Retatrutide and tirzepatide are consecutive designs from the same Eli Lilly programme, built on the same GIP-based backbone — tirzepatide is a dual GIP/GLP-1 receptor agonist, and retatrutide adds a third arm at the glucagon receptor. The other decisive difference is stage: tirzepatide is an approved medicine, retatrutide an investigational compound approved nowhere. This is a pharmacological comparison for research context; it makes no claims about human outcomes or suitability.
Side by side
| Tirzepatide | Retatrutide (LY3437943) | |
|---|---|---|
| Receptor targets | GIP + GLP-1 (dual agonist) | GIP + GLP-1 + glucagon (triple agonist) |
| Developer | Eli Lilly | Eli Lilly |
| Peptide backbone | GIP-based, non-coded substitutions | GIP-based, non-coded substitutions |
| Distinguishing arm | Adds GIP to the GLP-1 baseline | Adds GIP and glucagon |
| Half-life extension | C20 fatty-diacid acylation | C20 fatty-diacid, albumin binding |
| Approx. molecular weight | ~4,814 Da | ~4,731 Da |
| Dosing cadence (clinical) | Once weekly | Once weekly (trials) |
| Regulatory status | Approved medicine (MHRA, FDA) | Investigational — Phase 3, no approval anywhere |
The one receptor that separates them
Strip away the branding and the two molecules differ meaningfully in a single design decision: whether to engage the glucagon receptor. Tirzepatide stops at the two incretin axes — GIP and GLP-1 — both of which act on glucose-dependent insulin secretion and related signalling. Retatrutide keeps those two arms and adds glucagon-receptor agonism, a deliberately counter-intuitive choice since glucagon raises hepatic glucose output; the design bet is that the incretin arms offset that effect while the glucagon arm contributes mechanisms neither parent class accesses. How that bet is structured — and what remains unresolved about it — is covered in depth in our mechanism of action explainer.
Because tirzepatide came first and is approved, it functions as retatrutide’s natural reference point: same company, same backbone strategy, one fewer receptor. The published retatrutide trial figures — including the Phase 2 NEJM and Phase 3 TRIUMPH-1 results — are summarised neutrally in our overview of the clinical trial evidence; they are findings in separate study populations, not a head-to-head, which is why this page does not rank the two compounds.
Frequently asked questions
What is the difference between retatrutide and tirzepatide?
One receptor. Both are Eli Lilly incretin agonists engineered on a GIP-based peptide backbone with fatty-acid acylation for once-weekly dosing, and both engage the GIP and GLP-1 receptors. Retatrutide additionally activates the glucagon receptor, making it a triple rather than dual agonist. The other difference is regulatory: tirzepatide is an approved medicine, while retatrutide is investigational and not approved by any regulator.
Is retatrutide stronger than tirzepatide?
"Stronger" is not a claim that can responsibly be made. There are no published head-to-head trials between the two compounds, so any comparison rests on separate trials with different populations, designs, and endpoints. What can be said pharmacologically is that retatrutide engages one additional receptor — the glucagon receptor, associated in research with energy expenditure — and that its Phase 2 and Phase 3 trial results are summarised, with attribution, in our clinical evidence overview.
Are retatrutide and tirzepatide made by the same company?
Yes — both originate from Eli Lilly, and they are consecutive steps in the same design programme: tirzepatide demonstrated that adding GIP agonism to the GLP-1 baseline changes the metabolic profile, and retatrutide asks whether a third, mechanistically distinct glucagon arm can be added on top. That shared lineage is why their backbones and half-life chemistry look so similar on paper.
Can tirzepatide be bought as a research peptide like retatrutide?
Tirzepatide is an approved, prescription-only medicine in the UK, so its lawful route is a prescription — grey-market "research" tirzepatide sits in a different and murkier legal position than an unapproved compound. Retatrutide, having no marketing authorisation anywhere, is supplied only as a research compound. RETApro supplies retatrutide for laboratory research use only and does not sell tirzepatide.