. The molecule itself is not a stimulant, a fat-blocker or a diuretic. Its weight-loss effect comes entirely from working on receptors that the body already uses to regulate hunger and metabolism, as the NHS medicines page for semaglutide explains in its overview of how the medicine works.

This is worth sitting with for a moment. The medicine is not overriding your biology; it is amplifying a signal your body already knows. For many people that distinction matters.

Three things semaglutide does inside the body that reduce weight

The first pathway is central: semaglutide crosses into areas of the brain (particularly the hypothalamus) that govern hunger and satiety. It reduces the sensation of hunger between meals and, for many people, also quiets the background mental noise around food that makes dieting so hard to sustain. Cravings and portion sizes both tend to fall.

The second pathway is gastric: by slowing gastric emptying, semaglutide means the stomach stays fuller for longer after each meal. A smaller portion triggers the same fullness signal that previously needed a larger one. This is also why GI side effects (nausea in particular) are most common in the first few weeks; the stomach is adjusting to a different pace. Most people find keeping the pen in the fridge door (alongside whatever else lives there) and injecting on the same evening each week helps build the routine that makes the early weeks easier to manage.

The third pathway involves the reward system. Research suggests semaglutide reduces the dopamine-driven appeal of high-calorie foods, making it easier to choose differently, not through willpower, but through a genuine shift in how rewarding those foods feel neurologically. If you want to understand how Wegovy causes weight loss across all three of these pathways, that page covers the mechanisms in more detail.

What the trial evidence shows, and where the limits are

The STEP 1 trial, published in the New England Journal of Medicine, randomised 1,961 adults with obesity or overweight plus at least one weight-related condition. Over 68 weeks, participants taking semaglutide 2.4mg alongside lifestyle changes lost an average of around 15% of body weight, compared with roughly 2.4% in the placebo group. That is a meaningful clinical difference, but it is an average across a population. Individual responses vary, and the trial was conducted alongside a structured diet and activity programme, not medication alone.

The MHRA approved a higher-dose 7.2mg pen on 14 April 2026 for adults with a BMI of 30 or above; trials at that dose reported average weight loss of around 20.7% over 72 weeks, which narrows the gap with tirzepatide. If you want to understand what Wegovy weight loss looks like in practice and what the evidence actually shows, including how results vary by dose and individual circumstances, the evidence now answers that question fairly clearly, though it remains a conversation every prescriber has individually with each patient.

Wegovy is licensed in the UK 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 or type 2 diabetes. Lower thresholds apply for some ethnic backgrounds under UK guidance. The NICE appraisal of semaglutide for weight management is available at NICE TA875 for anyone who wants to read the committee's reasoning in full. If you are wondering about private access and what it costs, the Wegovy cost page sets out the context honestly.

Why Wegovy and Ozempic contain the same ingredient but serve different purposes

Both Wegovy and Ozempic contain semaglutide. The distinction is licensing, not chemistry. Ozempic is licensed in the UK for type 2 diabetes management; Wegovy is licensed for weight management. The doses and their intended therapeutic targets differ, and the MHRA is clear that Ozempic should not be used as a weight-loss medicine. Rybelsus (oral semaglutide) is also licensed for diabetes, not weight loss, though a separate oral formulation (Wegovy tablets) received MHRA approval for weight management in June 2026, making it the first oral GLP-1 medicine licensed for that purpose in the UK.

If you are exploring the broader landscape of semaglutide as a medicine, or looking at how semaglutide sits alongside tirzepatide on the weight-loss treatment overview, the clinical picture is richer than any single headline figure suggests. A prescriber looks at your individual health history before recommending either. Our clinical team at nume reviews every consultation personally, speak to our prescribers if you want to understand which option, if any, suits your situation.