Retatrutide Dosage Guide: The Triple Agonist Explained

What Is Retatrutide?

Retatrutide (LY3437943) is an investigational peptide developed by Eli Lilly that acts on three metabolic receptors simultaneously:

• GLP-1 (glucagon-like peptide-1) receptor
• GIP (glucose-dependent insulinotropic polypeptide) receptor
• Glucagon receptor

This triple agonism is what distinguishes retatrutide from its predecessors. Semaglutide acts on GLP-1 only. Tirzepatide acts on GLP-1 and GIP. Retatrutide adds the glucagon receptor, which may contribute to increased energy expenditure and fat mobilization.

Important: As of early 2026, retatrutide has not received FDA approval and remains in clinical trials. The information below is based on published clinical trial data and is provided for educational purposes.

The Science Behind Triple Agonism

GLP-1 Receptor Activation

The GLP-1 component provides the effects familiar from semaglutide: appetite suppression, delayed gastric emptying, improved insulin sensitivity, and reduced food intake. This is the best-characterized pathway for weight loss.

GIP Receptor Activation

GIP receptor agonism, also present in tirzepatide, enhances insulin secretion and may have additional effects on fat metabolism. The interaction between GLP-1 and GIP appears to be synergistic — dual agonism produces greater effects than GLP-1 alone.

Glucagon Receptor Activation

This is the novel component. Glucagon receptor agonism may contribute to:

• Increased energy expenditure — glucagon stimulates thermogenesis and metabolic rate
• Enhanced fat oxidation — glucagon promotes the breakdown of stored fat for energy
• Hepatic fat reduction — glucagon signaling reduces liver fat content

The glucagon component is a double-edged sword: while it enhances calorie burning, glucagon also raises blood glucose. The GLP-1 and GIP components counterbalance this hyperglycemic effect, which is why the triple combination works — you get the metabolic benefits of glucagon without uncontrolled glucose elevation.

Clinical Trial Dosing Data

The Phase 2 trial of retatrutide (published in the New England Journal of Medicine, 2023) tested multiple dose levels over 48 weeks. Here are the key results:

Doses Studied

The 12 mg dose group achieved a mean weight loss of 24.2% at 48 weeks — the largest weight reduction reported for any anti-obesity medication in a clinical trial to date.

Titration Schedule Used in Trials

The clinical trial used a gradual titration approach:

Some study arms used a 4 mg starting dose, but the 2 mg escalation schedule appeared to be better tolerated with fewer GI side effects.

Comparison to Semaglutide and Tirzepatide

Key Differences

• Retatrutide's glucagon component may provide additional metabolic benefits beyond weight loss, including liver fat reduction and increased energy expenditure
• The weight loss magnitude in Phase 2 trials exceeds both semaglutide and tirzepatide, though direct head-to-head trials have not been completed
• Side effect profiles are similar across all three, dominated by GI effects (nausea, vomiting, diarrhea, constipation)

Calculating Injection Volume

If you have access to compounded retatrutide, injection volume depends on the solution concentration. Use the Reconstitution Calculator for precise calculations.

Example: For a 10 mg/mL solution:

• 1 mg dose = 0.10 mL (10 units on a U-100 syringe)
• 4 mg dose = 0.40 mL (40 units)
• 8 mg dose = 0.80 mL (80 units)
• 12 mg dose = 1.20 mL (requires two injections or a larger syringe)

Side Effects Observed in Trials

The most common side effects were gastrointestinal and dose-dependent:

• Nausea — most common, especially during dose escalation
• Diarrhea — more common than with semaglutide alone
• Vomiting — typically transient during titration
• Constipation — less common than with semaglutide
• Decreased appetite — a therapeutic effect, but can become excessive at higher doses

Slower titration (4-week intervals between dose increases) appears to reduce the severity of GI side effects.

Half-Life and Dosing Frequency

Retatrutide has an estimated half-life of approximately 6 days (144 hours), which supports once-weekly dosing. This is similar to semaglutide (~7 days) and slightly longer than tirzepatide (~5 days).

Use the Half-Life Visualizer to model how retatrutide accumulates in the body with repeated weekly dosing.

Current Status and Future Outlook

Retatrutide is currently in Phase 3 clinical trials. If results confirm the Phase 2 findings, it could receive FDA approval for obesity and potentially type 2 diabetes. Additional studies are investigating its effects on metabolic dysfunction-associated steatotic liver disease (MASLD, formerly NAFLD).

Until FDA approval, retatrutide should be considered investigational. Access through compounding pharmacies may be available but should be approached with appropriate caution and medical supervision.

Disclaimer

This guide is for informational and educational purposes only and does not constitute medical advice. Retatrutide is an investigational compound that has not been approved by the FDA for any indication. Clinical trial data is preliminary and may not reflect final approved labeling. Use of investigational compounds should only occur under the supervision of a qualified healthcare provider, ideally within the context of a clinical trial.