Retatrutide and tirzepatide are the two most-watched incretin peptides of 2026. Tirzepatide is already a market-leading dual GLP-1 / GIP agonist; retatrutide is the next-generation triple agonist (GLP-1 / GIP / glucagon) that produced the highest weight loss ever recorded in a Phase 2 obesity trial. This comparison covers mechanism, efficacy, side effects, and current availability.
Mechanism Comparison
| Feature | Retatrutide | Tirzepatide |
|---|---|---|
| Drug class | Triple GLP-1 / GIP / Glucagon receptor agonist | Dual GLP-1 / GIP receptor agonist |
| Code name | LY3437943 | LY3298176 |
| Brand name | None (investigational) | Mounjaro (T2D), Zepbound (obesity, US) |
| Manufacturer | Eli Lilly | Eli Lilly |
| Administration | Weekly subcutaneous injection | Weekly subcutaneous injection |
| Highest trial dose | 12 mg weekly | 15 mg weekly |
Weight Loss Efficacy
No direct head-to-head trial has been published. Cross-trial comparison of the two largest readouts gives a useful estimate:
| Trial | Drug | Mean Weight Loss | Duration |
|---|---|---|---|
| Phase 2 obesity (NEJM, 2023) | Retatrutide 12 mg | 24.2% | 48 weeks |
| Phase 2 obesity | Retatrutide 8 mg | 22.8% | 48 weeks |
| SURMOUNT-1 (NEJM, 2022) | Tirzepatide 15 mg | 22.5% | 72 weeks |
| SURMOUNT-1 | Tirzepatide 10 mg | 19.5% | 72 weeks |
Retatrutide reached greater weight loss in 48 weeks than tirzepatide achieved in 72, and the weight-loss curve had not plateaued at study end, suggesting further loss with longer dosing. These are cross-trial estimates, so they carry the usual caveats around population differences and trial design.
Diabetes Control (HbA1c)
The Phase 2 type 2 diabetes trial of retatrutide (Rosenstock et al., Lancet, 2023) showed HbA1c reductions of up to 2.0% at the 12 mg dose at 36 weeks, comparable to or modestly exceeding tirzepatide's SURPASS-2 results against semaglutide. Phase 3 data is required for direct comparison.
Side Effect Comparison
| Side Effect | Retatrutide 12 mg (Phase 2) | Tirzepatide 15 mg (SURMOUNT-1) |
|---|---|---|
| Nausea | ~50% | 31% |
| Diarrhoea | ~27% | 23% |
| Vomiting | ~24% | 17% |
| Heart rate increase | Mild dose-dependent rise (glucagon-mediated) | Mild |
Retatrutide's higher trial doses ramped faster than typical tirzepatide titration, which likely amplified GI events. The glucagon receptor component is also associated with small heart rate increases, which will be a key Phase 3 safety endpoint.
Availability (June 2026)
| Retatrutide | Tirzepatide | |
|---|---|---|
| MHRA / FDA approval | No, Phase 3 (TRIUMPH) ongoing | Yes, T2D and obesity |
| NHS access | None | Expanding through specialist services |
| Research peptide market | Available for laboratory research only | Available for laboratory research only |
Research Peptide Context
Both compounds are widely studied as research peptides for in vitro and pre-clinical work. Retatrutide research material and tirzepatide research material are stocked in factory-direct vials at manufacturer-stated 99%+ purity for laboratory use only. Material sold for research is not for human consumption and must not be used as a substitute for an MHRA-approved prescription.
Which Is "Better"?
For raw efficacy in obesity, current data favours retatrutide, but it remains pre-approval and pre-Phase 3 readout. Tirzepatide is the proven, regulator-approved option today. The picture will sharpen significantly when the TRIUMPH-1, -2 and -3 results land in 2026-2027.
For the wider GLP-1 landscape including semaglutide, see our complete GLP-1 guide, or the tirzepatide vs semaglutide head-to-head.
Key References
- Jastreboff, A.M. et al. (2023). "Triple-Hormone-Receptor Agonist Retatrutide for Obesity (Phase 2)." NEJM, 389, 514-526.
- Rosenstock, J. et al. (2023). "Retatrutide, a GIP, GLP-1 and glucagon receptor agonist, for people with type 2 diabetes (Phase 2)." Lancet, 402, 529-544.
- Jastreboff, A.M. et al. (2022). "Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1)." NEJM, 387, 205-216.
- Frias, J.P. et al. (2021). "Tirzepatide versus semaglutide once weekly in patients with type 2 diabetes (SURPASS-2)." NEJM, 385, 503-515.