What Is Semaglutide? How It Works and What Research Shows

Semaglutide is a GLP-1 receptor agonist developed by Novo Nordisk. It is the active compound in Ozempic (approved for type 2 diabetes management) and Wegovy (approved for chronic weight management). It is the reference compound against which newer GLP-1 peptides like tirzepatide and retatrutide are compared. Understanding semaglutide is foundational to understanding the entire GLP-1 research space.

What GLP-1 Is and Why It Matters

GLP-1 (glucagon-like peptide-1) is a hormone naturally produced in the gut in response to food intake. It triggers several responses that regulate blood sugar and appetite: it stimulates insulin secretion from the pancreas, suppresses glucagon release, slows gastric emptying (so food moves more slowly from the stomach), and acts on appetite centers in the brain to promote satiety. The half-life of native GLP-1 is approximately 2 minutes, making it unsuitable as a drug without modification.

How Semaglutide Is Different from Native GLP-1

Semaglutide is a modified version of GLP-1 with several changes that extend its half-life. Modifications to the peptide backbone and the addition of a fatty acid chain that binds to albumin in the blood slow its clearance from approximately 2 minutes to approximately 1 week. This extended half-life allows once-weekly dosing, which is practical for clinical use.

What the STEP Trials Showed

The STEP (Semaglutide Treatment Effect in People with obesity) trials are the Phase 3 data foundation for semaglutide in weight management. STEP 1 enrolled adults with obesity without diabetes and showed average weight loss of approximately 14.9% over 68 weeks at 2.4mg weekly. STEP 2 studied adults with obesity and type 2 diabetes, showing approximately 9.6% weight loss. STEP 3 combined semaglutide with intensive behavioral intervention, showing approximately 16% loss.

The SELECT trial, published in 2023, showed that semaglutide reduced the risk of major cardiovascular events (heart attack, stroke, cardiovascular death) by approximately 20% in adults with obesity and existing cardiovascular disease. This cardiovascular benefit is a significant positive finding that has shaped how the compound is viewed beyond weight loss.

Oral vs. Injectable Semaglutide

Semaglutide is available in both subcutaneous injection form and oral tablet form. The oral form (Rybelsus) is less bioavailable than the injectable form due to challenges with peptide absorption in the gut, requiring higher doses and specific administration conditions (empty stomach, limited water). For research purposes, injectable semaglutide is the standard form with the most clinical trial data.

Side Effects Documented in STEP Trials

Gastrointestinal side effects (nausea, vomiting, diarrhea, constipation) are the most common adverse events, documented in 40 to 50% of participants at some point during treatment. They are most pronounced during dose escalation and generally diminish over time. The black box warning for thyroid C-cell tumors is based on rodent data and has not been confirmed in human studies. Gallbladder events are documented at a higher rate than placebo.

How Semaglutide Compares to Tirzepatide and Retatrutide

Semaglutide is a single receptor agonist (GLP-1 only). Tirzepatide adds GIP receptor agonism and produced higher average weight loss in separate trials (approximately 20% vs. 15%). Retatrutide adds both GIP and glucagon receptor agonism and produced even higher results in Phase 2 (approximately 24%). Semaglutide has the most clinical data, the longest safety track record, and the most accumulated real-world use data. Newer compounds offer higher efficacy but with less long-term data.

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