Mounjaro, Wegovy, Ozempic, Saxenda: they are all part of the GLP-1 class. But what exactly are they, and why do they work so differently from everything that came before?
What does GLP-1 stand for?
GLP-1 stands for glucagon-like peptide-1. It is a hormone produced naturally in your small intestine when food enters your gut. Its job is to signal your pancreas to release insulin, suppress glucagon (the hormone that raises blood sugar between meals), slow the passage of food through your stomach, and send fullness signals to the hypothalamus, the brain's appetite control centre.
Here is the key point: in many people with obesity, GLP-1 signalling is less effective. The body either produces less of the hormone or responds to it less efficiently. This can perpetuate overeating, not because of a lack of willpower, but because the biological feedback loop that creates the feeling of satiety is not functioning optimally.
What is a GLP-1 receptor agonist?
A GLP-1 receptor agonist is a medication that mimics GLP-1 by binding to the same receptors in your body, generating the same physiological responses, but more potently and for much longer. Natural GLP-1 has a half-life of only a few minutes. Pharmaceutical GLP-1 agonists are engineered to last days to weeks, providing sustained appetite suppression that the natural hormone simply cannot maintain.
Which medications are GLP-1 receptor agonists?
- Wegovy (semaglutide), licensed for weight management in the UK; once-weekly injection
- Ozempic (semaglutide), licensed for type 2 diabetes; the same molecule as Wegovy at lower doses
- Saxenda / Nevolat (liraglutide), licensed for weight management; once-daily injection
- Mounjaro (tirzepatide), a once-weekly injection that acts on both GLP-1 and GIP receptors, licensed for weight management and type 2 diabetes
Note: Mounjaro is a dual agonist. It activates both GLP-1 and GIP receptors simultaneously. In SURMOUNT-5 (New England Journal of Medicine, 2025), a 72-week trial comparing the two medicines directly in 751 adults with obesity but not diabetes, people on the highest dose they could tolerate of tirzepatide lost an average of 20.2% of their body weight, compared with 13.7% on semaglutide. Individual results vary.
How GLP-1 medications produce weight loss
- Appetite reduction: direct signals to the hypothalamus reduce your baseline hunger level
- Slower gastric emptying: food moves more slowly through your stomach, extending the feeling of fullness after meals
- Reduced food cravings: GLP-1 receptors in the brain's reward centres reduce the psychological pull of high-calorie foods; what patients often describe as "food noise" quietens significantly
- Blood glucose stabilisation: prevents the spikes and crashes in blood sugar that drive hunger and cravings between meals
The result: patients eat substantially less, not through willpower, but because the biological drive to eat has been reduced. In the STEP 1 trial (New England Journal of Medicine, 2021), 1,961 adults with overweight or obesity and without diabetes all had lifestyle support; those who took semaglutide 2.4mg lost an average of 14.9% of their body weight over 68 weeks, compared with 2.4% on placebo, counting everyone who started treatment. Individual results vary.
"GLP-1 medications are addressing the underlying hormonal drivers of obesity, not just providing caloric guidance. This is why they produce results that dietary advice alone cannot replicate."
What conditions are GLP-1 medications used for in the UK?
In the UK, the GLP-1 medicines licensed for weight management in adults with a BMI of 30 or more, or 27 or more with a weight-related condition, are Wegovy injections and tablets, Mounjaro, Foundayo, Saxenda and Nevolat. For type 2 diabetes, Ozempic contains the same medicine as Wegovy at lower doses, while Mounjaro and Foundayo are licensed for type 2 diabetes at the same doses as for weight management. Wegovy is also licensed for a type of fatty liver disease (MASH) with moderate to advanced scarring.
Wegovy 2.4mg is the only weight-management medicine with a UK licence to reduce the risk of heart attack, stroke and cardiovascular death, for adults with established cardiovascular disease and a BMI of 27 or more.
Emerging research is also investigating GLP-1 medications for heart failure and neurodegenerative conditions.
Are GLP-1 medications safe long-term?
Semaglutide trial data now extend to four years: in the SELECT trial, weight loss was sustained for up to four years (Nature Medicine, 2024). Tirzepatide extension studies are ongoing. The safety profile is reassuring: gastrointestinal side effects are the most common, and these are concentrated during the dose escalation phase.
In the SELECT trial (New England Journal of Medicine, 2023), 17,604 adults aged 45 or over with established cardiovascular disease, a BMI of 27 or more and no diabetes took semaglutide 2.4mg or placebo and were followed for an average of about 40 months; 6.5% of those on semaglutide had a heart attack, stroke or cardiovascular death, compared with 8.0% on placebo, counting everyone who took part. Individual results vary.
Who is eligible privately?
Adults with a BMI of 30 or above, or 27 or above with a weight-related health condition, can access Mounjaro or Wegovy privately through GPhC-registered pharmacies, including QuickCare Pharmacy through Solira, following a clinical consultation.