What Is Semaglutide?
Semaglutide is a glucagon-like peptide-1 (GLP-1) receptor agonist — a class of compound that has attracted significant scientific and clinical attention over the past decade. Originally developed for the management of type 2 diabetes, it is now approved in both formulations across the European Union: as Ozempic (0.5mg and 1mg weekly injection) for glycaemic control in adults with type 2 diabetes, and as Wegovy (2.4mg weekly injection) for chronic weight management in adults with obesity or overweight with at least one weight-related comorbidity.
In Ireland, both formulations are regulated by the Health Products Regulatory Authority (HPRA) and are classified as prescription-only medicines. This means semaglutide is among the most rigorously evaluated compounds in metabolic medicine — subject to the same pre-market clinical scrutiny as any HPRA-approved drug.
Structurally, semaglutide is a modified analogue of human GLP-1, with a half-life extended to approximately one week through fatty acid side-chain attachment. This allows for once-weekly subcutaneous dosing, which is a practical advantage over earlier GLP-1 agents that required daily injection.
How It Works — The Mechanism
To understand semaglutide, it helps to understand the role of GLP-1 itself. GLP-1 is a hormone produced in the gut in response to food intake. Its natural functions include stimulating insulin secretion from the pancreas (in a glucose-dependent manner), suppressing glucagon release, slowing gastric emptying, and signalling satiety to the brain.
Semaglutide mimics these actions by binding to GLP-1 receptors throughout the body. The key mechanisms relevant to both weight management and cardiovascular outcomes are:
- Appetite suppression: GLP-1 receptors in the hypothalamus respond to semaglutide by reducing hunger signalling and increasing feelings of fullness. Research participants consistently report reduced appetite and lower caloric intake.
- Gastric emptying: Semaglutide slows the rate at which food moves from the stomach to the small intestine, which blunts postprandial glucose spikes and prolongs satiety.
- Glucose regulation: In pancreatic beta cells, semaglutide enhances insulin secretion — but only when blood glucose is elevated. This glucose-dependent mechanism reduces hypoglycaemia risk compared with older diabetes treatments.
- Cardiovascular effects: GLP-1 receptors are also found in cardiac and vascular tissue. The mechanisms underlying cardiovascular benefit are still being studied, but are thought to involve direct anti-inflammatory and anti-atherosclerotic effects alongside improvements in metabolic risk factors.
It is worth noting that semaglutide does not act in isolation — its effects emerge from a complex interplay with the body's own hormonal environment, gut microbiome, and metabolic state. Researchers continue to investigate secondary mechanisms.
What Does the Research Say?
Semaglutide has one of the most robust human clinical trial records of any compound in metabolic medicine. The main evidence base comes from two large trial programmes: SUSTAIN (type 2 diabetes) and STEP (weight management), as well as the landmark SELECT cardiovascular outcomes trial.
STEP Trials — Weight Management in Humans
The STEP programme comprised several phase 3 randomised controlled trials evaluating semaglutide 2.4mg once weekly (Wegovy dosing) in adults with obesity or overweight. These were large, well-designed human trials — not animal studies. The headline finding across STEP 1 was a mean body weight reduction of approximately 14.9% over 68 weeks in the semaglutide group, compared with 2.4% in the placebo group. STEP 2 (participants with type 2 diabetes) showed a 9.6% reduction, acknowledging that metabolic disease modifies response. STEP 3, which combined semaglutide with intensive behavioural counselling, reported reductions approaching 16-17%.
These are human trial results at approved clinical doses, not mechanistic estimates from cell culture or rodent models. The participant populations were adults with BMI ≥30, or ≥27 with a weight-related condition — relevant to the real-world context in which the drug is prescribed.
SUSTAIN Trials — Diabetes Context
The SUSTAIN programme established semaglutide's efficacy and safety profile in type 2 diabetes management, demonstrating HbA1c reductions, cardiovascular non-inferiority, and in SUSTAIN-6, a statistically significant reduction in cardiovascular events versus placebo in a high-risk population.
SELECT Trial — Cardiovascular Outcomes
The SELECT trial is arguably the most significant recent development in semaglutide research. Published in 2023, this was a double-blind, randomised, placebo-controlled trial involving over 17,600 adults with pre-existing cardiovascular disease and overweight or obesity — but without diabetes. Participants received semaglutide 2.4mg weekly or placebo over a median follow-up of approximately 34 months. The primary outcome — a composite of cardiovascular death, non-fatal myocardial infarction, or non-fatal stroke — occurred in 6.5% of the semaglutide group versus 8.0% in the placebo group. This represents a 20% relative risk reduction.
It is an important finding because it suggests cardiovascular benefit independent of diabetes status and extends beyond what could be explained by weight loss alone. Researchers have noted this warrants further mechanistic investigation.
Honest Assessment of Limitations
No research programme is without limitations. The STEP trials were funded by Novo Nordisk, the manufacturer of semaglutide, which is standard for phase 3 trials but warrants acknowledgement. Long-term data beyond two years remains limited. Weight regain following discontinuation has been documented — the STEP 4 extension study showed that participants who stopped semaglutide regained approximately two-thirds of lost weight within a year, indicating the compound addresses ongoing physiology rather than conferring permanent change. Side effects — primarily nausea, vomiting, and gastrointestinal discomfort — are well-documented, particularly during dose escalation.
Context for Irish Researchers
For researchers and healthcare professionals in Ireland, semaglutide is a compound of particular relevance given its HPRA-approved status and the established prescribing infrastructure around it. Both Ozempic and Wegovy are licensed EU medicines — this distinguishes semaglutide from many peptide compounds that remain in research-use-only categories.
The Irish healthcare context is also relevant: obesity-related cardiovascular disease remains a significant public health burden, and the SELECT trial data has meaningful implications for how clinicians and researchers think about metabolic intervention. The National Clinical Programme for Diabetes and HSE weight management pathways are areas where semaglutide's evidence base is being actively considered.
From a research sourcing perspective, semaglutide's regulatory status means it is not available through peptide research supply channels — it is a fully licensed pharmaceutical. Irish researchers interested in GLP-1 biology, appetite regulation, or metabolic outcomes will find semaglutide's published trial data an exceptionally rich resource, given the scale and rigour of the available evidence compared with many investigational compounds.
Supply shortages, which have affected both Ozempic and Wegovy availability in Ireland in recent years, reflect broader global demand pressures and remain a practical consideration for prescribers and patients alike.
Key Takeaways
- Semaglutide is an HPRA-approved GLP-1 receptor agonist available as Ozempic (diabetes) and Wegovy (weight management) in Ireland and across the EU.
- It works by mimicking the gut hormone GLP-1 — reducing appetite, slowing gastric emptying, improving insulin response, and acting on cardiovascular tissue.
- The STEP trials (large-scale, randomised, human trials) demonstrated 15–17% body weight reduction over 68 weeks at the 2.4mg dose in adults with obesity.
- The SELECT trial showed a 20% relative reduction in major cardiovascular events in non-diabetic adults with cardiovascular disease — a finding that has shifted how researchers consider the compound's role beyond metabolic control.
- Evidence quality for semaglutide is high — phase 3 human RCTs at scale — distinguishing it from many compounds where evidence is limited to animal models or small mechanistic studies.
- Weight regain following discontinuation is well-documented; semaglutide appears to address ongoing physiology rather than cause permanent metabolic change.
- As a prescription medicine, it is not available through research peptide channels — Irish researchers will engage with it primarily through the published clinical literature.
- Side effects are predominantly gastrointestinal; they are well-characterised in the trial literature and typically emerge during dose escalation.
For research tools, protocol guides, and curated resources on GLP-1 biology and related peptide science, visit irishpeptides.ie/free-tools.