These three compounds dominate current metabolic research, and they are frequently discussed as though they were three strengths of the same thing. They are not. They engage different numbers of receptors, and that distinction changes both what they do and, more importantly for research, how confidently you can attribute an observed effect to anything.
Everything below describes published laboratory and clinical literature. These compounds are supplied for research use only.
The one-line version
Each step adds a receptor. The glucagon receptor on retatrutide is the genuinely new element, and it is the reason the compound is interesting rather than merely stronger.
- Semaglutide: single agonist. GLP-1 only.
- Tirzepatide: dual agonist. GIP + GLP-1.
- Retatrutide: tri-agonist. GIP + GLP-1 + glucagon.
Semaglutide: the reference molecule
Semaglutide is a modified analogue of human GLP-1, engineered for a roughly seven-day half-life through three changes: an Aib substitution at position 8 that blocks DPP-4 degradation, a lysine substitution at 34, and a C18 fatty-diacid chain that binds albumin reversibly. Native GLP-1 survives about two minutes; semaglutide survives about a week.
It has the deepest published record of any peptide in the class, including large cardiovascular outcome trials. That makes it the comparator the literature expects when you characterise anything new.
For mechanistic work its single-receptor nature is a feature. If you want to know what GLP-1 signalling does, a molecule that engages GLP-1 and nothing else is the clean instrument. Dual and triple agonists are more powerful but they confound attribution.
Tirzepatide: adding GIP
Tirzepatide is built on a modified GIP scaffold and engages both the GIP and GLP-1 receptors. GIP is the second incretin hormone, expressed on pancreatic beta cells and on adipose tissue.
The interesting thing about GIP is that the field has not fully settled whether agonism or antagonism is the productive direction. There is credible published work pointing both ways, which is unusual for a receptor this well studied. Tirzepatide is the main tool for investigating the agonist side of that question in combination with GLP-1.
Retatrutide: adding glucagon
Retatrutide (development code LY3437943) adds glucagon-receptor agonism on top of GIP and GLP-1. Glucagon is the counter-intuitive addition: classically it raises hepatic glucose output, which sounds like the opposite of what the incretin arms are doing.
The rationale in the literature is that glucagon-receptor signalling is also associated with hepatic lipid handling and total energy expenditure, and that the incretin arms act as a counterweight to its glucose-raising tendency. Whether that balance works out favourably is precisely the open question the compound exists to answer.
Its phase 2 data is published in major journals, which is unusual for a compound circulating in the research market. Most do not have that. Half-life is reported at around six days, via the same albumin-binding strategy the other two use.
Side by side
- Receptors. Semaglutide: GLP-1. Tirzepatide: GIP + GLP-1. Retatrutide: GIP + GLP-1 + glucagon.
- Backbone. Semaglutide: modified human GLP-1. Tirzepatide and retatrutide: modified GIP scaffold.
- Reported half-life: semaglutide ~7 days, retatrutide ~6 days, tirzepatide ~5 days. All three use fatty-diacid albumin binding to get there.
- Published depth: semaglutide has the largest dataset including outcome trials; tirzepatide has an extensive phase 3 programme; retatrutide has phase 2 reported with phase 3 ongoing.
| Property | Retatrutide | Semaglutide | Tirzepatide |
|---|---|---|---|
| Receptors | GIP + GLP-1 + glucagon | GLP-1 only | GIP + GLP-1 |
| Class | Tri-agonist | Single agonist | Dual agonist |
| Reported half-life | ~6 days | ~7 days | ~5 days |
| Half-life strategy | C20 fatty-diacid, albumin binding | C18 fatty-diacid, albumin binding | C20 fatty-diacid, albumin binding |
| Backbone | Modified GIP scaffold | Modified human GLP-1 | Modified GIP scaffold |
| Published stage | Phase 2 reported, phase 3 ongoing | Extensive phase 3 and outcome trials | Extensive phase 3 programme |
| Primary research interest | Combined incretin and glucagon signalling, energy expenditure | GLP-1 pathway, glycaemic and satiety endpoints | GIP/GLP-1 interaction |
Half-life figures are approximate values reported in published human pharmacokinetic studies and vary with study design. Provided for laboratory reference only.
Which one for which question
If the research question is about GLP-1 signalling specifically, use semaglutide. Adding receptors adds explanatory ambiguity you will then have to design around.
If the question is about how GIP and GLP-1 interact, tirzepatide is the purpose-built instrument.
If the question involves the glucagon arm: energy expenditure, hepatic lipid handling, or what combined tri-agonism does that dual agonism does not, retatrutide is the only widely available tool that gets you there in a single molecule with a single exposure curve.
That last point is worth dwelling on. You could in principle approximate a tri-agonist by co-administering separate compounds, but each has its own pharmacokinetics, and any effect you observe might be a timing artefact rather than a pharmacological one. A single molecule collapses three variables into one.
A practical note on sourcing
These three are the most counterfeited compounds in the research market, in roughly that order of severity. Retatrutide in particular is expensive to synthesise correctly and is widely misrepresented, under-content vials and outright substitution are both documented problems.
The defence is boring and effective: buy from someone who assays each lot by HPLC and mass spectrometry and will send you the certificate matching the lot number on your vial. If you are not sure how to read that document, we wrote a guide to it.
For research use only. Everything above describes laboratory practice and published research. It is not medical advice, not dosing guidance, and carries no implication of human or veterinary use.