Retatrutide (GLP-3) vs Tirzepatide (GLP-2): Mechanism, Specs & Reported Ranges Compared
Side-by-side comparison of Retatrutide (GLP-3) and Tirzepatide (GLP-2) — receptor mechanism, half-life, vial sizes, reconstitution and reported study arms.
By PX1 Research Team · August 4, 2026
Retatrutide (GLP-3) vs Tirzepatide (GLP-2) at a glance
| Retatrutide (GLP-3) | Tirzepatide (GLP-2) | |
|---|---|---|
| Class | GIP/GLP-1/glucagon triple receptor agonist | dual GIP/GLP-1 receptor agonist |
| Reported half-life | ~6 days | ~5 days |
| Vial sizes | 10 mg / 15 mg / 20 mg / 30 mg | 10 mg / 15 mg / 30 mg / 60 mg |
| Typical reconstitution | 10 mg + 2 mL bacteriostatic water = 5 mg/mL | 10 mg + 2 mL = 5 mg/mL |
| Research focus | metabolic and body-composition research | incretin and metabolic research |
Mechanistic difference
Retatrutide engages three receptors — GIP, GLP-1 and glucagon — while tirzepatide engages two (GIP and GLP-1). The glucagon arm is the structural difference, and it is why reported effect sizes and escalation caution differ between the two literatures.
Reported study ranges side by side
Retatrutide (GLP-3)
| Study arm | Reported amount | Cadence |
|---|---|---|
| Initiation | 1 mg | weekly × 4 |
| Step 1 | 2 mg | weekly × 4 |
| Step 2 | 4 mg | weekly × 4 |
| Step 3 | 8 mg | weekly × 4 |
| Maximal arms | 12 mg | weekly |
Tirzepatide (GLP-2)
| Study arm | Reported amount | Cadence |
|---|---|---|
| Initiation | 2.5 mg | weekly × 4 |
| Step 1 | 5 mg | weekly × 4 |
| Step 2 | 7.5 mg | weekly × 4 |
| Step 3 | 10 mg | weekly × 4 |
| Maximal arms | 15 mg | weekly |
Practical handling differences
Both arrive lyophilized and shelf-stable. Retatrutide (GLP-3) is typically prepared at 10 mg + 2 mL bacteriostatic water = 5 mg/mL; Tirzepatide (GLP-2) at 10 mg + 2 mL = 5 mg/mL. Where half-lives differ, the literature's dosing cadence differs accordingly — longer-acting molecules are described weekly, shorter-acting ones daily or split.
Which one appears in which research context
Retatrutide (GLP-3) shows up primarily in metabolic and body-composition research. Tirzepatide (GLP-2) is used in incretin and metabolic research. They are not interchangeable substitutes; the receptor profile, not the marketing category, determines where each is used.
Documentation and sourcing
Both are supplied at ≥99% HPLC purity with lot-level COAs at /coa. Product pages: Retatrutide (GLP-3) · Tirzepatide (GLP-2).
How PX1 documents every lot
Every PX1 lot ships with HPLC purity, LC-MS identity, residual solvent, moisture (Karl Fischer), sterility and endotoxin data. Lot documentation is published at /purity-reports, /endotoxin-reports and /coa. Synthesis, purification, lyophilization and vial fill happen in U.S. facilities — see /manufacturing-process.
Research use only. PX1 Research supplies compounds for laboratory and in-vitro research. Nothing here is medical advice, a dosing recommendation, or an offer for human or veterinary use. Figures cited are reported from published laboratory literature.