How the semaglutide injection mechanism of action changes appetite and weight

The decision most people arrive at: injection or tablet, and why mechanism matters to that choice

A question our prescribers hear most weeks is some version of: "Do I really need the injection, or is there a tablet that does the same thing?" It is a fair question, and the mechanism of action is actually central to answering it honestly.

Semaglutide is now available in two licensed forms for weight management in the UK. The weekly subcutaneous injection (Wegovy) delivers semaglutide directly into the bloodstream via fatty tissue, bypassing the gut almost entirely. That means near-complete bioavailability, consistent blood levels between doses, and the appetite-suppressing effects described above arriving reliably. The newer oral option achieves lower systemic exposure because the molecule must survive the gut environment, which is why the tablet's maintenance dose is 25 mg rather than 2.4 mg.

If you are weighing up the formats, the oral alternative to the injection has its own profile worth reading. But for most people on the injection, the mechanism itself is not really the decision — it is already working on their behalf once treatment starts. The decision is whether the clinical picture makes semaglutide the right molecule at all, and that is what the prescriber assesses.

The injection works subcutaneously, meaning under the skin rather than into muscle. Where exactly you place it is more relevant than it might seem, and the evidence on whether the injection site affects semaglutide's performance is worth understanding before you start.

What GLP-1 receptor activation actually does, step by step

GLP-1 is released naturally from cells in the small intestine after a meal. It is short-lived in the body, broken down within minutes by an enzyme called DPP-4. Semaglutide is an analogue of GLP-1 that resists that breakdown, which is why a single weekly injection can sustain the effect across seven days.

Once semaglutide binds to GLP-1 receptors, three things happen in parallel. First, insulin secretion from the pancreas rises in response to blood glucose, a glucose-dependent effect, which is why hypoglycaemia is uncommon in people without diabetes using Wegovy. Second, glucagon release is suppressed, which prevents the liver from adding more glucose into circulation after meals. Third, gastric emptying slows. That last effect is the one most people feel: food sits in the stomach for longer, physical fullness arrives earlier in a meal, and the urge to eat again comes later.

On top of the gut effects, semaglutide crosses into the central nervous system and acts on appetite-regulating circuits in the brain. The hypothalamus receives a sustained signal that energy reserves are adequate. Cravings and hedonic eating (eating for reward rather than hunger) are reported to reduce, something the trials measured indirectly through calorie-intake data. The NHS medicines page on semaglutide gives a patient-level overview of how the medicine works and what to expect.

The 68-week STEP 1 trial, published in the New England Journal of Medicine, showed an average body-weight reduction of around 15% at the 2.4 mg maintenance dose alongside lifestyle changes. That figure is a population average across thousands of participants, not a guarantee for any individual, biology, adherence, diet quality and starting weight all influence outcomes.

Why the titration schedule exists and what it tells you about the mechanism

Semaglutide's dose ladder (starting at 0.25 mg and moving up in monthly steps toward 2.4 mg) is not the prescriber being cautious for its own sake. It follows directly from the mechanism.

Slowed gastric emptying, the same effect that reduces hunger, is also responsible for the nausea, vomiting and reflux that some people experience early in treatment. Starting at a low dose gives the gut time to adjust to that slower transit before the full appetite-suppressing signal is established. Jumping straight to a therapeutic dose would, for most people, produce a rough first few weeks with little benefit in terms of weight loss over the longer course.

Once the 2.4 mg maintenance dose is reached, the appetite effects are broadly stable week to week. Blood levels of semaglutide peak around 24–48 hours after injection and decline gradually over the following days, which is why the same day each week is recommended. Storing the pen correctly during this time matters: temperature excursions outside the refrigerated range can degrade the active molecule. Our page on how to store your semaglutide injection covers the specifics from the SmPC.

If a dose is missed or delayed, the prescriber and the Patient Information Leaflet are the right reference, not general guidance, because the answer depends on timing and how long the gap has been. This is one practical reason that ongoing clinical oversight, rather than a one-off prescription, matters for GLP-1 treatment.

Side effects that follow from the mechanism, and when to act quickly

Knowing why side effects occur makes them easier to manage and easier to recognise when something is outside the expected range. The gastrointestinal effects (nausea, loose stools, constipation, reflux, burping) are direct consequences of slowed gastric motility. They tend to be most noticeable at the start of treatment or after a dose increase, and for most people settle over days to a couple of weeks as the gut adapts.

Staying well hydrated matters more than it sounds, because persistent vomiting or diarrhoea can lead to dehydration quickly, and dehydration compounds the nausea. Eating smaller, lower-fat meals during the adjustment phase is consistently reported to help.

Two signals require prompt medical attention. Severe, persistent stomach pain that spreads to the back (with or without vomiting) can indicate acute pancreatitis, a rare but serious effect that the MHRA highlighted in a Drug Safety Update in January 2026. Symptoms of gallbladder problems (sharp upper-right abdominal pain, fever, jaundice) also warrant same-day assessment. Neither is common, but neither should be waited out at home.

If you are at the point of weighing up costs alongside the clinical picture, our Wegovy cost page sets out what private treatment typically involves in the UK. And if you are already considering semaglutide or want the broader picture on how injectable semaglutide compares across different brand names and indications, that context is worth having before a consultation.