How the semaglutide injection works — and what the evidence says

What GLP-1 actually does, and why the injection outperforms the natural version

GLP-1 (glucagon-like peptide-1) is released naturally from cells in the small intestine within minutes of eating. It tells the pancreas to release insulin, slows the rate at which the stomach empties, and carries a satiety signal to the hypothalamus, the part of the brain that governs hunger. The problem is that GLP-1 is broken down in the bloodstream within about two minutes of release, so its natural effect is brief.

Semaglutide is a synthetic analogue: its molecular structure is close enough to human GLP-1 to bind the same receptors, but modified at two points so that enzymes cannot break it down quickly. The result is a molecule with a half-life of roughly one week, which is why a semaglutide injection sustains the effect across seven days. According to the NHS medicines page for semaglutide, that prolonged activity at GLP-1 receptors is what produces the consistent reduction in appetite seen in clinical trials. Slowed gastric emptying means food stays in the stomach longer, which extends the feeling of fullness after a meal. Reduced hunger signalling between meals means fewer and smaller eating episodes. Together those effects create a sustained calorie deficit without requiring conscious restriction at every meal.

One misconception worth setting aside: semaglutide is not simply a metabolism booster. It does not meaningfully increase the rate at which calories are burned. The weight loss comes from eating less, not burning more, and that distinction matters when patients and clinicians are setting realistic expectations.

What the STEP programme found, and where the 15% figure comes from

The STEP 1 trial enrolled 1,961 adults with a BMI of 30 or above, or 27 or above with at least one weight-related condition, and no type 2 diabetes. Participants received either 2.4mg semaglutide weekly or a placebo, both alongside lifestyle counselling, for 68 weeks. Those on semaglutide lost an average of roughly 15% of starting body weight; the placebo group lost around 2.4%. About a third of participants on semaglutide lost 20% or more. These are trial averages, individual responses vary, and the prescriber assesses how treatment is going at each review.

The STEP programme included further trials in people with type 2 diabetes, cardiovascular disease and other conditions. Across those trials, the consistency of the appetite-reduction effect gave regulators and NICE the confidence to recommend semaglutide injections for weight management. NICE's appraisal of semaglutide (TA875) concluded it was clinically effective when used within a specialist weight management service for up to two years, for adults with a BMI of 35 or above and at least one weight-related condition. Lower BMI thresholds apply for some ethnic backgrounds under that guidance.

If you are weighing up semaglutide against other options, details of how the broader landscape of weight management treatments compares can help frame that conversation with a prescriber.

How the injection schedule translates the mechanism into practice

Wegovy is injected once a week, subcutaneously (just under the skin) at the abdomen, thigh or upper arm. The dose starts low and is increased gradually by the prescriber, typically in roughly four-week steps, to give the body time to adjust. The starting dose is designed to minimise nausea while the GI system acclimatises to slowed gastric emptying; it is not the dose at which most of the therapeutic effect operates. The prescriber decides the pace and ceiling based on how treatment is tolerated.

You can read about the specific mechanics of using the pen (priming, needle attachment, injection angle) in our step-by-step guide to injecting semaglutide. The pen itself has a straightforward design; how the Wegovy pen works is covered in detail on a dedicated page if you want to understand the device before your first dose.

Side effects are mostly gastrointestinal and most noticeable at the start or after a dose increase: nausea, constipation, diarrhoea and indigestion are the most commonly reported, typically settling within days to a couple of weeks. Seek urgent medical attention for severe, persistent stomach pain that radiates to the back, this can be a sign of pancreatitis, which the MHRA highlighted as an infrequent but serious risk in its January 2026 Drug Safety Update for GLP-1 medicines. Any side effects you suspect are linked to treatment can be reported at the MHRA's Yellow Card scheme.

How long before the mechanism produces visible change

The appetite-reducing effect begins with the first injection, but measurable weight loss takes longer to accumulate. Most people notice changes within four to eight weeks; the larger reductions seen in trials emerged over months, not days. A question our prescribers hear regularly is whether the medicine has stopped working when progress slows around weeks eight to twelve. It usually has not, weight loss at this stage often reflects the body adapting energy expenditure, which is a normal physiological response rather than treatment failure.

For a realistic picture of the typical timeline, how long semaglutide takes to work sets out what the evidence says week by week. If you are curious about the background science (where semaglutide originates and how the molecule was developed) the origin of semaglutide is worth a read.

Wegovy is a prescription-only medicine. A clinical assessment is required, and the prescriber's job is to decide whether it is appropriate for you specifically, not every BMI, medical history or medication list makes it suitable. If you would like that assessment carried out by a GPhC-registered Independent Prescriber, the free consultation at nume is the place to start.