Semaglutide

Semaglutide is a 31-amino-acid analog of glucagon-like peptide-1 (GLP-1) with two structural modifications: an α-aminoisobutyric acid substitution at position 8 that resists degradation by DPP-4, and a C18 fatty acid chain attached at position 26 through a γ-glutamic acid spacer that binds plasma albumin and extends the half-life to about a week ([Lau et al., *J Med Chem* 2015, 58:7370–80](https://doi.org/10.1021/acs.jmedchem.5b00726)). It acts as a full agonist at the GLP-1 receptor, which is expressed on pancreatic β-cells, on enteric neurons, and on hypothalamic and brainstem appetite-regulating circuits. Three downstream effects matter clinically: glucose-dependent insulin secretion, slowed gastric emptying, and centrally mediated appetite suppression. The central effect is the dominant driver of weight loss, not the peripheral metabolic ones.

Semaglutide is the cultural gateway peptide. More people search for "Ozempic" than for every other peptide on this site combined, and the broader interest in peptide therapeutics that supports the rest of this corpus exists in part because semaglutide opened that door. The evidence base is also genuinely deep. The pivotal STEP 1 trial (Wilding et al., *NEJM* 2021) randomized 1,961 adults with overweight or obesity, without diabetes, to once-weekly 2.4 mg subcutaneous semaglutide or placebo plus lifestyle intervention for 68 weeks: the treatment arm lost 14.9% of body weight versus 2.4% in placebo, with 86.4% of treated patients reaching at least 5% loss. Subsequent STEP trials extended the result across populations and regimens, and the SELECT trial confirmed cardiovascular benefits in adults with established cardiovascular disease. Three boundaries on this story matter, all three addressed below rather than only the comfortable one. The first is gastrointestinal side effects. STEP 1 reported 44.2% nausea and 31.5% diarrhea on semaglutide versus 17.4% and 15.9% on placebo; 4.5% of treated patients discontinued for GI reasons. These are typically dose-escalation phenomena and improve with time, but they are not minor. The second is body composition. Lean-mass changes during semaglutide-induced weight loss vary substantially across studies — some report 15–25% of total weight lost as lean mass (in line with what any equivalent caloric deficit produces), others report 40–60%. A 2024 systematic review found heterogeneity rather than a settled answer; the SEMALEAN study reported preserved lean mass when patients ate adequate protein and trained. The honest reading is that the muscle-loss concern is real but variable, and that mitigation is mostly diet and exercise rather than a property of the drug itself. The third is discontinuation. In the STEP 1 extension (Wilding et al., *Diabetes, Obesity and Metabolism* 2022), participants who stopped semaglutide regained roughly two-thirds of their prior loss within a year, and cardiometabolic improvements reverted toward baseline in parallel. Whether semaglutide is therefore a chronic therapy or a tool with a defined exit strategy is an open clinical and economic question, not a settled answer.