The semaglutide drug class: GLP-1 receptor agonists explained

The biggest misconception: semaglutide is not a new type of drug invented for weight loss

The class that semaglutide belongs to (GLP-1 receptor agonists) has been used clinically since the mid-2000s, primarily to treat type 2 diabetes. The idea that these are brand-new, poorly understood compounds is one of the most common misunderstandings our prescribers encounter. Exenatide, the first approved GLP-1 agonist, launched in 2005. Liraglutide followed. Semaglutide came later, with a longer half-life that made once-weekly dosing possible rather than the daily injections earlier versions required.

The weight-loss application is newer, yes. But the underlying drug class has a substantial body of evidence built over nearly two decades. When researchers began studying higher doses in people without diabetes, the weight-loss results were striking enough to prompt entirely new licensing decisions, not because the mechanism was reinvented, but because the dose turned out to matter far more than anyone had initially expected.

Understanding this history matters because it changes the risk calculus. You are not volunteering to test an experimental substance; you are taking a medicine whose class mechanism is well characterised, with long-term safety data from millions of patients with diabetes informing how clinicians think about it today. That context tends to reassure people who are understandably cautious.

What GLP-1 receptor agonists actually do in the body

GLP-1 (glucagon-like peptide-1) is a hormone released naturally from cells in your gut when you eat. It has several jobs: it nudges the pancreas to release insulin in response to rising blood sugar, slows the rate at which food leaves the stomach, and sends signals to the brain that reduce appetite. In people with obesity, this hormone system can become less effective over time.

GLP-1 receptor agonists are synthetic molecules designed to bind to the same receptors the natural hormone targets, producing those same effects but lasting far longer. Semaglutide, for instance, has a half-life of around one week, which is why a single weekly injection sustains therapeutic levels throughout the interval. You can find a fuller account of how semaglutide works as a medicine in its own right on this site.

One practical habit worth building early: if you are prescribed any GLP-1 medicine, take a moment to check the patient information leaflet that comes in the box before your first injection. It takes less than a minute to confirm the storage temperature (these medicines are refrigerated) and the information is specific to the brand and dose you have been given, which matters more than any general guide online.

The class does not work by blocking fat absorption (that is orlistat's mechanism) or by stimulating the central nervous system the way older appetite suppressants did. The appetite reduction is peripheral and central (gut and brain together) which is why many patients describe feeling less preoccupied with food rather than feeling suppressed or jittery.

Where Wegovy fits within the class, and why the drug class matters for UK licensing

Semaglutide exists in more than one brand and more than one formulation, and the distinction matters legally. Ozempic is also semaglutide, but it carries a UK licence for type 2 diabetes, not weight management. Wegovy (the brand name you will see discussed in the context of weight loss) is licensed by the MHRA specifically for adults with a BMI of 30 or above, or from 27 with a weight-related health condition, used alongside a reduced-calorie diet and increased physical activity. The Wegovy overview page covers its licensed doses and UK approval in detail.

NICE recommended semaglutide (Wegovy) for use in specialist weight management services under Technology Appraisal 875, published in March 2023. The recommendation comes with specific criteria (including a maximum duration of two years and multidisciplinary support) so the NHS pathway is narrower than private prescribing. Privately, a prescriber can assess suitability against the licensed eligibility criteria without those additional constraints, subject to a full clinical review.

Wegovy also now exists as an oral tablet, the MHRA approved oral semaglutide for weight management in June 2026, making it the first licensed oral GLP-1 in the UK for this purpose. If you are curious about the difference between injection and tablet options, our treatment overview sets out what is currently available.

It is also worth knowing that semaglutide sits alongside, but is distinct from, tirzepatide (the active ingredient in Mounjaro) which is a dual GIP and GLP-1 receptor agonist. That is a different class, or more precisely a class extension: two receptors instead of one. You can read more about how the two medicines compare in clinical context if you are weighing options. The key point here is that what medication class a medicine belongs to is not just academic, it shapes how it is licensed, how it is monitored, and which patients are most likely to benefit from it.

Side effects and safety: what the class profile looks like in practice

GLP-1 receptor agonists share a broadly consistent side-effect profile across the class, and semaglutide is no exception. The most common effects are gastrointestinal: nausea, loose stools, constipation, reflux and, less often, vomiting. These are most noticeable in the early weeks after starting or after a dose increase, and they typically settle as the body adjusts. The NHS medicines page for semaglutide lists the full frequency data in plain language and is worth reading before you start.

Less common but more serious effects include acute pancreatitis. The MHRA issued guidance on this risk across the GLP-1 class in January 2026, advising patients to seek urgent medical attention for severe stomach pain that radiates to the back, with or without vomiting. That signal does not mean the risk is common (it is not) but it is the kind of thing that warrants knowing before you begin. Similarly, gallbladder problems occur at a higher rate in people losing weight rapidly, regardless of whether a medicine is involved, and GLP-1 treatment can accelerate that process.

The class is not recommended during pregnancy, when breastfeeding, or for anyone trying to conceive. It is not licensed for under-18s. If you are taking oral contraceptives and starting tirzepatide specifically, additional contraceptive precautions apply during dose increases, though the equivalent evidence for semaglutide is less clear-cut. These are the kinds of details a prescriber works through with you individually, not something to determine from a webpage. If you would like a clinical view on whether semaglutide (as a Wegovy prescription) could be appropriate for you, speaking to our prescribers is the right next step.