How semaglutide was discovered, and why a Gila monster changed medicine

The hormone hiding in your gut that researchers almost overlooked

Picture a researcher in the early 1980s trying to understand why eating triggers insulin release. The obvious candidates (glucose, fat) were well known. What wasn't well understood was that the gut itself sends hormonal signals to the pancreas the moment food arrives. That signalling molecule, glucagon-like peptide-1 (GLP-1), was identified by scientists including Joel Habener and colleagues and published through the mid-1980s. GLP-1 turned out to do several things at once: stimulate insulin only when blood sugar was actually elevated, suppress glucagon, slow the rate at which the stomach empties, and reduce appetite. It was, on paper, a near-ideal metabolic target.

The practical problem was brutal: the human body destroys its own GLP-1 within two to three minutes. An enzyme called DPP-4 breaks it down almost immediately after secretion. Injecting GLP-1 itself as a medicine would be pointless, it would be gone before it reached its receptors in any meaningful quantity. The challenge became: how do you design a molecule that behaves like GLP-1 but survives long enough to work? You can read more about the foundations of this research in our overview of the semaglutide discovery timeline.

The Gila monster's unexpected contribution

The pivotal clue came from an unusual source. John Eng, a physician-scientist at the Veterans Affairs Medical Center in New York, spent years cataloguing bioactive compounds in animal venoms and salivas. In the late 1980s and early 1990s he isolated a compound from the saliva of the Gila monster (a large venomous lizard native to the American southwest) and found it bore a striking structural resemblance to human GLP-1. Crucially, it was far more resistant to enzymatic breakdown. Eng named it exendin-4.

Exendin-4 proved that nature had already solved the stability problem. It bound to the same receptors as human GLP-1 and lasted long enough to produce measurable, sustained effects. Novo Nordisk and other researchers took notice. The first GLP-1 analogue to reach patients, liraglutide, was developed partly in response to this insight, and you can explore that earlier chapter in more detail on our page covering the development of liraglutide and semaglutide. Semaglutide came next, designed to go further: once-weekly rather than once-daily dosing, with even tighter receptor binding.

The chemical engineering that made semaglutide once-weekly

Novo Nordisk's chemists made three key structural changes to the GLP-1 backbone to create semaglutide. First, they substituted a single amino acid at position 34 (lysine replaced by arginine) to block DPP-4 from cleaving the molecule. Second, they attached a long fatty acid chain via a linker at position 26, which allows the molecule to bind loosely to albumin (the most abundant protein in blood) acting as a slow-release reservoir. Third, they modified position 8 (alanine replaced by aminoisobutyric acid) for additional DPP-4 resistance. Together these changes extended the half-life from under three minutes to approximately one week: a transformation that makes a once-weekly injection biologically practical.

The resulting molecule, semaglutide, was first approved for type 2 diabetes management (as Ozempic). The 2.4mg weekly dose used in Wegovy (higher than the diabetes formulations) was then tested in dedicated weight-management trials including the STEP programme, the results of which are detailed on the semaglutide overview page. The STEP 1 trial, published in the New England Journal of Medicine, reported an average weight reduction of around 15% over 68 weeks. That evidence base supported the MHRA granting UK marketing authorisation for Wegovy, and subsequently NICE recommended it for use in specialist weight management services under NICE technology appraisal TA875.

What the discovery means for people considering treatment today

Understanding the science is genuinely useful for anyone weighing up treatment. Semaglutide does not force weight loss through stimulants or metabolism suppression; it works by restoring and amplifying signals the body already uses, signals that regulate appetite, satiety and digestion. If you're curious about the researchers whose work made this possible, our page on who discovered semaglutide traces the key scientists and institutions behind the medicine. The forty-year gap between isolating GLP-1 and producing a weekly medicine approved for weight management reflects just how many obstacles had to be solved: stability, receptor selectivity, dosing, and manufacturing at scale.

In the UK, Wegovy is available on the NHS through specialist weight management services, and privately through regulated pharmacies. If you're not sure which route is open to you, or want to understand the full picture of how Wegovy works and who it is licensed for, that page covers both. The weight-loss treatment overview also sets out the options side by side. Oral semaglutide is now part of that picture too: the Wegovy tablet, approved by the MHRA on 11 June 2026, uses the same molecule in a once-daily pill taken before the kettle goes on in the morning (no injection required) though your prescriber would assess which format suits your circumstances.

One practical note: semaglutide, however it's taken, remains a prescription-only medicine. A prescriber reviews your health history, current medicines, and BMI before recommending it, and if you are thinking about stopping treatment it is worth reading our guidance on semaglutide discontinuation so you understand what to expect and how to plan that conversation with your prescriber. If you'd like that conversation, our free consultation connects you with a GPhC-registered prescriber who reads your answers the same day. There is no algorithm in that step, and no obligation.