What is a GLP-3 receptor agonist?

There is no clinically recognized 'GLP-3 receptor.' The term is a misnomer. What people call 'GLP-3' typically refers to triple-agonist drugs like retatrutide that activate three receptors: GLP-1, GIP, and glucagon. The glucagon-like peptide family only includes GLP-1 and GLP-2.

How much weight can you lose on retatrutide?

In the Phase II trial, participants on the highest dose (12 mg weekly) lost an average of 24.2% body weight over 48 weeks, with some losing over 30%. This exceeds results seen with semaglutide (~15%) and tirzepatide (~22.5%). Phase III trials are ongoing to confirm these results.

When will retatrutide be available?

Retatrutide is currently in Phase III clinical trials (the TRIUMPH program). If results are positive, FDA approval could come as early as 2027-2028. It is not yet available by prescription and is not legally sold as a research peptide in the same way as non-drug peptides.

Is retatrutide better than tirzepatide?

Phase II data suggests retatrutide may produce greater weight loss (~24% vs ~22.5%) due to the addition of glucagon receptor agonism, which increases energy expenditure. However, head-to-head trials have not been conducted, and Phase III data is needed to confirm superiority. Side-effect profiles may also differ.

What is the difference between dual and triple agonist obesity drugs?

Dual agonists like tirzepatide target two receptors (GLP-1 + GIP), while triple agonists like retatrutide target three (GLP-1 + GIP + glucagon). The glucagon receptor adds energy expenditure and hepatic fat oxidation benefits, potentially driving greater weight loss and liver fat reduction, but requires careful balancing to prevent hyperglycemia.

Can I use research peptides instead of GLP-1 drugs for weight loss?

Research peptides like AOD-9604, tesamorelin, and CJC-1295/ipamorelin work through different mechanisms than GLP-1 drugs. They may support fat metabolism, GH secretion, or body composition through pathways that don't involve appetite suppression. However, their clinical evidence for weight loss is generally less robust than FDA-approved GLP-1 agonists. Consult a physician to evaluate your options.

GLP-3 Receptor Agonists: What Is Retatrutide & How Does It Work?

GLP-3 Receptor Agonists: Clearing Up the Confusion

If you have searched for "GLP-3 receptor agonist," you have likely encountered confusing or misleading information. Strictly speaking, there is no clinically recognized GLP-3 receptor. The glucagon-like peptide family includes GLP-1 and GLP-2, each with distinct receptors and functions. What many people call "GLP-3" is actually a reference to triple-agonist drugs — compounds that activate three receptors simultaneously: GLP-1, GIP, and glucagon. The most prominent of these is retatrutide (LY3437943), developed by Eli Lilly. This article is for educational purposes only and does not constitute medical advice.

The Incretin Receptor Landscape

The incretin hormone family is well-characterized. GLP-1 and GLP-2 are produced by L-cells in the gut and have distinct receptors:

GLP-1 receptor: Targeted by drugs like semaglutide, liraglutide, and tirzepatide. Regulates appetite, insulin secretion, and gastric emptying.
GLP-2 receptor: Targeted by teduglutide (Gattex). Promotes intestinal growth and nutrient absorption. Used for short bowel syndrome.
GIP receptor: Glucose-dependent insulinotropic polypeptide receptor. Targeted alongside GLP-1 by tirzepatide (Mounjaro/Zepbound).
Glucagon receptor: Activated by glucagon. Promotes hepatic glucose output, thermogenesis, and fat oxidation.

When people say "GLP-3," they typically mean the third agonist added on top of GLP-1 and GIP — which is the glucagon receptor. This creates a triple agonist, not a "GLP-3 agonist." The terminology is a misnomer that has gained traction in online discussions.

What Is Retatrutide?

Retatrutide (LY3437943) is an investigational triple-hormone receptor agonist developed by Eli Lilly. It simultaneously activates:

GLP-1 receptors — suppresses appetite, improves insulin sensitivity
GIP receptors — enhances insulin secretion, may improve fat metabolism
Glucagon receptors — increases energy expenditure, promotes hepatic lipid oxidation, drives thermogenesis

The addition of glucagon receptor agonism is what distinguishes retatrutide from tirzepatide (which is only a dual GLP-1/GIP agonist). The glucagon component increases energy expenditure and drives greater fat oxidation, potentially producing superior weight loss. For more on this compound, see our retatrutide triple agonist guide.

Phase 2 Trial Results

The Phase 2 trial of retatrutide, published in the New England Journal of Medicine in 2023, produced remarkable results^[1]:

Dose (Weekly SC)	Weight Loss at 24 Weeks	Weight Loss at 48 Weeks
Placebo	-2.1%	-3.2%
1 mg	-7.2%	-8.7%
4 mg (escalated from 2 mg)	-12.9%	-17.1%
4 mg (fixed)	-13.4%	-13.4%
8 mg (escalated from 2 mg)	-16.4%	-22.8%
8 mg (fixed)	-16.3%	-19.9%
12 mg (escalated from 2 mg)	-18.2%	-24.2%

The 12 mg dose group achieved 24.2% mean body weight loss at 48 weeks, with some participants losing over 30%. This exceeds the results seen with semaglutide (~15%) and tirzepatide (~22.5%), making retatrutide potentially the most effective weight-loss drug ever studied. The weight loss curve had not plateaued at 48 weeks, suggesting even greater losses with longer treatment.

For context on how these results compare to existing therapies, see our retatrutide vs tirzepatide comparison and our retatrutide vs semaglutide comparison.

Triple Agonism: Why Three Receptors?

Each receptor contributes a distinct metabolic effect:

GLP-1 Component

The GLP-1 component provides the appetite-suppressing backbone familiar from semaglutide. It slows gastric emptying, enhances satiety signaling in the hypothalamus, and improves glycemic control. This is the "proven" foundation of the triple-agonist approach.

GIP Component

GIP's role in obesity therapy was controversial until tirzepatide demonstrated that dual GLP-1/GIP agonism outperformed GLP-1 alone. GIP appears to potentiate the effects of GLP-1 on beta-cell function and may independently modulate fat storage and adipocyte metabolism^[2].

Glucagon Component

The glucagon receptor component is the key differentiator. Glucagon increases hepatic glucose production (necessitating the GLP-1 component to counterbalance), but it also:

Increases resting energy expenditure (thermogenesis) by 10-15% in animal models
Promotes hepatic lipid oxidation (potentially beneficial for fatty liver disease / MASH)
Stimulates amino acid catabolism
May reduce hepatic fat content more effectively than GLP-1 alone

The balance between these three signals is carefully calibrated. Too much glucagon would cause hyperglycemia; the GLP-1 and GIP components counteract this, allowing the thermogenic benefits of glucagon without the metabolic risk.

Comparison: From Mono to Triple Agonists

Drug	Receptors Targeted	Peak Weight Loss (Trials)	Status
Semaglutide (Wegovy)	GLP-1	~15%	FDA Approved
Tirzepatide (Zepbound)	GLP-1 + GIP	~22.5%	FDA Approved
Retatrutide	GLP-1 + GIP + Glucagon	~24.2%	Phase III
Survodutide	GLP-1 + Glucagon	~19%	Phase III
Mazdutide	GLP-1 + Glucagon	~17%	Approved (China)

Side Effects and Safety Considerations

The Phase 2 safety profile of retatrutide was generally consistent with the GLP-1 agonist class. For comprehensive coverage of side effects across all weight loss compounds, see our peptide side effects guide.

Gastrointestinal: Nausea (25-46%), diarrhea (16-26%), vomiting (9-19%), constipation (9-16%) — dose-dependent, most events mild-to-moderate
Injection site reactions: Less common than with other formulations
Heart rate increase: Small mean increase observed (2-4 bpm)
Hepatic effects: Increases in ALT observed in some participants, particularly at higher doses, though generally transient
Hypoglycemia: Rare, despite the glucagon component

Slow dose titration (starting at 2 mg and escalating monthly) significantly reduced GI side effect severity. Discontinuation rates due to adverse events were 6-10%, comparable to other GLP-1-based therapies.

Dosing and Titration

Based on the Phase 2 protocol, retatrutide was administered as a once-weekly subcutaneous injection. The most effective titration schedule was:

Weeks 1-4: 2 mg once weekly
Weeks 5-8: 4 mg once weekly
Weeks 9-12: 8 mg once weekly
Weeks 13+: 12 mg once weekly (if tolerated)

For general principles on peptide dosing, consult our peptide dosage guide.

Phase 3 Program and Timeline

As of early 2026, retatrutide is in Phase 3 clinical trials (the TRIUMPH program) across multiple indications:

TRIUMPH-1: Obesity/overweight without type 2 diabetes
TRIUMPH-2: Obesity with type 2 diabetes
TRIUMPH-3: Obesity with obstructive sleep apnea
TRIUMPH-4: MASH with fibrosis

If Phase 3 results confirm the Phase 2 data, retatrutide could receive FDA approval as early as 2027-2028. For the latest on the clinical trial program, see our retatrutide phase 3 update.

Retatrutide vs Peptide Approaches to Weight Loss

For those exploring the broader peptide landscape, it is worth comparing retatrutide's pharmaceutical approach with research peptides used for body composition. Compounds like AOD-9604, tesamorelin, and growth-hormone-releasing peptides (CJC-1295/ipamorelin) work through different pathways — stimulating lipolysis, GH secretion, or metabolic rate rather than suppressing appetite. For a deeper look, see our weight-loss peptides overview and AOD-9604 vs semaglutide comparison.

The Bottom Line

The "GLP-3" label may be inaccurate, but the compound it usually refers to — retatrutide — represents a genuine leap forward in obesity pharmacology. The addition of glucagon receptor agonism to the established GLP-1/GIP platform addresses a limitation of existing therapies: they reduce caloric intake without substantially increasing energy expenditure. Retatrutide does both. Phase 2 results are among the most impressive ever seen for a weight loss medication, and the Phase 3 TRIUMPH program will determine whether these results hold at scale. For anyone tracking the evolution of weight loss peptides, retatrutide is the most important compound in development.