# Incretin Peptides FAQ — NextGen Peptides > Answers to common questions about semaglutide, tirzepatide, and retatrutide: how they work, how they compare, approval status, safety signals, and what the trial evidence actually shows. ## What is semaglutide? Semaglutide is a 31-amino-acid synthetic peptide analogue of GLP-1, one of the gut hormones released after eating. Chemical modifications — including a fatty-acid chain at position 26 — give it a plasma half-life of roughly one week, enabling once-weekly dosing. It is FDA-approved as a prescription medicine for type 2 diabetes, chronic weight management, cardiovascular risk reduction in adults with obesity and established cardiovascular disease, and (2025) metabolic liver disease. ## What is semaglutide used for? In clinical practice, semaglutide is used for type 2 diabetes glycemic control, chronic weight management in adults with obesity or overweight with a weight-related condition, and reduction of major cardiovascular events in adults with established cardiovascular disease and overweight/obesity. The STEP 1 trial documented a mean -14.9% body-weight change at 68 weeks [4]; the SELECT trial documented a 20% reduction in cardiovascular events [3]; the FLOW trial documented a 24% reduction in major kidney-disease events in type 2 diabetes with CKD [2]. It is a prescription-only medicine. ## How does semaglutide work? Semaglutide activates the GLP-1 receptor in the pancreas (stimulating glucose-dependent insulin secretion and suppressing glucagon), the gut (slowing gastric emptying), and the brain's appetite circuits (the hypothalamic arcuate nucleus, the area postrema, and the brainstem parabrachial nucleus) [6]. The central effect — reducing food intake and quieting appetite — is considered the primary driver of the weight-loss signal rather than any change in energy expenditure. ## How does semaglutide work for weight loss? The weight-lowering mechanism is primarily central: semaglutide reaches hypothalamic and brainstem circuits that regulate hunger, activating neurons that signal fullness and inhibiting neurons that drive appetite [6]. The downstream effect is that people eat substantially less. Slowed gastric emptying contributes by prolonging the sensation of fullness after a meal. In the STEP 1 trial, the mean weight change at 68 weeks was -14.9% for semaglutide 2.4 mg versus -2.4% for placebo [4]. ## What is tirzepatide? Tirzepatide is a 39-amino-acid synthetic peptide that activates both the GIP receptor and the GLP-1 receptor — making it a dual incretin agonist (sometimes called a "twincretin"). It is FDA-approved for type 2 diabetes (2022), chronic weight management (2023), and obstructive sleep apnea in adults with obesity. It consistently outperforms selective GLP-1 agonism on weight endpoints in head-to-head trials. ## How does tirzepatide work? Tirzepatide activates both the GIP receptor and the GLP-1 receptor simultaneously from a single peptide. GLP-1R activation suppresses appetite, slows gastric emptying, and promotes glucose-dependent insulin secretion. GIPR activation adds to the insulin-secretion signal and appears to enhance insulin sensitivity in adipose tissue. The combined effect produces larger glycemic and weight reductions than selective GLP-1 agonism in clinical trials [8][11]. ## What does tirzepatide do in the body? At the pancreas it stimulates insulin secretion (glucose-dependently) and suppresses glucagon. At the stomach it slows gastric emptying. In the brain it activates satiety circuits that reduce food intake. In clinical trials — most notably SURMOUNT-1 (n=2,539) — these combined effects produced a mean body-weight reduction of -20.9% at 15 mg over 72 weeks versus -3.1% with placebo [10]. In SURPASS-2, it outperformed semaglutide 1 mg on both HbA1c and body weight across all doses [11]. ## What is tirzepatide used for? Tirzepatide is FDA-approved for: type 2 diabetes mellitus (glycemic control); chronic weight management in adults with obesity (BMI ≥30) or overweight (BMI ≥27) with at least one weight-related comorbidity; and moderate-to-severe obstructive sleep apnea in adults with obesity. Weight-loss use was initially off-label relative to the 2022 diabetes approval, but received a dedicated obesity indication in November 2023. ## What does retatrutide do? Retatrutide activates three receptors — GLP-1R, GIPR, and the glucagon receptor (GCGR) — from a single peptide. The GLP-1 and GIP arms suppress appetite and improve insulin secretion. The glucagon receptor arm adds thermogenesis (increased energy expenditure) and lipid mobilization from fat tissue, mechanisms not available to dual or single agonists. In its Phase 2 obesity trial, this combination produced a mean -24.2% body-weight change at 48 weeks at the highest dose [15]. ## How does retatrutide work? Retatrutide engages three receptors simultaneously: GLP-1R, GIPR, and GCGR. Cryo-EM structural work confirmed the triple simultaneous binding and characterized relative potencies — approximately 8.9-fold at GIPR, 0.3-fold at GCGR, and 0.4-fold at GLP-1R versus the respective endogenous hormones [13]. The calibrated glucagon component drives thermogenesis and hepatic lipid clearance without full hyperglycemic risk. The combined effect in Phase 2 MASLD data reduced liver fat by -82.4% at 24 weeks [14]. ## How to reconstitute retatrutide? Retatrutide is an investigational compound and has no approved pharmaceutical formulation or labeling. All reconstitution data come from clinical trial protocols, which are not publicly disclosed as consumer instructions. Research-channel formulations are not manufactured under pharmaceutical GMP standards and carry documented identity, purity, and sterility risks. This site provides no preparation instructions for any compound. ## Is retatrutide FDA approved? No. As of mid-2026, retatrutide has not been approved by the FDA or any regulatory agency. It remains under investigation in Phase 3 clinical trials (the TRIUMPH series) run by Eli Lilly. Phase 3 data are pending; no results have been reported. The Phase 2 data cited on this desk are real and peer-reviewed, but Phase 2 results are not an approval basis [12][15]. --- A literature digest of peer-reviewed incretin research — effect sizes reported as published, not as clinical guidance.