Retatrutide vs Mounjaro: how the triple agonist compares to tirzepatide
Mounjaro (tirzepatide) is the closest thing to retatrutide already available in the UK — both are multi-receptor agonists from Eli Lilly. The difference is one extra receptor, and a materially different trial result.
| Mounjaro (tirzepatide) | Retatrutide | |
|---|---|---|
| Receptors targeted | GLP-1 + GIP | GLP-1 + GIP + glucagon |
| Best reported weight loss | ~20–22% (SURMOUNT trials) | Up to 30.3% (TRIUMPH-1, phase 3) |
| UK status | Licensed & available | Not licensed anywhere — phase 3 only |
| Manufacturer | Eli Lilly | Eli Lilly |
| Real-world safety data | Years of post-launch data | None yet — trial-only, medically supervised |
The one receptor that changes the picture
Both drugs share the same maker and two of the same three receptor targets. The difference comes down to glucagon.
What Mounjaro does
Tirzepatide activates GLP-1 and GIP receptors together — the mechanism that made it the strongest licensed weight-loss option in the UK, outperforming semaglutide-only drugs in head-to-head trials.
What retatrutide adds
Retatrutide keeps both of those and adds glucagon receptor activation, which raises resting energy expenditure and increases fat breakdown in the liver — pathways tirzepatide doesn't directly engage. That's the leading explanation for retatrutide's larger trial results.
Why this comparison matters more than most
Because they're both from Eli Lilly and share two receptor targets, retatrutide is widely seen as tirzepatide's likely successor once it's licensed — not a different category of drug, but the next step in the same lineage.