What Is Retatrutide?

Summary: Retatrutide is Eli Lilly's investigational once-weekly injection that activates three receptors at once (GIP, GLP-1, and glucagon), produced about 24% weight loss in Phase 2, and is still working through Phase 3 trials in 2026.

This content is for informational purposes only and is not medical advice. Always consult a qualified healthcare provider before starting, changing, or stopping any medication.

Retatrutide is Eli Lilly's investigational triple hormone receptor agonist, a single once-weekly injection that activates the GIP, GLP-1, and glucagon receptors at the same time. It is not approved by the FDA. It is in Phase 3 trials. In the Phase 2 obesity trial published in the New England Journal of Medicine in 2023, the highest dose produced an average weight loss of around 24% over 48 weeks, more than any FDA-approved weight loss drug to date ^[1].

That is the headline. The rest of this page is the detail behind it: what the three receptors actually do, what the clinical program looks like in 2026, how retatrutide compares to tirzepatide and semaglutide, what is known and not known about safety, and the legal status of the "research peptide" market that has formed around the drug.

The short version

Other names you will see in literature and on supplier sites: LY3437943, "triple G," "Triple G agonist," sometimes informally and incorrectly "GLP-3." Lilly itself prefers "triple hormone receptor agonist." There is no brand name yet because there is no approved product yet ^[3].

What retatrutide is (and is not)

Retatrutide is a synthetic 39-amino-acid peptide engineered to bind and activate three different metabolic hormone receptors. Its development code at Eli Lilly is LY3437943. A fatty acid chain is attached to slow clearance so it can be dosed once a week, the same trick Lilly uses on tirzepatide and that Novo Nordisk uses on semaglutide.

It is not a vitamin. It is not a supplement. It is not a generic version of any other GLP-1 drug. It is an experimental molecule that has cleared Phase 1 safety testing, demonstrated meaningful results in Phase 2, and is now being tested at full scale in Phase 3 trials across obesity, type 2 diabetes, knee osteoarthritis pain, obstructive sleep apnea, chronic low back pain, cardiovascular and renal outcomes, and metabolic dysfunction-associated steatotic liver disease ^[3].

Until those trials wrap and the FDA reviews the data, retatrutide is legally available only to clinical trial participants in the US. Lilly has been explicit about that point and has warned the public that anything else sold as retatrutide is not from them and is not authorized for human use ^[3].

The three receptors and why glucagon matters

The whole point of retatrutide is that it does not pick one pathway. It activates three at once.

GLP-1 receptor: the appetite axis

Glucagon-like peptide-1 is the receptor that started this drug class. Activating GLP-1 receptors in the brain reduces appetite. It also slows gastric emptying, so meals feel filling for longer. And it stimulates insulin release in a glucose-dependent way, so blood sugar gets better without the hypoglycemia risk of older diabetes drugs. Semaglutide (Ozempic, Wegovy) is the pure-GLP-1 example of how much you can do with this pathway alone.

GIP receptor: the metabolic shaping piece

Glucose-dependent insulinotropic polypeptide does several things. It improves insulin sensitivity, it shifts fat storage away from visceral depots and toward subcutaneous depots, and at least in animal data it appears to dampen some of the GI side effects you get from hammering GLP-1 in isolation. Tirzepatide (Mounjaro, Zepbound) is the dual GLP-1 plus GIP agonist that proved adding GIP produced more weight loss than GLP-1 alone.

Glucagon receptor: the energy expenditure piece

This is the one that makes retatrutide different. Glucagon is usually thought of as the hormone that raises blood sugar when you have not eaten, the counter-regulatory partner of insulin. That is part of what it does. The other part, the part Lilly is exploiting here, is that glucagon receptor activation increases resting energy expenditure. It promotes lipolysis, the breakdown of stored fat. It directly mobilizes liver fat. People taking retatrutide do not just eat less; their body burns more.

The catch is that activating glucagon also tends to raise blood sugar, which would be bad in a diabetes drug. The engineering trick is balancing the three receptor potencies so that the GLP-1 and GIP arms more than compensate. The Phase 2 diabetes data, published alongside the obesity data, showed retatrutide actually improved A1c despite the glucagon component ^[2]. That balance is the molecule's central design achievement.

A clean way to think about the contributions: GLP-1 reduces how much you eat. GIP optimizes how your body handles what you eat. Glucagon increases how much you burn. Three independent levers, pulled simultaneously.

Phase 2 results (the NEJM 2023 trial)

The Phase 2 trial that put retatrutide on the map was published by Jastreboff and colleagues in the New England Journal of Medicine on June 26, 2023 ^[1]. It enrolled 338 adults with obesity or with overweight plus at least one weight-related condition, randomized them to placebo or one of several retatrutide doses (1 mg, 4 mg, 8 mg, or 12 mg weekly), and ran for 48 weeks.

The headline weight loss numbers at 48 weeks:

At the 12 mg dose, every participant in the analysis lost at least 5% of body weight, and the curve had not plateaued at week 48. Translation: people were still losing weight when the trial ended. That is unusual. It suggests the asymptote of retatrutide weight loss is higher than 24% if you keep dosing for longer.

The trial also tracked metabolic markers. Systolic blood pressure fell. Triglycerides dropped roughly 30%. HDL went up. Liver fat fell by about 50%, larger than the overall body weight loss percentage. Visceral fat dropped about 42%. A1c improved even in participants who did not have diabetes. The metabolic effects were not just proportional to weight; they were disproportionately favorable, which is what you would predict from the glucagon receptor component preferentially mobilizing the metabolically dangerous fat depots.

The parallel Phase 2 trial in type 2 diabetes, published in The Lancet the same summer, showed retatrutide produced A1c reductions up to about 2 percentage points and weight loss up to about 17% at 36 weeks ^[2]. That established the molecule could deliver glycemic control along with weight loss, which matters for the eventual FDA filings.

Phase 3 TRIUMPH program status

Lilly's Phase 3 program for retatrutide runs under two umbrellas. TRIUMPH covers obesity and obesity-related conditions. TRANSCEND covers type 2 diabetes ^[3].

The TRIUMPH trials include:

• TRIUMPH-1 in adults with obesity or overweight and at least one weight-related comorbidity.
• TRIUMPH-2 in adults with obesity and type 2 diabetes.
• TRIUMPH-3 evaluating major cardiovascular and renal outcomes.
• TRIUMPH-4 in adults with obesity and knee osteoarthritis. Topline results released in December 2025 showed average weight loss of up to about 71.2 lbs (around 28.7% at the 12 mg dose over 68 weeks) along with substantial reduction in osteoarthritis pain ^[5].
• TRIUMPH-5 in obstructive sleep apnea, plus trials in chronic low back pain and MASH.

TRANSCEND-T2D-1 reported positive topline Phase 3 results in March 2026, with significant A1c and weight reductions ^[3].

That is roughly the status as of mid-2026. TRIUMPH-4 and TRANSCEND-T2D-1 are the first two Phase 3 wins on record. Remaining trials in obesity, cardiovascular outcomes, and MASH are still reading out.

When will retatrutide be available

This depends on which trial completes and on what indication Lilly files for first. The standard playbook is to file with the strongest, fastest-completing trial and add indications later. Based on the TRIUMPH-4 result in late 2025 and the diabetes Phase 3 result in early 2026, an initial FDA submission is plausibly within reach in 2026, with an FDA decision somewhere in the 2026 to 2027 window for the first indication. A broader obesity label or the cardiovascular outcome data would land later.

These timelines move. Trial readouts slip. The FDA review clock starts when Lilly actually submits a Biologics License Application or NDA, not when topline data hits a press release. The most accurate way to track real status is Lilly's investor relations site and ClinicalTrials.gov.

How retatrutide compares to tirzepatide and semaglutide

There is no head-to-head trial of retatrutide versus tirzepatide or versus semaglutide. Every comparison you see in print, including the one below, is cross-trial. Cross-trial comparisons are a useful sketch and a misleading proof. The patient populations, baseline weights, trial durations, and titration speeds all differ between studies, and that affects the raw numbers more than people sometimes appreciate.

With that warning attached:

The trend across the class is consistent: adding receptors has produced more weight loss at each step. GLP-1 alone gets you about 15%. GLP-1 plus GIP gets you about 21%. GLP-1 plus GIP plus glucagon gets you into the mid-20s and possibly higher. That progression makes mechanistic sense, but until a head-to-head trial runs, the absolute size of retatrutide's advantage is an estimate, not a proven number.

What retatrutide is not: the "research peptide" gray zone

Search "retatrutide for sale" and you will find dozens of websites selling vials labeled R-10 (10 mg) or R-30 (30 mg), often with claims of HPLC testing, certificates of analysis, and 99% purity. These vendors are operating in a gray zone, and the gray zone matters because it carries real risk.

Here is what is actually true:

• Retatrutide is not FDA-approved. There is no legal prescription pathway in the US.
• Compounding pharmacies cannot legally compound retatrutide. Under FDA rules, a substance has to either be FDA-approved or appear on the official bulk substances list for 503A or 503B compounding. Retatrutide is on neither. Lilly has been explicit that no compounded retatrutide is authorized ^[3].
• "Research peptide" sites typically sell vials labeled "for research use only, not for human consumption." That disclaimer is a regulatory fig leaf, not a legal protection. It does not change the chemistry in the vial, and it does not protect the buyer who uses it on themselves.
• There is no guarantee of identity, purity, or sterility for any vial labeled retatrutide that did not come from a Lilly clinical trial. Third-party COAs help but do not eliminate the risk of contamination, underdosing, or wrong-compound substitution.

This site does not provide instructions for sourcing or using research-grade retatrutide. The mechanism, the trial data, and the development timeline are all legitimate health topics. The actual buying and using of unapproved injectables is a separate decision with separate consequences, and one most readers should make in conversation with a clinician rather than a forum.

Safety signals so far

The safety profile reported across Phase 1, Phase 2, and the Phase 3 readouts available so far looks broadly similar to the rest of the GLP-1 class, with a few class-specific notes.

The most common adverse events at the 12 mg dose in the Phase 2 obesity trial were:

• Nausea, around 45% during titration.
• Diarrhea, around 25%.
• Vomiting, around 20%.
• Constipation, around 15%.

These were mostly mild to moderate and concentrated in the dose escalation phase. Discontinuation due to side effects ran around 16% at 12 mg, comparable to tirzepatide. A slower titration reduces dropouts in real-world use.

Notable items that are specific to retatrutide because of the glucagon component:

• A small increase in heart rate of about 2 to 3 beats per minute, similar in size to what other incretin drugs produce.
• Cardiac arrhythmia was reported in roughly 11% of the 12 mg group in Phase 2, more than placebo. This is being watched.
• One isolated case of acute pancreatitis was reported in the highest dose group in Phase 2.
• Lipase elevations were observed, similar to other GLP-1 drugs.

No formal FDA boxed warnings exist because there is no FDA label. The thyroid C-cell tumor warning that applies to GLP-1 drugs in rodent models will likely carry over to retatrutide's eventual label, given the class. Long-term safety beyond the 48 to 68 weeks already studied is genuinely unknown.

What retatrutide does (and the conditions it might help)

Beyond raw weight loss, retatrutide is being investigated for several conditions where the underlying problem is closely linked to weight or to metabolic dysfunction.

• Obesity with knee osteoarthritis. TRIUMPH-4 showed substantial pain reduction alongside weight loss ^[5].
• Type 2 diabetes. TRANSCEND-T2D-1 showed strong A1c and weight reductions ^[3].
• Obstructive sleep apnea. TRIUMPH-5 is testing whether the weight loss translates into AHI reductions large enough to matter clinically.
• MASH/MASLD (formerly NASH/NAFLD). The 50% liver fat reduction in Phase 2 makes this a high-probability indication if the Phase 3 liver biopsy data hold up.
• Cardiovascular and renal outcomes. TRIUMPH-3 is testing whether retatrutide reduces major adverse cardiovascular events similar to semaglutide's SELECT result.
• Chronic low back pain. Being studied as a function of weight-related load and inflammation.

If even half of these indications pan out, retatrutide ends up labeled across a broader set of conditions than any single GLP-1 drug to date.

Pros and cons of retatrutide at a glance

Development history and timeline

Retatrutide first entered humans in a 2019 Phase 1 study in Singapore, with 47 participants receiving weekly doses from 0.1 mg up to 6 mg. The early data showed dose-dependent effects on appetite, food intake, and body weight, with a tolerable safety profile, and Lilly advanced the molecule into Phase 2.

The Phase 2 obesity trial (NCT04881760) ran from 2021 to 2022, and the results were presented at the 2023 American Diabetes Association meeting and published in NEJM in June 2023 ^[1]. The parallel Phase 2 type 2 diabetes trial appeared in The Lancet around the same time ^[2].

The Phase 3 TRIUMPH program enrolled starting in 2023. TRIUMPH-4 read out in December 2025 ^[5], TRANSCEND-T2D-1 in March 2026 ^[3], and additional readouts are expected through the rest of 2026 and into 2027. Lilly's filing strategy and the FDA review cycle determine when an actual prescription becomes available.

Where to find updates

The fastest sources for accurate retatrutide news, ranked by signal quality:

1. Lilly's investor relations page and press releases. Phase 3 topline data and FDA filings are announced here first.
2. ClinicalTrials.gov, searching for "retatrutide" or "LY3437943." This is where trial completion dates and protocol details are updated in real time.
3. New England Journal of Medicine, The Lancet, JAMA, and Diabetes Care for peer-reviewed trial publications.
4. The FDA's Drugs@FDA database, which will list retatrutide once an application is accepted.

Marketing pages, telehealth blogs, and peptide vendor sites tend to lag behind these primary sources and frequently mix in unsourced numbers.

Common questions about retatrutide

What is retatrutide in one sentence?

Retatrutide is Eli Lilly's investigational once-weekly injection that activates the GIP, GLP-1, and glucagon receptors at the same time to drive weight loss and improve metabolic health.

What 3 receptors does retatrutide affect?

GLP-1 (glucagon-like peptide-1), GIP (glucose-dependent insulinotropic polypeptide), and glucagon. All three are activated by a single molecule.

What does glucagon do in retatrutide?

The glucagon receptor component increases resting energy expenditure, promotes the breakdown of stored fat, and directly mobilizes liver fat. It is the addition that distinguishes retatrutide from tirzepatide.

What is retatrutide also called?

The development code is LY3437943. It is informally called "triple G" or "triple hormone receptor agonist." The label "GLP-3" sometimes appears in media but is not scientifically accurate.

Is retatrutide FDA approved?

No. As of 2026 retatrutide is investigational and in Phase 3 trials. FDA approval is possible in the 2026 to 2027 window for the first indication, depending on when Lilly submits and how the review proceeds.

How much weight can you lose on retatrutide?

In the Phase 2 obesity trial at 12 mg weekly, mean weight loss was 24.2% at 48 weeks. In TRIUMPH-4 at 12 mg weekly, mean weight loss reached 28.7% at 68 weeks. Individual results vary.

Is retatrutide better than tirzepatide?

Cross-trial comparisons suggest 3 to 8 percentage points more weight loss with retatrutide at top doses, plus larger reductions in visceral fat and liver fat. No head-to-head trial exists yet, so the gap is estimated, not proven.

Can I buy retatrutide as a research peptide?

Vendors sell vials labeled retatrutide, often marked R-10 or R-30, but these products are not FDA-approved, not from Lilly, and not authorized for human use. Lilly has warned that no compounded or research-supplied retatrutide is legitimate.

When will retatrutide be available by prescription?

There is no confirmed launch date. The earliest plausible US approval is 2026 to 2027 for the first indication, with broader labels following. Track Lilly press releases for definitive updates.

Is retatrutide safe?

The 48-week and 68-week safety data so far look similar to other GLP-1 drugs, with GI side effects most common. A small heart rate increase and a cardiac arrhythmia signal at high doses are being watched. Long-term safety beyond 1.5 years is not yet established.

Does retatrutide work for fatty liver?

Phase 2 showed roughly 50% reduction in liver fat, larger than the overall weight loss percentage. Dedicated MASH/MASLD trials are ongoing. The glucagon receptor component appears to drive this effect specifically.

Is retatrutide a GLP-1?

It acts on the GLP-1 receptor but is not a pure GLP-1 drug. It is more accurately called a triple agonist, since it activates GLP-1, GIP, and glucagon receptors simultaneously.

What this article does not cover

This is the pillar overview. Dose-by-dose protocols, specific side effect management, reconstitution math, and direct comparisons to specific drugs each have their own dedicated pages on this site. If you are looking for the receptor mechanism, the trial numbers, and the regulatory state of play, this page covers it. If you are looking for "how much R-10 do I draw for a 4 mg dose," that is a different article, and not one this site writes, because retatrutide does not have an FDA-sanctioned dose to write about.