Mechanism of Action.
Semaglutide is a GLP-1 receptor agonist with 94% structural homology to native human GLP-1. It incorporates two key modifications: an amino acid substitution (Aib at position 8) that resists DPP-4 degradation, and a C-18 fatty di-acid chain that enables albumin binding. These changes extend its half-life to approximately 7 days, permitting once-weekly dosing. Semaglutide activates GLP-1 receptors in the pancreas (enhancing glucose-dependent insulin secretion and suppressing glucagon), the gastrointestinal tract (slowing gastric emptying), and the hypothalamus (reducing appetite and food intake through central satiety signaling). The weight loss effect is primarily driven by reduced caloric intake rather than increased energy expenditure.
Research Summary.
Semaglutide has the most extensive clinical evidence base of any peptide in the weight management space. The STEP trial program (STEP 1-5) demonstrated mean weight loss of 14.9% at 2.4mg weekly over 68 weeks. The SELECT cardiovascular outcomes trial showed a 20% reduction in major adverse cardiovascular events in overweight/obese adults with established cardiovascular disease. Research continues into semaglutide for NASH/MASH, Alzheimer disease, addiction, and kidney disease. The oral formulation (Rybelsus) uses an absorption enhancer (SNAC) for gastric absorption but achieves lower bioavailability than injection.
Side Effects & Safety.
Gastrointestinal effects are the most common side effects and are dose-dependent: nausea (44%), diarrhea (30%), vomiting (24%), and constipation (24%) reported in STEP 1. These typically diminish with dose escalation over 16-20 weeks. More serious but rare adverse effects include pancreatitis, gallbladder events, and potential thyroid C-cell concerns (boxed warning based on rodent studies). There is a risk of lean mass loss concurrent with fat loss, which can be mitigated with resistance training and adequate protein intake. Semaglutide is contraindicated in patients with a personal or family history of medullary thyroid carcinoma or MEN2 syndrome.
Legal Status & Access.
Semaglutide is FDA-approved under multiple brand names: Ozempic for type 2 diabetes (0.5mg, 1mg, 2mg weekly injection), Wegovy for chronic weight management (2.4mg weekly injection), and Rybelsus for type 2 diabetes (oral, 7mg and 14mg daily). It is a prescription-only medication. Research-grade semaglutide is available from peptide suppliers and specialized GLP-1 vendors for laboratory research.