What are semaglutide medicines and how do they work?

You've heard the name, but what is semaglutide actually doing inside your body?

Picture the moment you finish a meal. Your gut releases a hormone called GLP-1 (glucagon-like peptide-1) that tells your brain you've had enough, slows the pace at which food leaves your stomach, and nudges your pancreas to release insulin. Semaglutide is a synthetic version of that hormone, engineered to bind the same receptors but to last far longer in the body than the natural hormone does. A single injection persists for roughly a week; a daily tablet sustains a lower but continuous level throughout the day.

The practical effect for most people is a meaningful reduction in appetite and, over time, in calorie intake, not through willpower, but through the biology of satiety signalling. That's an important distinction. Weight gain driven by hormonal and metabolic factors doesn't respond reliably to effort alone, and semaglutide addresses some of those underlying mechanisms directly. For a broader look at how the medicine fits into weight management, the semaglutide overview on this site covers the clinical picture in more depth.

The medicine also has effects on blood sugar regulation, which is why it was originally developed for type 2 diabetes. That dual action shapes both its benefits and some of the checks a prescriber will run before approving it for you.

Wegovy and Ozempic both contain semaglutide, so why aren't they the same thing?

This is the question our prescribers hear most weeks, and the answer matters. The UK drugs that contain semaglutide are Wegovy and Ozempic (injection) and Wegovy tablets and Rybelsus (oral). Same molecule; different licences, doses and purposes.

Wegovy injection (doses from 0.25 mg up to 2.4 mg (or the newer 7.2 mg maintenance dose approved by the MHRA in April 2026)) is licensed for weight management in adults with a BMI of 30 or above, or 27 and above with a weight-related condition such as high blood pressure or high cholesterol. Ozempic, by contrast, is licensed for type 2 diabetes, not weight loss. The doses are different, the clinical criteria are different, and a UK prescriber cannot lawfully issue Ozempic for weight management. Rybelsus (3 mg, 7 mg, 14 mg tablets) is similarly a diabetes medicine; it is not the same product as the Wegovy weight-loss tablet, which follows its own dosing schedule entirely.

The confusion between brands is understandable given media coverage, but prescribers are bound by the licensed indications. If you're interested in the injection schedule in more detail, the Wegovy dose breakdown explains each step. For the tablet route specifically, what semaglutide is licensed to treat sets out both indications clearly.

What does the trial evidence actually say about weight loss?

The headline figure most people land on is around 15% average body-weight reduction over 68 weeks in the STEP 1 trial, a large phase-3 study of the 2.4 mg weekly injection in adults with obesity who did not have type 2 diabetes, published in the New England Journal of Medicine. That's an average; individuals in the trial ranged widely. Trial participants also received lifestyle support alongside the medicine, which is part of what the licence requires.

The Wegovy tablet, approved in the UK in June 2026 as the first oral GLP-1 for weight management, showed approximately 13.6% average weight loss in its phase-3 trial (with around 17% among those who adhered fully to treatment, that second figure carries an important caveat, because it reflects a subset who didn't miss doses). The tablet's appeal is straightforward: no injections, no refrigeration. Keep it in your bag or on the kitchen counter rather than remembering to take it out of the fridge door each morning. You take it on an empty stomach with a small amount of water, then wait at least 30 minutes before eating or drinking anything else.

These are clinical trial results under controlled conditions. What happens in practice depends on individual metabolism, dose tolerability, diet and activity. A prescriber doesn't just check a box; they assess your full picture before recommending which product and starting dose fits your situation. You can read more about how Wegovy is prescribed in the UK and what the clinical criteria look like.

Side effects, monitoring and the practical realities

The most common side effects reported in trials and in practice are gastrointestinal: nausea, loose stools, constipation, indigestion, belching and sometimes vomiting. They're typically most pronounced in the first weeks of treatment or after a dose increase, and they tend to settle as the body adjusts. Drinking enough water and eating smaller, lower-fat meals helps most people get through the adjustment period.

Less common but more serious effects require prompt medical attention. The MHRA highlighted acute pancreatitis in a January 2026 Drug Safety Update: severe stomach pain that radiates to the back, with or without vomiting, needs urgent assessment. Gallbladder problems are also associated with rapid weight loss on GLP-1 medicines, and prescribers weigh this in the initial assessment. Anyone who experiences symptoms of a serious allergic reaction (swelling of the face, tongue or throat, difficulty breathing) should call 999.

Semaglutide is not recommended during pregnancy, while breastfeeding, or for anyone trying to conceive. It isn't licensed for under-18s. People with a personal or family history of medullary thyroid carcinoma should not use it. These aren't edge cases; they're standard clinical exclusions that a thorough consultation process checks for routinely. If you want to understand the injection format and routine in more detail before starting, the guide on Wegovy dosing frequency is a useful next step. Suspected side effects can be reported directly to the MHRA through the Yellow Card scheme.

If you'd like to explore whether treatment is right for you, speak to our prescribers through a free consultation, reviewed the same day by a real clinician, not an algorithm.