The three generations of GLP-1 receptor agonists — semaglutide, tirzepatide, and retatrutide — represent the most effective class of compounds for metabolic research ever studied. This guide compares all three by mechanism, clinical weight loss data, side effect profiles, and sourcing.
GLP-1 receptors in the hypothalamus reduce hunger signals and increase satiety. This is the primary mechanism shared by all three compounds.
GLP-1 agonism slows gastric emptying, prolonging the feeling of fullness after meals and reducing caloric intake.
GIP agonism (tirzepatide, retatrutide) and glucagon agonism (retatrutide only) increase thermogenesis and resting energy expenditure beyond appetite suppression alone.
Each generation adds a new receptor mechanism on top of the previous one. Semaglutide targets GLP-1 only. Tirzepatide adds GIP, which amplifies insulin secretion and adipose tissue signaling. Retatrutide adds glucagon, which directly promotes lipolysis and thermogenesis — making it the most potent fat-loss compound studied to date.
GLP-1 agonist
GLP-1 + GIP dual agonist
GLP-1 + GIP + glucagon triple agonist
| Metric | Semaglutide | Tirzepatide | Retatrutide |
|---|---|---|---|
| Mean weight loss | ~15% | ~22% | ~24% |
| Receptors targeted | GLP-1 | GLP-1 + GIP | GLP-1 + GIP + Glucagon |
| Thermogenic effect | No | Mild (GIP) | Yes (glucagon) |
| Visceral fat reduction | Moderate | Strong | Strongest |
| Phase 3 data | Yes (STEP) | Yes (SURMOUNT) | In progress |
| Cardiovascular data | Yes (FLOW) | Yes (SURPASS-CVOT) | Not yet |
| PCOS evidence | Moderate | Strong | Limited |
| Starting dose | 0.25 mg/wk | 2.5 mg/wk | 2 mg/wk |
| Max dose | 2.4 mg/wk | 15 mg/wk | 12 mg/wk |
Retatrutide produced the highest weight loss in clinical trials — approximately 24% of body weight at 48 weeks in Phase 2. Tirzepatide produced ~22% at 72 weeks (SURMOUNT-1), and semaglutide produced ~15% at 68 weeks (STEP 1). However, retatrutide has only completed Phase 2 trials, while semaglutide and tirzepatide have extensive Phase 3 data.
Semaglutide is a GLP-1 receptor agonist. Tirzepatide adds GIP receptor agonism (dual agonist). Retatrutide adds glucagon receptor agonism on top of both (triple agonist). Each generation adds a mechanism that increases energy expenditure and fat loss beyond the previous generation.
Appetite suppression typically begins within the first week. Measurable weight loss (2–4 kg) is usually seen within 4 weeks. The most rapid weight loss phase occurs between weeks 4–24. Maximum weight loss is typically achieved at 48–72 weeks depending on the compound.
Semaglutide has the longest safety record, with cardiovascular outcome trials showing benefit in high-risk populations. Tirzepatide has 2+ years of Phase 3 data. Retatrutide has only 48-week Phase 2 data. All three share the GLP-1 class side effect profile: GI effects (nausea, vomiting, diarrhea), rare pancreatitis risk, and thyroid C-cell considerations.
Purgo Labs carries pharmaceutical-grade semaglutide, tirzepatide, and retatrutide with third-party COAs. All compounds are sold for research purposes only. Use code HEALTH for 15% off your first order.
Purgo Labs carries pharmaceutical-grade semaglutide, tirzepatide, and retatrutide with third-party COAs. All compounds are sold for research purposes only. Use code HEALTH for 15% off.
Medical Disclaimer: All content on this site is for educational and research purposes only. Research peptides are not FDA-approved for human use. Always consult a qualified healthcare professional before considering any peptide or supplement protocol. Nothing on this site constitutes medical advice, diagnosis, or treatment.