What is Retatrutide?
Retatrutide (LY3437943) is an investigational peptide developed by Eli Lilly that functions as a simultaneous agonist at three incretin-related receptors: GLP-1, GIP, and the glucagon receptor. This triple-agonist mechanism distinguishes it from dual agonists like tirzepatide and represents the most aggressive pharmacological approach to obesity currently in clinical development. The compound is a 39-amino acid peptide with structural modifications that enable once-weekly dosing through albumin binding.
The addition of glucagon receptor agonism introduces a thermogenic component absent in GLP-1/GIP-only agents. Glucagon increases energy expenditure through hepatic mechanisms, promotes lipolysis in adipose tissue, and increases thermogenesis. While glucagon also raises blood glucose, the concurrent GLP-1 and GIP receptor activation counterbalances this hyperglycemic effect, resulting in a net glucose-neutral or glucose-lowering profile with enhanced fat oxidation.
Phase 2 trial results published in the New England Journal of Medicine (2023) demonstrated unprecedented weight loss outcomes. At the highest dose (12 mg), participants achieved a mean body weight reduction of 24.2% at 48 weeks. Approximately 26% of participants in the 12 mg group lost more than 30% of their body weight. These results exceed any previously reported outcomes for a pharmacological weight loss intervention and approach the efficacy of bariatric surgery. Phase 3 trials (TRIUMPH program) are currently underway.
Mechanism of Action
Retatrutide's triple-agonist design targets three distinct but complementary metabolic pathways simultaneously, creating a synergistic effect on weight loss that exceeds what any single receptor agonist can achieve.
GLP-1 Receptor Agonism
GLP-1 receptor activation reduces appetite through hypothalamic and brainstem signaling, slows gastric emptying (increasing meal-related satiety), and enhances glucose-dependent insulin secretion from pancreatic beta cells. This mechanism is shared with semaglutide and liraglutide and is responsible for the core appetite-suppressive effect [1].
GIP Receptor Agonism
GIP receptor activation enhances the insulinotropic response to meals and may improve lipid metabolism in adipose tissue. In combination with GLP-1 agonism, GIP activation appears to amplify weight loss beyond what GLP-1 alone achieves — a finding first demonstrated with tirzepatide. GIP may also improve tolerability of GLP-1 effects, potentially reducing nausea [2].
Glucagon Receptor Agonism
The glucagon receptor component is what distinguishes Retatrutide from all approved therapies. Glucagon activates hepatic glycogenolysis and gluconeogenesis (raising blood glucose), but critically also increases energy expenditure through thermogenesis and promotes lipolysis. In the context of simultaneous GLP-1/GIP agonism, the hyperglycemic effect of glucagon is counterbalanced, while the thermogenic and lipolytic effects provide additive fat-burning activity [3].
Synergistic Energy Balance Effects
The triple-agonist approach affects both sides of the energy balance equation: reducing energy intake (appetite suppression via GLP-1 and gastric slowing) while simultaneously increasing energy expenditure (glucagon-driven thermogenesis and fat oxidation). Preclinical data from the foundational triagonist research demonstrated that this dual-arm approach produces weight loss exceeding the sum of individual receptor contributions [4].
Dosing Protocol
Retatrutide follows a gradual dose-escalation schedule to minimize gastrointestinal side effects. The protocol below reflects the Phase 2 trial design. Final approved dosing may differ.
| Phase | Dose | Frequency | Duration | Notes |
|---|---|---|---|---|
| Initiation | 1 mg | Once weekly | Weeks 1–4 | Titration dose; GI adaptation |
| Escalation 1 | 2 mg | Once weekly | Weeks 5–8 | Continued titration |
| Escalation 2 | 4 mg | Once weekly | Weeks 9–12 | Low therapeutic range |
| Escalation 3 | 8 mg | Once weekly | Weeks 13–20 | Mid-range therapeutic dose |
| Target | 12 mg | Once weekly | Week 21+ | Highest studied dose; maximum weight loss |
- Dose escalation is critical — starting at the full dose causes severe GI side effects in most individuals.
- Inject on the same day each week. If a dose is missed, administer as soon as remembered if within 3 days of the scheduled dose.
- The 4 mg dose showed clinically meaningful weight loss (~17% at 48 weeks); not all individuals need to reach 12 mg.
- Slow the escalation if GI side effects (nausea, vomiting) are severe. Tolerability generally improves at each dose level after 2–4 weeks.
Reconstitution Guide
Reconstitute lyophilized Retatrutide with bacteriostatic water. Swirl gently until fully dissolved. Never shake the vial.
- Remove the plastic cap from the vial and wipe the rubber stopper with an alcohol swab. Allow to dry completely.
- Draw the appropriate volume of bacteriostatic water into a sterile syringe based on vial size (see concentrations below).
- Insert the needle through the rubber stopper at a slight angle. Inject the water slowly against the inner wall of the vial — do not spray directly onto the peptide powder.
- Allow the vial to sit for 2–3 minutes. Gently swirl if needed. Do not shake or vortex.
- The solution should be completely clear and colorless. Discard if you observe any cloudiness, particulate matter, or discoloration.
5 mg vial + 1 mL BAC water: Concentration = 5 mg/mL
2 mg dose = 40 units (0.4 mL) on a 100-unit insulin syringe
10 mg vial + 2 mL BAC water: Concentration = 5 mg/mL
4 mg dose = 80 units (0.8 mL) on a 100-unit insulin syringe
30 mg vial + 3 mL BAC water: Concentration = 10 mg/mL
8 mg dose = 80 units (0.8 mL) on a 100-unit insulin syringe
12 mg dose = 60 units + 60 units (split into two 0.6 mL injections for comfort)
- 2 vials Retatrutide 5 mg — covers weeks 1–8 (1–2 mg doses)
- 2 vials Retatrutide 10 mg — covers weeks 9–12 (4 mg doses)
- 3 vials Retatrutide 30 mg — covers weeks 13–24 (8–12 mg doses)
- 3 vials bacteriostatic water (30 mL each)
- 24 insulin syringes (29–31 gauge, 100-unit)
- Alcohol prep pads
Injection Technique
Retatrutide is administered via once-weekly subcutaneous injection. The compound's albumin-binding properties provide sustained receptor activation throughout the week.
- Clean the injection site with an alcohol swab and allow it to air dry completely (approximately 30 seconds). Common sites: abdomen (2 inches from navel), upper thigh, or back of upper arm.
- Draw the dose. Insert the needle into the vial through the rubber stopper. Invert the vial and draw the calculated number of units slowly. Tap the syringe to move any air bubbles to the top, then push them out gently.
- Pinch the skin at the injection site to create a fold of subcutaneous tissue. Insert the needle at a 45-degree angle in a quick, smooth motion. Release the skin fold.
- Inject slowly. Depress the plunger steadily over 10–15 seconds (larger volume at higher doses). Withdraw the needle at the same angle it was inserted. Apply gentle pressure with a clean swab if needed.
Rotate injection sites weekly. Alternate between abdomen, thigh, and arm. At higher doses (8–12 mg), the injection volume may exceed 0.8 mL; consider splitting into two separate injection sites for comfort. Allow at least 2 inches between simultaneous injection sites.
Storage & Stability
Proper storage is essential for maintaining Retatrutide potency. The compound should be protected from light and temperature extremes at all times.
- Keep vials upright and away from direct light.
- If condensation forms on a cold vial, allow it to reach room temperature before opening to prevent moisture contamination.
- Never re-freeze a reconstituted vial. Discard if left at room temperature for more than 4 hours.
- Label reconstituted vials with the date to track the 28-day use window.
Side Effects & Considerations
The adverse effect profile is similar to other incretin-based therapies, with gastrointestinal symptoms predominating during dose escalation. The glucagon component introduces additional metabolic considerations.
Gastrointestinal (Most Common)
- Nausea — reported in 16–45% of participants depending on dose, generally mild-to-moderate and improving with continued use.
- Diarrhea and vomiting — most frequent during dose escalation periods, typically resolving at each new dose level after 2–4 weeks.
- Decreased appetite and early satiety — a pharmacological effect of the compound and a primary mechanism of weight loss.
- Constipation — less common than nausea but reported across dose groups.
Cardiovascular & Metabolic
- Small mean increase in heart rate (2–4 bpm observed in trials), consistent with GLP-1 class effects.
- Glucagon component may theoretically affect hepatic glucose output in some individuals, though net glucose effect is neutral or improved due to GLP-1/GIP counterbalance.
Rapid Weight Loss Considerations
- Gallstone formation — rapid weight loss is a known risk factor for cholelithiasis. Monitoring is recommended.
- Nutritional deficiencies — significant appetite reduction can lead to inadequate protein and micronutrient intake. Dietary monitoring is important.
- Lean mass preservation — adequate protein intake (1.2–1.6 g/kg) and resistance training are recommended to minimize muscle loss during rapid weight reduction.
Retatrutide is an investigational compound currently in Phase 3 clinical trials. It is not FDA-approved for any indication. Long-term safety data is still being collected. All information presented here reflects published clinical and preclinical research and should not be construed as medical advice.
Retatrutide (RT3) is available in 5 mg, 10 mg, and 30 mg vials from Heritage Labs USA, a U.S.-based research peptide supplier with batch-level purity verification.
- Third-party purity testing (HPLC & MS)
- U.S.-based fulfillment
- Published COAs per lot
Literature & Citations
- Jastreboff AM, Kaplan LM, Frias JP, et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial. N Engl J Med. 2023;389(6):514-526. PubMed
- Rosenstock J, Frias JP, Jastreboff AM, et al. Retatrutide, a GIP, GLP-1 and glucagon receptor agonist, for people with type 2 diabetes: a randomised, double-blind, placebo and active-comparator-controlled, parallel-group, phase 2 trial. Lancet. 2023;402(10401):529-544. PubMed
- Coskun T, Urva S, Roell WC, et al. LY3437943, a novel triple glucagon, GIP, and GLP-1 receptor agonist for glycemic control and weight loss. Cell Metab. 2022;34(9):1234-1247. PubMed
- Finan B, Yang B, Ottaway N, et al. A rationally designed monomeric peptide triagonist corrects obesity and diabetes in rodents. Nat Med. 2015;21(1):27-36. PubMed
- Tan TM, Field BCT, McCullough KA, et al. Coadministration of glucagon-like peptide-1 during glucagon infusion in humans results in increased energy expenditure and amelioration of hyperglycemia. Diabetes. 2013;62(4):1131-1138. PubMed
- Day JW, Ottaway N, Patterson JT, et al. A new glucagon and GLP-1 co-agonist eliminates obesity in rodents. Nat Chem Biol. 2009;5(10):749-757. PubMed
- Habegger KM, Heppner KM, Geary N, et al. The metabolic actions of glucagon revisited. Nat Rev Endocrinol. 2010;6(12):689-697. PubMed