The Three Generations

GLP-1 class peptides have evolved through three generations of receptor targeting: Generation 1 (GLP-1 monoagonists): semaglutide, liraglutide. Generation 2 (GIP/GLP-1 dual agonists): tirzepatide. Generation 3 (GIP/GLP-1/GCGR triple agonists): retatrutide, mazdutide. Each generation adds a receptor pathway, with the goal of improving weight loss magnitude and metabolic outcomes beyond what a single receptor can achieve.

Mechanism Comparison

Semaglutide: GLP-1-R agonism only. Appetite suppression via hypothalamic GLP-1R, insulin secretion, slowed gastric emptying, cardiovascular protection (SUSTAIN/SOUL trials). Tirzepatide: GLP-1-R + GIP-R co-agonism. Adds improved insulin sensitivity via GIP-R (especially in adipose tissue and muscle), with synergistic weight loss beyond GLP-1 alone. Retatrutide: GLP-1-R + GIP-R + GCGR triple agonism. Adds hepatic fat oxidation and elevated basal metabolic rate via GCGR — the first agent in this class to meaningfully increase energy expenditure rather than solely reducing intake.

Clinical Trial Weight Loss Data

Semaglutide 2.4mg (STEP-1, 68 weeks): 14.9% mean body weight loss. Tirzepatide 15mg (SURMOUNT-1, 72 weeks): 20.9% mean body weight loss. Retatrutide 12mg (Phase 2, 48 weeks): 24.2% mean body weight loss. Note: these trials differ in duration, population, and dose — direct comparison is imprecise. However, the directional trend across generations is clear, and the magnitude increase with each added receptor is consistent.

Half-Life and Dosing Frequency

All three compounds use fatty acid modifications for albumin binding and extended half-life. Semaglutide: ~165 hours (~7 days) — once weekly dosing. Tirzepatide: ~5 days — once weekly dosing. Retatrutide: ~6 days — once weekly dosing. The similar half-lives across the class reflect the albumin-binding strategy shared by all three.

Research Design Considerations

For research isolating GLP-1 receptor effects specifically: semaglutide is the appropriate compound. For studying GIP/GLP-1 co-agonism and its additive effects: tirzepatide. For studying the contribution of glucagon receptor activation to metabolic outcomes: retatrutide provides the cleanest model. Because retatrutide is the newest compound, less published data exists on its long-term effects — making it the most exploratory research target of the three.