Retatrutide vs Tirzepatide
Retatrutide (LY3437943) and Tirzepatide (LY3298176) are both investigational incretin peptides from the same development lineage. Tirzepatide is already approved as Mounjaro and Zepbound; Retatrutide is the next-generation triple-agonist follow-on currently in phase 3 trials. This page compares the two as research compounds.
| Retatrutide | Tirzepatide | |
|---|---|---|
| Developmental code | LY3437943 | LY3298176 |
| Receptor targets | GLP-1, GIP, glucagon (triple agonist) | GLP-1, GIP (dual agonist) |
| Approval status | Phase 3, investigational only | Approved as Mounjaro and Zepbound |
| Phase 2 body-weight reduction (48 weeks) | Up to 24.2% mean reduction | Up to 17% mean reduction |
| Administration frequency | Once weekly | Once weekly |
| Peptx dose sizes | 10mg to 60mg lyophilised vials | 5mg to 30mg lyophilised vials |
| Research-grade availability | Specialist research suppliers only | Specialist research suppliers only |
Mechanism of action, what the third receptor changes
Tirzepatide was the first incretin peptide to simultaneously activate the GLP-1 and GIP receptors in a single molecule. The dual mechanism produces larger metabolic effects than selective GLP-1 agonists like Semaglutide because the GIP arm contributes additional insulin-sensitivity and lipid-handling effects on top of the GLP-1 driven satiety and glucose-disposal effects.
Retatrutide adds a third arm, agonism at the glucagon receptor. Counterintuitively, glucagon receptor activation in this context drives additional adipose-mass change through hepatic glucose output, lipolysis, and increased energy expenditure, while the simultaneous GLP-1 and GIP activation neutralises the hyperglycaemia that pure glucagon activation would otherwise cause. The result in published phase 2 data is the largest sustained body-weight reduction effect seen in any pharmacological intervention to date.
Published phase 2 efficacy data
Tirzepatide's SURPASS and SURMOUNT phase 3 programmes established dose-dependent body-weight reduction reaching 17% to 22.5% at the highest 15mg weekly dose over 72 weeks. These are the figures that supported the FDA approvals.
Retatrutide's phase 2 TRIUMPH-1 trial reported mean weight reductions of 24.2% at the 12mg weekly dose by week 48, with the dose-response curve still climbing at the trial's end-point. The trial was not yet head-to-head against Tirzepatide, but the cross-trial comparison is striking, particularly given that Retatrutide hit those numbers in less time than Tirzepatide took to reach its lower ceiling.
Side effect and tolerability differences
Both compounds share the GLP-1-class gastrointestinal profile of nausea, vomiting and constipation, particularly at dose escalation. Tirzepatide's published data shows roughly 25% of patients reporting nausea at higher doses; Retatrutide's reported rates in phase 2 were broadly similar, suggesting the third receptor target does not meaningfully worsen tolerability.
The most distinctive Retatrutide-specific signal in phase 2 data was a modest increase in heart rate (around 6 to 11 beats per minute at higher doses) attributed to the glucagon agonism arm. This signal is being followed closely in phase 3.
Bottom line for research labs
For most research-lab work modelling next-generation incretin pharmacology, Retatrutide is the more interesting target because its triple-receptor mechanism is producing pharmacology distinct from any earlier-generation compound. Tirzepatide remains the better choice for replication studies that need to anchor against the published SURPASS and SURMOUNT trial data.