Comparison
Semaglutide vs Tirzepatide
vs Retatrutide
The three most important GLP-1 class drugs compared head-to-head. Single receptor, dual receptor, triple receptor — each generation adds a new mechanism and produces more weight loss.
The Pattern
Semaglutide (GLP-1) → 15% weight loss. Tirzepatide (GIP + GLP-1) → 21%. Retatrutide (GIP + GLP-1 + Glucagon) → 24%. Each additional receptor target adds ~3-5% more weight loss.
| Semaglutide | Tirzepatide | Retatrutide | |
|---|---|---|---|
| Brand names | Ozempic, Wegovy, Rybelsus | Mounjaro, Zepbound | N/A (investigational) |
| FDA status | Approved (2017/2021) | Approved (2022/2023) | Phase 3 trials |
| Receptor targets | GLP-1 | GIP + GLP-1 | GIP + GLP-1 + Glucagon |
| Mean weight loss | 14.9% (STEP 1) | 20.9% (SURMOUNT-1) | 24.2% (Phase 2) |
| Trial duration | 68 weeks | 72 weeks | 48 weeks |
| Max dose studied | 2.4 mg/week | 15 mg/week | 12 mg/week |
| Half-life | ~7 days | ~5 days | ~6 days |
| Dosing | Once weekly | Once weekly | Once weekly |
| Administration | SubQ injection (oral available) | SubQ injection | SubQ injection |
| Nausea rate | ~44% | ~31% | ~45% |
| HbA1c reduction | Up to 1.8% | Up to 2.4% | Up to 2.02% |
| Cardiovascular data | SELECT: 20% MACE reduction | SURPASS-CVOT ongoing | No CV outcomes data yet |
| Liver fat reduction | Moderate | Significant | 82–86% (Phase 2) |
| Developer | Novo Nordisk | Eli Lilly | Eli Lilly |
How They Work
All three drugs belong to the incretin receptor agonist class, but they target different combinations of receptors. Semaglutide activates only the GLP-1 receptor. Tirzepatide adds GIP receptor activation. Retatrutide adds glucagon receptor activation on top of both.
GLP-1 receptor activation reduces appetite and slows gastric emptying. GIP receptor activation enhances insulin sensitivity and appears to improve tolerability. Glucagon receptor activation increases energy expenditure and promotes liver fat oxidation — explaining retatrutide's exceptional liver fat reduction data.
Weight Loss Comparison
Direct comparison is complicated by different trial designs. STEP 1 (semaglutide) ran 68 weeks. SURMOUNT-1 (tirzepatide) ran 72 weeks. Retatrutide's Phase 2 ran only 48 weeks — and the weight loss curve was still descending, suggesting final numbers could be even higher in longer trials.
The only true head-to-head data is SURPASS-2, where tirzepatide 15 mg produced significantly more weight loss than semaglutide 1 mg in type 2 diabetes patients. No head-to-head trials exist between retatrutide and the others yet.
Side Effects
The gastrointestinal side effect profile is similar across all three. Nausea, vomiting, and diarrhea are the most common adverse events, primarily during dose escalation. Tirzepatide may have slightly better GI tolerability than semaglutide, possibly due to the GIP component. Retatrutide's Phase 2 data shows comparable GI side effects to semaglutide.
All three carry a class warning for medullary thyroid carcinoma risk based on animal studies, though this has not been observed in human trials.
Availability
Semaglutide and tirzepatide are FDA-approved and available by prescription, though supply constraints have been an ongoing issue. Retatrutide is only available through Phase 3 clinical trial enrollment. If approved, retatrutide is expected to reach market in 2026-2027.
The Bottom Line
Each generation of incretin agonist produces more weight loss by targeting additional receptor pathways. Semaglutide transformed obesity treatment. Tirzepatide improved on it. Retatrutide may improve again. The key question for retatrutide is whether Phase 3 data confirms Phase 2 results — and whether the additional glucagon receptor activation introduces any long-term safety signals.