Retatrutide (GLP-3 / Reta) -- The Triple Agonist Research Guide

What is Retatrutide -- and why is it called GLP-3 and Reta?

Retatrutide (development code LY3437943) is a 39-amino acid synthetic peptide developed by Eli Lilly. In the research and biohacking communities, two informal names have taken hold: "Reta" (or "Ret") as a shorthand for the compound name, and "GLP-3" as a functional classification indicating it targets three GLP-related receptor systems. Neither name is an official pharmacological designation -- they arose organically as the research community mapped the compound's position in the evolving GLP agonist class.

The "GLP-3" label is particularly revealing. GLP-1 mono-agonists (semaglutide, liraglutide) activate one receptor system. GLP-1/GIP dual agonists (tirzepatide) activate two. Retatrutide activates three: GLP-1, GIP, and glucagon. The "3" in GLP-3 reflects the receptor count, not a new hormone -- glucagon-like peptide 3 does not exist as a biological molecule. In some research supply catalogs, Retatrutide is listed as "GLP-3 R" (R for receptor, distinguishing it from GLP-1 and GLP-2). The Retatrutide database entry includes full dosing specifications and an interactive dose calculator.

The three receptor mechanisms -- how triple agonism works

GLP-1 receptor agonism (appetite suppression). Retatrutide activates GLP-1 receptors in the hypothalamus and brainstem, triggering satiety signaling that reduces caloric intake -- the foundational mechanism shared with semaglutide and tirzepatide. GLP-1 receptor agonism also enhances glucose-dependent insulin secretion and slows gastric emptying, contributing to sustained post-meal fullness. This mechanism alone produces approximately 15% mean weight loss in isolation (STEP 1 trial, semaglutide).

GIP receptor co-agonism (metabolic efficiency + GI tolerability). Simultaneous GIP receptor activation enhances insulin secretion through a complementary intracellular pathway that converges on overlapping cellular machinery in the pancreas, brain, and adipose tissue. The key practical consequence: GIP co-agonism significantly improves GI tolerability of the GLP-1 component, which is why both tirzepatide and Retatrutide cause less nausea than semaglutide at weight-loss-equivalent doses. GIP receptor agonism also appears to help preserve lean mass -- an important distinction from mono-agonists, where a higher proportion of weight lost includes lean tissue.

Glucagon receptor agonism (energy expenditure). This is the third and distinguishing mechanism -- the one neither semaglutide nor tirzepatide has. Glucagon receptor activation increases resting metabolic rate and drives hepatic fat oxidation. Historically, glucagon was considered an unlikely addition to a weight loss compound because it raises blood glucose; but at the doses used in Retatrutide, the glucagon-driven glucose rise is buffered by concurrent GLP-1 and GIP-mediated insulin sensitization. The net result is an energy expenditure increase operating independently of appetite suppression, producing additive weight loss magnitude that neither dual nor mono agonism can achieve. This mechanism is also thought to underlie the energy and cognitive benefits many research participants report alongside weight reduction.

These three mechanisms operate simultaneously across the brain, liver, pancreas, and adipose tissue. They are not sequential -- they are concurrent -- which is why the efficacy hierarchy shows clean additive benefit: ~15% (mono) → ~21% (dual) → 26-29% (triple).

How Retatrutide (GLP-3 / Reta) compares to GLP-1 and GLP-1 Dual Agonist

Based on Phase 3 data across the class, the efficacy hierarchy is clear. GLP-1 mono-agonism (semaglutide, STEP 1): approximately 15% mean weight reduction at 68 weeks, plateau typically reached by week 60-68. GLP-1/GIP dual agonism (tirzepatide, SURMOUNT-1): approximately 21% mean weight reduction at 72 weeks, plateau reached by week 72. Triple agonism (Retatrutide, TRIUMPH-4): 26-29% mean weight reduction at 48-68 weeks, no plateau observed at trial end -- participants were still losing weight when the study concluded.

Beyond weight loss magnitude: Retatrutide shows substantially greater liver fat reduction (80%+ reduction in liver fat content; MASLD normalization in approximately 93% of participants in sub-studies) compared to both classes above. Lean mass preservation is better than semaglutide and comparable to tirzepatide. Gastrointestinal tolerability is comparable to tirzepatide and better than semaglutide. For a full mechanistic comparison of all three GLP receptor agonist classes, see the GLP-1 vs Dual Agonist vs GLP-3 Journal article.

Phase 3 clinical trial data -- TRIUMPH-4

The TRIUMPH-4 Phase 3 trial enrolled adults with obesity (BMI ≥30) or overweight (BMI ≥27) with at least one weight-related complication, including type 2 diabetes participants in sub-cohorts. Key findings at 48 weeks in the highest dose group (8mg weekly): mean body weight reduction of approximately 26.3%. At 68 weeks with extended data: approximately 29% mean weight loss reported in updated analyses. Critically, the weight loss curve had not plateaued at either timepoint -- distinguishing Retatrutide from semaglutide and tirzepatide, where plateau is observed within the trial window.

Milestone data: approximately 83% of participants achieved ≥10% weight loss; approximately 69% achieved ≥20%; approximately 46% achieved ≥30% -- a threshold that had essentially never been reached in a pharmaceutical obesity trial before Retatrutide's Phase 3 data. Liver outcomes in sub-studies: greater than 80% reduction in liver fat content measured by MRI; MASLD normalization in approximately 93% of participants with baseline steatosis. These liver outcomes are substantially stronger than those observed with either semaglutide or tirzepatide and are attributed to the glucagon receptor-mediated hepatic fat oxidation mechanism.

GI adverse effects: nausea, vomiting, and diarrhea were the most common during titration and diminished as dose stabilized. Profile was comparable to tirzepatide at equivalent weight-loss-producing doses and better than semaglutide. Importantly, participants frequently reported subjective improvements in energy levels and reduced cognitive fatigue alongside weight reduction -- effects attributed to glucagon receptor-mediated metabolic activation.

Research protocol and dosing -- slow titration is critical

The Phase 3 titration schedule: 0.5mg weekly subcutaneous for weeks 1-4, 1mg for weeks 5-8, 2mg for weeks 9-12, 4mg for weeks 13 and beyond, with a maximum of 8mg. The slow titration schedule is non-negotiable -- GI adverse effects in the triple agonist class are dose-escalation dependent, meaning they are largely a function of how fast dose increases occur rather than the maintenance dose itself. Research protocols in the community typically use lower maintenance doses (1-4mg weekly) given that meaningful outcomes appear well below the clinical trial maximum.

Same day each week, subcutaneous injection, fed or fasted (no meaningful difference demonstrated). Same day each week is standard across all GLP-class compounds due to their extended half-lives. Reconstituted vials should be stored refrigerated at 2-4°C. Use the interactive dose calculator for unit calculations at your target dose and concentration.

Regulatory status as of May 2026

As of May 2026, Retatrutide's NDA (New Drug Application) has not been submitted to the FDA. Eli Lilly has indicated a filing timeline but no specific submission date has been confirmed publicly. It remains a research compound in all jurisdictions. This distinguishes it from tirzepatide (FDA-approved as Zepbound for obesity and Mounjaro for T2D) and semaglutide (FDA-approved as Wegovy for obesity and Ozempic for T2D). In some research supply catalogs it is listed as "GLP-3 R". Researchers should be aware of and comply with all applicable regulations in their jurisdiction.

Research references

• Jastreboff AM, et al. (2023). Triple hormone receptor agonist retatrutide for obesity -- NEJM via PubMed
• Drucker DJ (2020). Mechanisms of action and therapeutic application of GLP-1 -- PubMed