Wegovy (semaglutide) explained — how it works and how it compares for weight loss

What semaglutide actually is

Semaglutide is a once-weekly injectable peptide that mimics GLP-1 (glucagon-like peptide-1), one of the gut hormones the body releases in response to food. GLP-1 helps regulate appetite, satiety, gastric emptying, and insulin response. Natural GLP-1 is broken down within minutes; semaglutide is engineered to last about a week per dose, so the signals stay on for much longer than the body's own version.

The same molecule is sold as Ozempic for type 2 diabetes and as Wegovy at higher doses for chronic weight management. Same drug, different licence, different dose ladder, different intended use. That distinction matters: in the UK, the MHRA and clinicians have actively discouraged using Ozempic off-label for weight loss because it diverts supply from people with diabetes and isn't prescribed inside a structured weight management programme.

Who the UK licence is for

The MHRA licence for Wegovy in weight management covers adults with a BMI of 30 or more, or 27–30 with at least one weight-related comorbidity (such as type 2 diabetes, hypertension, dyslipidaemia, obstructive sleep apnoea, or cardiovascular disease). For NHS access via NICE TA875, eligibility is tighter: BMI ≥35 (≥32.5 in some ethnic groups, including South Asian, Chinese, Black African, and African-Caribbean) with at least one comorbidity, and treatment capped at two years through specialist weight management services.

Private prescribing follows the MHRA licence. A responsible service will only initiate after a full consultation that confirms BMI, comorbidities, contraindications, and prior lifestyle effort.

The dose escalation

Semaglutide is escalated over 16 weeks to a maintenance dose of 2.4 mg weekly. The standard ladder is 0.25 mg → 0.5 mg → 1 mg → 1.7 mg → 2.4 mg, each held for 4 weeks before stepping up. The starter dose is not a treatment dose — it's there to let the gut adapt.

Slow escalation matters. Rushing it almost always means worse nausea and a higher chance of stopping early. Some patients tolerate a slower pace than the standard ladder and that's fine — the right pace is the one you can stay on long enough to get the benefit.

What the first few months feel like

The first 1–2 doses usually don't feel like much. By weeks 2–4, most people notice that their appetite is quieter — meals feel adequate sooner, snacking becomes effortful rather than automatic, and cravings for energy-dense foods fade. The scale typically starts moving after the second or third dose.

The most common early side effects are nausea (often after larger or fattier meals), mild constipation, occasional reflux, fatigue, and sometimes a metallic taste. These tend to peak shortly after each dose step and settle within 1–2 weeks as the body adapts. The single most useful behaviour change in the early weeks: smaller portions, slower eating, stop when you feel full. The medication is telling you to stop; pushing through usually means feeling unwell within the hour.

Where Wegovy fits now

For a while, semaglutide was effectively the only well-evidenced GLP-1 option for non-diabetic weight management. Then tirzepatide arrived — a dual GIP/GLP-1 agonist with larger average weight loss in head-to-head trials. That doesn't make Wegovy obsolete. It does mean the conversation in consultations has shifted.

Wegovy still makes sense if you've previously tolerated semaglutide well and want to continue, if tirzepatide isn't appropriate for you, if cost or supply favours it at the time, or simply if you prefer the older, more familiar option with the longer real-world track record. Cardiovascular outcome data on semaglutide (SELECT trial) is also stronger and longer than on tirzepatide, which can matter clinically.

What the structured programme means in 2026

Following MHRA action on prescribing standards, every UK Wegovy prescription now sits inside a structured weight management programme. The prescription is one component of a service that should include clinical consultation before initiation, dietary and behavioural support, monitoring through escalation, side effect management, and a clear plan for maintenance and discontinuation.

Pharmacist-led services suit this well: you get continuity, in-person access, and the ability to adjust in real time rather than waiting weeks for a remote review.

Side effects to know about

The common, expected side effects (nausea, mild constipation, reduced appetite, occasional reflux, mild fatigue) are usually manageable with portion control, hydration, and time. They tend to be at their worst after each dose increase and settle within 1–2 weeks.

The serious ones are rare but important. Pancreatitis presents as severe, persistent upper abdominal pain, often radiating through to the back — stop the medication and seek urgent care. Gallbladder problems can occur, partly driven by the rate of weight loss itself. Severe dehydration is a risk if vomiting is heavy. And like all GLP-1-family medicines, semaglutide carries a precaution around personal or family history of medullary thyroid cancer or MEN2.

Cost, plainly

Prices aren't listed in this guide because they vary by dose, supply, and what's included in a programme. The principle to apply: a responsible private price should bundle consultation, ongoing monitoring, dietary support, and clinician access for side effects — not just hand you a pen. A pen-only price tag with no wraparound isn't a programme.

The 'rebound' question

The STEP-4 trial answered this clearly: when people on full-dose semaglutide were switched to placebo, average weight gradually returned over the following year. That isn't a failure of the medicine — it tells you what the medicine is doing. While you take it, the appetite signal is amplified. When you stop, your underlying physiology returns to where it was.

The honest framing: the medication gives you a window — usually 12–18 months of strong appetite control — to rebuild eating, activity, and behavioural patterns so the new baseline holds when you taper. The medication alone doesn't change behaviour permanently. Ongoing support during loss and maintenance is where durable results come from.

When not to use it

Wegovy isn't appropriate for everyone. Absolute contraindications include personal or family history of medullary thyroid carcinoma, MEN2 syndrome, and known hypersensitivity to semaglutide. Cautions and shared-decision conversations apply if you have a history of pancreatitis, severe gastrointestinal disease (including gastroparesis), severe renal impairment, are pregnant or planning pregnancy, or breastfeeding.

If you're under 18 (Wegovy is only licensed from 12 in specific scenarios), on insulin, or have type 1 diabetes, talk to a specialist clinician before considering treatment.

The next step

The most useful single step if you're considering Wegovy is a 20–30 minute consultation: it's where eligibility gets confirmed, expectations get set, the right starting dose gets agreed, and you find out whether the structured programme actually fits how you live. It's also where the choice between Wegovy and Mounjaro stops being a marketing question and becomes a clinical one.