Retatrutide and tirzepatide are two of the most closely followed incretin peptides in current research. Tirzepatide is an established dual GLP-1 / GIP agonist; retatrutide is the next-generation triple agonist (GLP-1 / GIP / glucagon) that produced the highest body-weight reduction recorded to date in a Phase 2 obesity trial. This comparison covers mechanism, trial efficacy, reported adverse events, and current regulatory 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 tested cohort | Highest-dose cohort | Highest-dose cohort |
Body-Weight Reduction 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 Body-Weight Reduction | Duration |
|---|---|---|---|
| Phase 2 obesity (NEJM, 2023) | Retatrutide, highest-dose cohort | 24.2% | 48 weeks |
| Phase 2 obesity | Retatrutide, second-highest-dose cohort | 22.8% | 48 weeks |
| SURMOUNT-1 (NEJM, 2022) | Tirzepatide, highest-dose cohort | 22.5% | 72 weeks |
| SURMOUNT-1 | Tirzepatide, mid-dose cohort | 19.5% | 72 weeks |
Retatrutide reached greater body-weight reduction in 48 weeks than tirzepatide achieved in 72, and the body-weight reduction curve had not plateaued at study end, suggesting further reduction with continued treatment duration. 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% in the highest-dose cohort at 36 weeks, comparable to or modestly exceeding tirzepatide's SURPASS-2 results against semaglutide. Phase 3 data is required for direct comparison.
Adverse Events Reported in Published Trials
| Adverse Event | Retatrutide, highest-dose cohort (Phase 2) | Tirzepatide, highest-dose cohort (SURMOUNT-1) |
|---|---|---|
| Nausea | ~50% | 31% |
| Diarrhoea | ~27% | 23% |
| Vomiting | ~24% | 17% |
| Heart rate increase | Mild dose-dependent rise (glucagon-mediated) | Mild |
The retatrutide trial's dose escalation ramped faster than typical tirzepatide titration schedules used in its trials, which likely amplified reported gastrointestinal 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.
Comparative Summary
For raw efficacy in obesity trials, 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 research landscape including semaglutide, see the research overview, 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.