Semaglutide Guide: What It Is, How It Works, and What the Evidence Shows

Semaglutide is a synthetic analogue of human glucagon-like peptide-1 (GLP-1) with 94% structural homology to the naturally occurring hormone. It is a long-acting GLP-1 receptor agonist developed by Novo Nordisk and approved under three brand names: Ozempic (injectable, type 2 diabetes), Wegovy (injectable and oral, weight management and cardiovascular risk), and Rybelsus (oral tablet, type 2 diabetes).

Unlike native GLP-1, which has a circulating half-life of just 1-2 minutes, semaglutide persists in the body for approximately one week. This extended duration comes from two structural modifications: a fatty acid chain that promotes albumin binding (reducing renal clearance) and amino acid substitutions that protect it from degradation by the DPP-4 enzyme. The result is a once-weekly injection or once-daily oral tablet rather than the continuous infusion that native GLP-1 would require.

Semaglutide was first approved by the FDA on December 5, 2017 (Ozempic, for type 2 diabetes). It is now approved for four distinct indications across its three brands, making it one of the most broadly indicated peptide therapeutics ever developed.

The bottom line: Semaglutide is an FDA-approved, prescription-only GLP-1 receptor agonist available in both injectable and oral formulations across three brand names, each targeting different clinical indications.

Semaglutide works by selectively binding to and activating the GLP-1 receptor, the same receptor targeted by the body’s own GLP-1 hormone. This activation triggers multiple downstream effects across three primary organ systems.

Pancreatic Effects

GLP-1 receptor activation stimulates glucose-dependent insulin secretion from pancreatic beta cells. Critically, this is glucose-dependent – insulin release increases when blood sugar is high and decreases when it is normal, which significantly reduces the risk of hypoglycaemia compared to older diabetes medications. Semaglutide also suppresses glucagon secretion from alpha cells in a glucose-dependent manner, reducing the liver’s output of glucose into the bloodstream.

Gastrointestinal Effects

Semaglutide causes a minor delay in gastric emptying during the early postprandial phase. This slows the rate at which glucose enters the bloodstream after eating, contributing to both improved blood sugar control and increased feelings of fullness during meals.

Central Nervous System Effects

The weight loss effects of semaglutide appear to be primarily driven through the brain. GLP-1 receptors in the hypothalamus and hindbrain regions – particularly the area postrema and nucleus of the solitary tract – regulate appetite and satiety signals. Systemically injected semaglutide shows limited direct brain penetration but activates these circuits primarily through circumventricular organs that lack a complete blood-brain barrier. Research published in 2025 confirmed that semaglutide drives weight loss through cAMP-dependent signalling pathways in GLP-1 receptor-expressing hindbrain neurons, though the precise mechanisms remain an active area of investigation.

The bottom line: Semaglutide reduces appetite and blood sugar through GLP-1 receptor activation in the pancreas, gut, and brain, with its weight loss effects primarily mediated through central nervous system appetite suppression.

Semaglutide has one of the most extensive clinical trial programmes of any peptide therapeutic, with data from multiple Phase 3 programmes across different indications. All evidence discussed below comes from completed, peer-reviewed human clinical trials.

STEP Programme (Weight Management)

The Semaglutide Treatment Effect in People with Obesity (STEP) programme is the foundation for Wegovy’s approval. In the landmark STEP 1 trial (n=1,961), participants receiving semaglutide 2.4 mg weekly achieved a mean weight loss of 14.9% at 68 weeks, compared to 2.4% with placebo. 86% of semaglutide-treated participants lost at least 5% of their body weight (Phase 3 human RCT, published in NEJM). The STEP 5 trial confirmed these results were sustained over two years, with mean weight loss of 15.2% maintained at 104 weeks and 77.1% of participants achieving 5% or greater weight loss (Phase 3 human RCT, published in Nature Medicine).

In May 2025, topline results from the STEP UP trial showed that a higher 7.2 mg dose of semaglutide achieved 20.7% mean weight loss at 72 weeks, with 33% of patients losing more than 25% of their body weight – results that narrow the gap with tirzepatide’s highest-dose outcomes (Phase 3 human RCT, results pending full publication).

SELECT Trial (Cardiovascular Outcomes)

The SELECT trial was a landmark cardiovascular outcome study enrolling 17,604 adults with pre-existing cardiovascular disease and overweight or obesity, without diabetes. Over a mean follow-up of approximately 40 months, semaglutide 2.4 mg reduced major adverse cardiovascular events (cardiovascular death, non-fatal heart attack, or non-fatal stroke) by 20% compared to placebo (HR 0.80, 95% CI 0.72-0.90, p<0.001). Weight loss was sustained for up to 4 years, with a mean reduction of 10.2% at 208 weeks (Phase 3 human RCT, published in NEJM and Nature Medicine). This was the first trial to demonstrate that a GLP-1 agonist could reduce cardiovascular events independently of its diabetes effects.

ESSENCE Trial (Liver Disease)

Based on Part 1 of the Phase 3 ESSENCE trial (72-week data), the FDA granted accelerated approval to Wegovy for noncirrhotic MASH (metabolic dysfunction-associated steatohepatitis) with moderate to advanced liver fibrosis (stages F2-F3) on August 15, 2025. Semaglutide became the first GLP-1 receptor agonist approved for this liver condition and only the second MASH treatment ever authorised by the FDA (after resmetirom/Rezdiffra in March 2024). Part 2 of the ESSENCE trial, evaluating liver-related clinical events at 240 weeks, is ongoing (Phase 3 human RCT, Part 1 data published).

Trial Programme	Indication	Key Result	Evidence Level
STEP 1 (n=1,961)	Weight management	14.9% mean weight loss at 68 weeks	Phase 3 RCT (NEJM)
STEP 5 (n=304)	Long-term weight maintenance	15.2% sustained at 104 weeks	Phase 3 RCT (Nature Medicine)
STEP UP (n=1,407)	Higher-dose weight loss	20.7% at 72 weeks (7.2 mg dose)	Phase 3 RCT (topline 2025)
SELECT (n=17,604)	Cardiovascular outcomes	20% reduction in MACE over ~40 months	Phase 3 RCT (NEJM)
ESSENCE (Part 1)	MASH with liver fibrosis	Significant fibrosis improvement at 72 weeks	Phase 3 RCT (accelerated approval)
SUSTAIN 1-10	Type 2 diabetes	Superior A1C reduction vs comparators	Phase 3 RCT programme
PIONEER 1-10	Oral semaglutide for T2D	First oral GLP-1 with CV benefit	Phase 3 RCT programme

The bottom line: Semaglutide has the broadest evidence base of any GLP-1 agonist, with Phase 3 data supporting approvals for type 2 diabetes, weight management, cardiovascular risk reduction, and MASH liver disease.

Semaglutide is fully approved as a prescription medicine in all three major markets. However, the regulatory picture for compounded versions has changed dramatically since 2024.

Country	Regulator	Branded Status	Compounded Status
United States	FDA	Approved (Ozempic, Wegovy, Rybelsus)	503B proposed exclusion (Apr 2026); shortage resolved Feb 2025
United Kingdom	MHRA	Approved (Ozempic, Wegovy incl. 7.2 mg)	Not applicable (no 503B equivalent)
Australia	TGA	Approved (ARTG-listed, Ozempic and Wegovy)	Schedule 4 (prescription only); Wegovy PBS listing expected mid-2026
EU	EMA	Approved (Ozempic 2018, Wegovy 2022)	N/A

The US Compounding Crackdown

Semaglutide was placed on the FDA drug shortage list in 2022 amid explosive demand. During the shortage, compounding pharmacies legally produced compounded versions at approximately $150-300/month versus the branded price of over $1,000/month. At peak in 2024, compounded GLP-1s represented roughly 30% of US supply. For how the US compounding framework works, see our Category 1 vs Category 2 explainer.

The shortage was resolved in February 2025, triggering phased enforcement deadlines for 503B compounders. Then on April 30, 2026, the FDA proposed formally excluding semaglutide (along with tirzepatide and liraglutide) from the 503B Bulks List entirely, citing no clinical need for outsourcing facilities to compound these drugs. The public comment period closes June 29, 2026. If finalised, this would permanently close the pathway for large-scale compounding regardless of future market conditions. The FDA has received more than 455 adverse event reports linked to compounded semaglutide as of early 2025, many involving dosing errors from patients self-administering from multi-dose vials.

For the broader US regulatory landscape, see our guide to US peptide legality. For UK and Australian regulation, see our UK peptide legality guide and our Australian peptide law guide.

The bottom line: Branded semaglutide is fully approved in the US, UK, and Australia, but the large-scale compounding pathway that provided affordable access during shortages is being permanently shut down by the FDA.

Semaglutide’s safety profile is extensively characterised across the SUSTAIN, PIONEER, STEP, and SELECT trial programmes. The most common adverse effects are gastrointestinal, and most safety concerns are shared across the GLP-1 receptor agonist class rather than being specific to semaglutide.

Common Side Effects (Clinical Trial Data)

In the STEP weight management trials, 73% of semaglutide-treated patients reported gastrointestinal adverse events, compared to approximately 47% on placebo. Severe GI reactions occurred in 4.1% of treated patients. The most frequently reported events were nausea, diarrhoea, vomiting, constipation, and abdominal pain. These were generally mild to moderate in severity, typically most pronounced during the dose-escalation phase, and tended to diminish with continued use. Adverse events led to treatment discontinuation in 4.3-7.1% of participants across the STEP programme.

Serious Safety Concerns

• Thyroid C-cell tumours (boxed warning): Semaglutide caused dose-dependent thyroid C-cell tumours in rodent studies at clinically relevant exposures. Human studies have not confirmed this risk, but semaglutide is contraindicated in patients with a personal or family history of medullary thyroid carcinoma or Multiple Endocrine Neoplasia syndrome type 2 (MEN 2).
• Acute pancreatitis: Reported in clinical trials at a rate of 0.27 cases per 100 patient-years (semaglutide) versus 0.33 per 100 patient-years (placebo) in one 2-year trial – no significant increase. Postmarketing reports include fatal and necrotising pancreatitis.
• Gallbladder disease: Cholelithiasis and cholecystitis have been reported at an incidence of approximately 2-4%, likely associated with rapid weight loss increasing biliary cholesterol supersaturation.
• Diabetic retinopathy: Rapid glucose improvement has been associated with temporary worsening of existing diabetic retinopathy. Patients with this condition should be monitored during dose escalation.
• Acute kidney injury: Postmarketing reports, typically associated with severe dehydration from vomiting or diarrhoea.
• Pulmonary aspiration: Due to delayed gastric emptying, semaglutide increases the risk of residual gastric content during general anaesthesia. Both the MHRA and TGA have issued specific safety updates on this risk.

Overall, reviews of the safety literature conclude that semaglutide has a favourable risk-benefit profile for its approved indications. The SELECT trial notably found that semaglutide-treated patients had fewer serious adverse events overall compared to placebo.

The bottom line: Semaglutide’s most common side effects are gastrointestinal and typically mild; its most serious monitored risks include a theoretical thyroid tumour concern (boxed warning), pancreatitis, gallbladder disease, and aspiration risk during anaesthesia.

Semaglutide sits within a broader landscape of incretin-based therapies. Key related compounds include:

• Tirzepatide – Eli Lilly’s dual GIP/GLP-1 receptor agonist (Mounjaro/Zepbound). Achieved 22.5% weight loss at the highest dose in SURMOUNT-1 and demonstrated superiority over semaglutide in the head-to-head SURPASS-2 and SURMOUNT-5 trials.
• Retatrutide – Eli Lilly’s investigational triple agonist (GIP/GLP-1/glucagon receptors). Phase 2 data showed up to 24.2% weight loss at 48 weeks. Phase 3 TRIUMPH trials ongoing, with no regulatory approval expected before 2027-2028.
• Liraglutide – Novo Nordisk’s earlier GLP-1 agonist (Victoza/Saxenda). Requires daily injection and produces less weight loss than semaglutide. Liraglutide remains on the FDA drug shortage list and can still be compounded under that exemption.

Date	Brand	Indication
Dec 2017	Ozempic	Type 2 diabetes (subcutaneous)
Sep 2019	Rybelsus	Type 2 diabetes (first oral GLP-1)
Jun 2021	Wegovy	Chronic weight management (injection)
Mar 2024	Wegovy	Cardiovascular risk reduction in adults with CVD + obesity/overweight
Aug 2025	Wegovy	Noncirrhotic MASH with liver fibrosis (accelerated)
Oct 2025	Rybelsus	CV risk reduction in type 2 diabetes (oral)
Dec 2025	Wegovy	Weight management (first oral GLP-1 for obesity, 25 mg tablet)