RESEARCH GUIDE

Retatrutide: The Triple Agonist Explained

A comprehensive, honest guide to the investigational triple-receptor agonist setting new records for weight loss in clinical research — what it is, how it works, and what the data actually shows.

Please read before continuing
This guide provides general educational information only. Nothing on this page is medical advice, a prescription, or a dosing recommendation. Retatrutide is an investigational compound not approved by the FDA, EMA, or Costa Rica’s Ministerio de Salud. Always consult a qualified healthcare professional. Products sold by Peptides Costa Rica are intended for research purposes only.

What's on this page

What Is Retatrutide?

Retatrutide, originally known by its development code LY3437943, is an investigational peptide developed by Eli Lilly. It belongs to a new class of metabolic medicines called multi-receptor agonists, and it is currently being tested in late-stage clinical trials for obesity, type 2 diabetes, knee osteoarthritis, sleep apnea, cardiovascular disease, fatty liver disease, and several other conditions.

What makes Retatrutide unique is that it activates three different hormone receptors at the same time. To understand why this matters, it helps to know what those receptors do.

Why the Word "Triple" Matters

Most weight loss peptides on the market today activate either one or two hormone receptors.

Retatrutide activates three. Here is what each one does:

GLP-1 (glucagon-like peptide-1): This is the same hormone that semaglutide (Ozempic, Wegovy) targets. It slows down stomach emptying, reduces appetite, and helps regulate blood sugar.
GIP (glucose-dependent insulinotropic polypeptide): This is the second receptor that tirzepatide (Mounjaro, Zepbound) hits. GIP supports insulin release in response to food and appears to enhance fat metabolism when activated alongside GLP-1.
Glucagon: This is the new one. Glucagon receptor activation is associated with increased energy expenditure — meaning your body burns more calories at rest — and contributes to improvements in fat metabolism and liver health.

By activating all three at once, Retatrutide combines appetite reduction, blood sugar control, and increased energy expenditure into a single molecule. That combination is what researchers refer to when they call it a “triple agonist” or “triple G” agonist.

How Retatrutide compares to similar peptides

Semaglutide activates one receptor (GLP-1). Tirzepatide activates two (GLP-1 and GIP). Retatrutide activates three (GLP-1, GIP, and glucagon). Each generation of these peptides has produced larger average weight reductions than the last in clinical research.

How Retatrutide Works in the Body

Retatrutide works on multiple systems simultaneously. The combined effect is much more than the sum of its parts.

1. It Reduces Appetite

GLP-1 activation signals the brain regions that control hunger and fullness. Users typically report feeling satisfied with much smaller portions and experiencing fewer food cravings. Stomach emptying slows down, which extends the feeling of fullness after meals.

2. It Improves Blood Sugar Control

Both GLP-1 and GIP enhance insulin release when blood sugar rises after eating. This means glucose is cleared from the bloodstream more efficiently, leading to lower average blood sugar levels and reduced A1C in research participants with diabetes.

3. It Increases Energy Expenditure

This is the unique contribution of the glucagon component. Glucagon receptor activation increases the body’s resting metabolic rate, meaning the body burns more calories even when not active. It also promotes fat mobilization from storage and supports liver health by reducing fat accumulation in the liver.

4. It Supports Fat Loss Specifically

The combination of reduced calorie intake and increased calorie expenditure creates conditions for sustained fat loss. Importantly, research has shown that Retatrutide also produces significant reductions in visceral fat (the deep abdominal fat that surrounds organs and is most associated with metabolic disease) and liver fat.

What the Research Shows

Retatrutide has been studied in one of the most rigorous clinical trial programs in recent metabolic medicine. Here is what the published data shows.

Phase 2 Obesity Trial (NEJM, 2023)

The Phase 2 trial led by Dr. Ania Jastreboff and colleagues at Yale, published in the New England Journal of Medicine in August 2023, enrolled 338 adults with obesity (BMI of 30 or higher) or overweight (BMI 27+ with at least one weight-related condition). Participants received once-weekly subcutaneous injections of Retatrutide at doses of 1 mg, 4 mg, 8 mg, or 12 mg, or placebo, over 48 weeks.

The headline results: at 48 weeks, participants on the 12 mg dose lost an average of 24.2% of their body weight, compared to 2.1% in the placebo group. The weight reduction at the 8 mg dose was 22.8%, and at 4 mg it was 17.1%.

To put 24.2% in perspective: this was the largest average weight reduction ever published for an anti-obesity medication at the time. Tirzepatide had reached approximately 22.5% in its 72-week Phase 3 trial. Semaglutide had reached approximately 15%. Retatrutide hit 24.2% in just 48 weeks — and the trial researchers noted that the weight loss curves had not yet plateaued, suggesting more reduction may have occurred with longer treatment.

Additional Phase 2 Findings

At 48 weeks on the 12 mg dose, 100% of participants achieved at least 5% weight loss, 93% achieved at least 10%, and 83% achieved at least 15%.
Liver fat reductions were substantial: in a sub-study of participants with fatty liver disease (MASLD), the 12 mg dose produced an 82.4% relative reduction in liver fat at 24 weeks, with 86% of participants reaching normal liver fat levels.
Improvements were also seen in blood pressure, cholesterol, and triglyceride levels.

Phase 3 Trials (2025 and 2026)

Retatrutide has now produced positive results in multiple Phase 3 trials.

TRIUMPH-4 (December 2025): In adults with obesity or overweight plus knee osteoarthritis, Retatrutide produced average weight reductions of up to 71.2 lbs (around 28.7%) at 68 weeks, along with significant reductions in knee pain.

TRANSCEND-T2D-1 (March 2026): In adults with type 2 diabetes, the 12 mg dose produced average A1C reductions of up to 2.0% and average weight loss of 36.6 lbs (16.8%) over 40 weeks. These results are competitive with or superior to current GLP-1 medications in head-to-head comparisons.

Additional Phase 3 trials are ongoing in cardiovascular outcomes, obstructive sleep apnea, chronic low back pain, and metabolic dysfunction-associated steatotic liver disease.

Side Effects and Safety Profile

Retatrutide’s safety profile in clinical trials has been broadly consistent with other peptides in this class. The most commonly reported side effects in research have been gastrointestinal.

Most Common Side Effects

Nausea — especially during the first few weeks of treatment and when increasing the dose
Vomiting
Diarrhea or constipation
Reduced appetite (often the desired effect, but can become uncomfortable)
Indigestion or stomach discomfort
Mild injection site reactions

In the Phase 2 trial, these effects were typically mild to moderate in severity and most occurred during the dose escalation period. They generally improved as users adjusted to the medication.

Less Common but Important Considerations

Heart rate may increase slightly in some users
Blood pressure has shown decreases in research, which is generally considered favorable
As with other GLP-1-related compounds, there is theoretical concern about thyroid effects based on animal studies — though no such effects have been confirmed in humans
Hypoglycemia (low blood sugar) is uncommon in non-diabetic users but can occur in those also taking insulin or sulfonylureas

An honest note on safety

Because Retatrutide is still investigational, the full long-term safety profile is not yet known. Research is ongoing. The information here reflects what has been published in peer-reviewed clinical trials through early 2026. Anyone considering Retatrutide should discuss it with a qualified healthcare professional who can evaluate the risks and benefits in their specific situation.

Dosing Information

In clinical trials, Retatrutide has been administered as a once-weekly subcutaneous injection with a gradual dose escalation. The trial protocols started low and increased slowly to minimize gastrointestinal side effects.

Common Dose Escalation Schedule in Research

Weeks	Dose	Notes
Weeks 1–4	2 mg per week	Starting dose
Weeks 5–8	4 mg per week	First escalation
Weeks 9–12	4 mg or 8 mg per week	Depending on tolerance
Weeks 13+	8 mg or 12 mg per week	Maintenance dose, depending on goals and tolerance

This is the escalation pattern used in Eli Lilly’s Phase 2 obesity trial. The exact schedule used in research can vary depending on the protocol. The most important principle is starting low and escalating slowly — going too fast significantly increases the risk of nausea and other side effects.

A reminder

The doses above reflect what has been used in clinical trials. They are not dosing recommendations for any specific individual. Personal dosing decisions should always be made with a qualified healthcare professional who can evaluate your specific health context.

How Retatrutide Is Available Through Peptides Costa Rica

We carry Retatrutide in multiple vial strengths to give our customers flexibility. Each vial is lyophilized (freeze-dried powder) and arrives sealed and ready to reconstitute with bacteriostatic water.

Available Vial Sizes

Retatrutide 10 mg
Retatrutide 12 mg
Retatrutide 15 mg
Retatrutide 20 mg
Retatrutide 24 mg
Retatrutide 30 mg
Retatrutide 40 mg

Larger vial sizes are typically more economical per milligram and are popular with customers who plan to use Retatrutide over several months. Smaller sizes are useful for first-time buyers who want to start with a smaller commitment.

Quality and Documentation

Every product comes with a Certificate of Analysis (COA) on request, showing identity confirmation and purity testing
Purity is verified by HPLC and mass spectrometry to industry-standard levels
All inventory is stored in climate-controlled conditions before shipping — critical in Costa Rica’s tropical climate
Domestic shipping eliminates the customs delays and cold chain risks of international orders

What Buyers Should Consider Before Purchasing

Retatrutide is one of the most promising peptides in modern research, but it is not the right fit for everyone. Here are the most important things to think through honestly before purchasing.

It Is Sold as a Research Compound

Like most peptides in our catalog, Retatrutide is sold for research purposes only. It is not a prescription medication, and the legal framework for buying it is different from how you would buy a pharmacy medicine. We have a complete guide on this in our pillar article on peptides in Costa Rica.

Side Effects Are Real, Especially Early On

Nausea during the first few weeks of dosing is very common. Most users find it tolerable and report that it diminishes as their body adjusts, but some find it severe enough to discontinue. Starting at the lowest possible dose and increasing slowly makes a significant difference in tolerability.

It Works Best With Lifestyle Changes

Every successful research participant in Retatrutide trials received nutrition counseling and exercise guidance alongside the medication. The peptide is not a replacement for healthy habits — it is most effective as a tool that supports those habits.

Storage Matters in Our Climate

Lyophilized Retatrutide vials should be refrigerated. Once reconstituted with bacteriostatic water, the solution should be kept refrigerated at 2 to 8 degrees Celsius at all times. In Costa Rica’s tropical climate, this discipline matters more than in cooler countries.

Common Questions About Retatrutide

How does Retatrutide compare to semaglutide or tirzepatide?

In clinical trial data, Retatrutide has produced larger average weight reductions than either semaglutide or tirzepatide. Semaglutide is a single-receptor agonist (GLP-1), tirzepatide is a dual agonist (GLP-1 and GIP), and Retatrutide is a triple agonist (GLP-1, GIP, and glucagon). However, no head-to-head clinical trials have directly compared all three, so direct comparison is limited to what has been observed in separate trials.

How often do I need to inject?

Retatrutide is designed for once-weekly subcutaneous injection. The same day of the week is recommended for consistency. Many users choose a day when mild nausea (a common early side effect) would not interfere with important plans.

Can I use Retatrutide if I don't have diabetes or obesity?

Most published research on Retatrutide has focused on adults with obesity, overweight with comorbidities, or type 2 diabetes. The safety and efficacy profile in healthy adults outside those populations has not been well-studied. This is a question for a qualified healthcare professional.

How long does a vial last?

It depends on the vial strength and your dose. A 10 mg vial used at 1 mg per week lasts about 10 weeks. A 40 mg vial at 4 mg per week lasts about 10 weeks. Larger vials offer better value per milligram for users planning longer-term use. Once reconstituted, the solution remains stable for around 28 days when properly refrigerated.

Can I stop suddenly?

Stopping any GLP-1-related medication abruptly may lead to a rebound in appetite and gradual regain of any weight lost, particularly without lifestyle support. Most protocols recommend gradually tapering off rather than stopping abruptly. Discuss any decision to start or stop with a qualified healthcare professional.

Why are there so many different vial sizes?

Different users have different dosing needs and budgets. A 10 mg vial gives flexibility for someone starting out or running a shorter cycle. A 40 mg vial offers significantly better cost-per-milligram for someone running an extended cycle. Multiple sizes make Retatrutide accessible to a wider range of buyer needs.

How should I store it?

Unopened lyophilized vials should be kept refrigerated at 2 to 8 degrees Celsius and protected from light. Once reconstituted, store the same way and use within 28 days. Our reconstitution and storage guide has a dedicated section on handling peptides in Costa Rica’s tropical climate.

Still have questions?

We respond to every customer message personally — usually within hours. If you’re considering Retatrutide and want a real human to walk you through the details, just reach out.

Important disclaimer: The information in this guide is for educational and research purposes only. It is not medical advice and should not be used to diagnose, treat, cure, or prevent any disease. Products sold by Peptides Costa Rica are intended for laboratory and research purposes only. Always consult a qualified healthcare professional before beginning any new health regimen.