Retatrutide

Summary

• Retatrutide is a single peptide that activates three receptors — GIP, GLP-1 and glucagon. The third is what separates it from every approved drug in the class, and it works by raising energy expenditure rather than by further suppressing appetite.6

• In the Phase 2 obesity trial, mean weight reduction at 48 weeks was 24.2 percent on 12 mg against 2.1 percent on placebo. At that dose 83 percent of participants lost at least 15 percent of body weight.1

• Phase 3 has reported, but mostly by press release. Four TRIUMPH trials have released topline figures and one Phase 3 trial has been published in full, in a diabetes population.4 Topline numbers are not the same evidentiary object as a published paper.

• The liver-fat result is the most striking secondary finding. In a substudy of participants with fatty liver disease, liver fat fell 82.4 percent at 24 weeks on 12 mg, and 86 percent of that group reached a normal liver-fat level.3

• Gastrointestinal side effects are dose-related and common, and discontinuation because of adverse events ran between 6 and 16 percent across retatrutide doses against zero on placebo.1

• Heart rate rose in a dose-dependent way, peaking at week 24 before declining. A composite cardiac arrhythmia category was recorded in 6 percent of participants overall.1

• No randomized trial has compared retatrutide against semaglutide or tirzepatide. Every comparison in circulation is indirect.10

• Nobody has studied what happens when it stops. There is no retatrutide withdrawal or maintenance trial, which is the single largest gap in the record for a drug of this class.

• Every published trial was funded by Eli Lilly. There is no independently funded human evidence.

• It is approved nowhere and cannot be lawfully bought. Lilly's position is that it is legally available only to participants in its clinical trials.12

Basics and Mechanism

What Is Retatrutide?

An investigational once-weekly injectable peptide, developed by Eli Lilly under the code LY3437943.

Structurally it is a single peptide chain carrying non-natural amino acids — α-amino isobutyric acid and α-methyl-L-leucine — which resist the enzymes that would otherwise break it down. A fatty diacid is attached through a linker to the lysine at position 17. That chain binds albumin in the blood, which is what stretches the half-life to roughly six days and makes weekly dosing possible.78

Cryo-electron microscopy has resolved it bound to all three of its receptors. It adopts a single continuous helix: the first thirteen residues penetrate the core of the receptor, while residues 14 to 30 engage the receptor's extracellular domain.7

How Does It Work?

One molecule, three receptors, and they do different jobs.

GLP-1 and GIP receptor activity reduce calorie intake, which is the mechanism the approved drugs in this class already use. Glucagon receptor activity does something different: in obese animals, weight loss was augmented by adding glucagon-driven increases in energy expenditure on top of the intake reduction produced by the other two.6

It is not equally potent at all three. Measured against each receptor's own natural ligand, retatrutide is about 0.3 times as potent as glucagon at the glucagon receptor, 0.4 times as potent as GLP-1 at the GLP-1 receptor, and 8.9 times as potent as GIP at the GIP receptor.6

That imbalance is deliberate. The compound is heavily weighted toward GIP activity, with balanced and comparatively modest activity at the other two.

In obese mice, retatrutide produced greater weight loss than tirzepatide, and the difference was attributed to that added energy expenditure through glucagon receptor activation.6 That is the origin of the whole design case for a third receptor — and it is a mouse result, which is where the case still rests, because the equivalent human comparison has never been run.

Why Agonize the Glucagon Receptor in a Diabetes Drug?

This is the question the mechanism invites, because glucagon is the hormone that raises blood glucose. Deliberately activating its receptor in a drug intended to lower blood glucose looks like a contradiction.

The resolution is partly conceptual and partly empirical. Conceptually, counter-regulation of hypoglycemia is only one of glucagon's roles; it also contributes to meal-size reduction and to energy expenditure.1 Empirically, the glucose-lowering driven by the GLP-1 and GIP arms appears to more than offset it: no cases of clinically significant hypoglycemia were reported at any dose in the Phase 2 obesity trial,1 and no severe hypoglycemia was reported in either the Phase 2 or the Phase 3 diabetes trial.24

The trials answered the objection rather than the theory doing it.

How Does It Compare to Semaglutide and Tirzepatide by Design?

Adding receptors is a design choice, not a guarantee of a better result. It is a reasonable prior, and the trial figures so far are consistent with it, but the only way to establish it is a head-to-head trial and none has been run.

Dosing as Studied

What Doses Were Tested?

Phase 2 tested 1, 4, 8 and 12 mg once weekly.1 Phase 3 narrowed to 4, 9 and 12 mg.4

Every schedule started low and escalated. In TRIUMPH-1, participants began at 2 mg weekly and stepped up every four weeks to their assigned dose.12

Why Does the Escalation Schedule Matter?

Because the Phase 2 trial tested it as a variable, and it changed the side-effect profile.

Two of the dose groups were run twice, once starting at 2 mg and once starting at 4 mg. Gastrointestinal adverse events were dose-related and were partially mitigated by the lower starting dose.1

The practical reading is that how fast someone arrives at a dose affects how well it is tolerated, independently of the dose itself. That is a finding about the schedule, not about the drug.

What the Trials Found

Phase 2, Obesity

338 adults with obesity and without diabetes, randomized against placebo for 48 weeks.1

The proportions reaching each threshold at 48 weeks are the more useful figures, because a mean conceals how many people it applies to. At 12 mg, 100 percent of participants lost at least 5 percent of body weight, 93 percent lost at least 10 percent, and 83 percent lost at least 15 percent. On placebo the same figures were 27, 9 and 2 percent.1

Weight was still falling at 48 weeks in the higher-dose groups, which is why the Phase 3 trials ran to 80 weeks.

Phase 2, Type 2 Diabetes

281 adults with type 2 diabetes, 36 weeks, with dulaglutide 1.5 mg as an active comparator rather than placebo alone — a stronger design than a placebo-only trial.2

HbA1c fell 2.02 percentage points at 12 mg at 24 weeks, against 1.41 on dulaglutide and 0.01 on placebo. Weight fell 16.94 percent at 12 mg at 36 weeks, against 2.02 percent on dulaglutide and 3.00 percent on placebo.2

The weight comparison is the striking one. Against an active GLP-1 comparator, the gap was roughly eightfold.

Phase 3: What Is Published, and What Is Not

This distinction matters more for retatrutide than for most compounds, and it is routinely collapsed in coverage.

One Phase 3 trial has been published in a peer-reviewed journal. TRANSCEND-T2D-1 randomized 537 adults with type 2 diabetes inadequately controlled by diet and exercise, for 40 weeks. HbA1c fell 1.94 percentage points at 12 mg against 0.81 on placebo; weight fell 15.3 percent against 2.6 percent. 91 percent completed treatment on study drug, and discontinuation due to adverse events ran 2 to 5 percent against zero on placebo.4

The four TRIUMPH obesity trials have not been published. What exists for them is company topline announcements:

In TRIUMPH-1, Lilly also reported that 45.3 percent of participants on 12 mg lost at least 30 percent of body weight, and that participants with a baseline BMI of 35 or above who continued into an extension reached 30.3 percent at 104 weeks.12

A topline announcement is a company's summary of its own unpublished data. It is not fraudulent and it is not peer review. Until the papers appear, the methods, the estimands and the full safety tables cannot be inspected by anyone outside the sponsor.

The TRIUMPH Program

The program uses a basket design: four Phase 3 randomized trials across more than 5,800 participants, with nested sub-protocols for obstructive sleep apnea and knee osteoarthritis running inside the weight-management trials. Primary endpoints are percentage body-weight change, the apnea-hypopnea index, and the WOMAC pain subscale respectively.9

Beyond Weight

Liver Fat

The strongest secondary result in the published record.

A prespecified substudy enrolled 98 participants from the Phase 2 obesity trial who had metabolic dysfunction-associated steatotic liver disease and at least 10 percent liver fat. At 24 weeks, relative liver fat fell 42.9 percent on 1 mg, 57.0 percent on 4 mg, 81.4 percent on 8 mg and 82.4 percent on 12 mg, against a 0.3 percent increase on placebo.3

A normal liver-fat level, under 5 percent, was reached by 86 percent of the 12 mg group and none of the placebo group.3

Two limits belong with that. The trial excluded people with advanced fibrosis or cirrhosis, so it says nothing about the patients with the most to lose. And liver fat is a marker rather than an outcome — reducing it is not the same as preventing liver disease progression, which takes years to demonstrate.

Knee Osteoarthritis

TRIUMPH-4 tested retatrutide in people with obesity and knee osteoarthritis. Lilly reported WOMAC pain scores reduced by up to an average of 4.5 points, or 75.8 percent, and that more than one in eight treated patients were completely free of knee pain at the end of the trial.11

Unpublished, and pain endpoints are subjective and placebo-sensitive, which makes the full paper worth waiting for.

Body Composition and Muscle

The muscle-loss question has one published answer, and it comes from the diabetes cohort rather than the obesity one.

A DXA substudy of 189 participants found total fat mass reduced 26.1 percent on 8 mg and 23.2 percent on 12 mg at 36 weeks, against 4.5 percent on placebo and 2.6 percent on dulaglutide.5 The authors concluded that the proportion of lean mass loss to weight loss was similar to other obesity treatments, and framed this as reassurance that a greater proportion of lean mass is not lost despite the larger overall weight loss.5

Only 103 participants completed both scans, so this is a small dataset answering a large question. No equivalent substudy of the obesity population has been published.

Cardiovascular Outcomes

Not established, and not yet testable from what exists.

TRIUMPH-3 enrolled people with established cardiovascular disease, but it was a weight-management trial rather than a cardiovascular outcomes trial. The dedicated outcomes trial, TRIUMPH-Outcomes, is event-driven and still running.15

Weight loss and improved metabolic markers are not the same as demonstrated cardiovascular benefit. Semaglutide needed a dedicated outcomes trial to establish that, and retatrutide will too.

Safety

Gastrointestinal Effects

Common, dose-related, and the main reason people stopped.

In the Phase 2 obesity trial, nausea reached 60 percent in the highest-exposure group against 11 percent on placebo. Vomiting reached 26 percent against 1 percent, diarrhea 20 percent against 11 percent, and constipation 16 percent against 3 percent.1

Discontinuation due to adverse events ran between 6 and 16 percent across the retatrutide groups, against zero on placebo.1 In the published Phase 3 diabetes trial the equivalent figure was 2 to 5 percent, in a population on lower doses.4

Heart Rate and Cardiac Findings

Heart rate rose in a dose-dependent manner, peaked at week 24, and declined by weeks 36 and 48.1 The published paper describes the shape without stating a magnitude in the main text, so figures circulating for the number of beats per minute should be checked against the supplementary appendix rather than taken from summaries.

A composite category covering supraventricular arrhythmias and conduction disorders was recorded in 19 of 337 participants, 6 percent overall, reaching 14 percent in one dose group.1

This is the finding most plausibly attributable to the glucagon arm, since both GLP-1 and glucagon receptor activity have positive chronotropic effects. It is also the finding least well characterized by a trial that ran 48 weeks in 338 people.

Hypoglycemia

No cases of clinically significant hypoglycemia were reported at any dose in the obesity trial, and no severe hypoglycemia in either diabetes trial.124

Skin Sensitivity

An unusual signal, and a real one rather than a rumor.

Cutaneous hyperesthesia and skin sensitivity adverse events were reported in 7 percent of retatrutide recipients against 1 percent on placebo. None were severe or serious, none were associated with visible skin findings, none led to discontinuation, and the authors reported that they did not appear related to the magnitude or rate of weight loss.1

It has no established mechanism. It is listed here because it is specific to this compound's record and is easy to mistake for something unrelated.

Liver, Pancreas and Gallbladder

Transient ALT elevation above three times the upper limit of normal occurred in 1 percent of retatrutide recipients, and mean liver enzymes were unchanged or lower at 48 weeks.1 The liver-fat substudy reported no hepatotoxicity signal through 48 weeks.3

One serious adverse event of acute pancreatitis occurred in the 12 mg group, and three biliary events were recorded across the trial.1 These are small numbers in a small trial, which means they neither establish nor exclude a class effect.

Note the contrast with the gray market, where a state health department has recorded acute liver injury in people using products labelled as retatrutide, with a contaminant suspected rather than the molecule. The trial record and the black-market record are describing different things.

Comparisons

Has It Been Compared Head to Head?

No. No randomized trial has compared retatrutide against semaglutide or tirzepatide.

Every claim that retatrutide beats either of them rests on comparing numbers from separate trials with different populations, durations, comparators and endpoints. That is not a comparison in the sense that SURMOUNT-5 was a comparison of tirzepatide and semaglutide.

What the Indirect Comparisons Say

A 2025 Bayesian network meta-analysis pooled 19 randomized trials and 29,506 adults across liraglutide, semaglutide, survodutide, tirzepatide, retatrutide and placebo.10

It found retatrutide and the dual agonists achieving equivalent mean weight loss of 11.0 kg, ahead of the GLP-1 receptor agonists at 9.0 kg, with retatrutide having the highest odds of reaching at least 15 percent weight loss. It also reported that retatrutide had the highest adverse event risk of any agent compared.10

Both halves of that finding travel together. A network meta-analysis is still an indirect comparison, and its efficacy ranking and its safety ranking carry the same caveats.

Stopping

What Happens After Treatment Ends?

Nobody has published an answer for this compound.

There is no retatrutide randomized-withdrawal or weight-maintenance trial, of the kind run for semaglutide as STEP 4 and its extension and for tirzepatide as SURMOUNT-4. Post-treatment weight trajectory is not a reported endpoint in any published retatrutide trial.

What the class data show is that in both semaglutide and tirzepatide trials, stopping led to substantial regain and continuing did not. Whether retatrutide behaves the same way is a reasonable expectation, not a finding. Treating it as established is exactly the kind of read-across this site tries not to make silently.

Regulatory Status and Availability

Where Does It Stand?

Investigational everywhere and approved nowhere. Lilly stated in July 2026 that it plans to submit a Biologics License Application to the FDA in Q1 2027.13

The only lawful route to receiving it is enrollment in a trial.12

What About the Gray Market?

Retatrutide is sold widely online despite all of the above, and the regulatory position on that is unambiguous. The FDA states that retatrutide cannot be used in compounding under federal law, that it is not a component of any FDA-approved drug, and that it has not been found safe and effective for any condition.14

The enforcement record, the identity question and the documented harms have their own page: retatrutide sourcing and quality. What it costs and why there is no price is covered at cost and access.

How Good Is the Evidence

Who Funded It

Eli Lilly funded every published retatrutide trial identified for this page — the Phase 1b, both Phase 2 trials, the liver-fat and body-composition substudies, the published Phase 3, and the program design papers. Lilly employees appear as co-authors on all of them.

No independently funded human trial of retatrutide has been published. That is normal for a compound at this stage, and it is still a limitation worth stating: there is no equivalent here of the WHO-funded Cochrane review that exists for semaglutide.

What Is Certain and What Is Not

What Remains Unstudied

No withdrawal or maintenance trial. No head-to-head against another incretin. No completed cardiovascular outcomes trial. No peer-reviewed data beyond 48 weeks. No published data in adolescents, in pregnancy, or in people with advanced liver fibrosis. And no independent replication of anything, because every trial has the same sponsor.

What Would Change This Page

Three things, in order of how much they would move it.

Publication of the TRIUMPH trials in peer-reviewed journals would convert the largest figures on this page from company toplines into inspectable evidence. A regulatory decision following the stated Q1 2027 submission would change the availability position entirely.13 And the completion of TRIUMPH-Outcomes would establish or fail to establish cardiovascular benefit, which is the question that separated semaglutide from a weight-loss drug.15

Related ONPEPS Coverage

• Retatrutide sourcing and quality: the enforcement record and what a buyer can verify

• Retatrutide cost and access: no price, and no legal route

• Tirzepatide: the approved dual agonist

• Semaglutide: the approved GLP-1 comparator

References

1. Jastreboff AM, Kaplan LM, Frías JP, et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial. N Engl J Med. 2023;389(6):514-526. Paywalled. pubmed.ncbi.nlm.nih.gov/37366315

2. Rosenstock J, Frias J, Jastreboff AM, et al. Retatrutide, a GIP, GLP-1 and glucagon receptor agonist, for people with type 2 diabetes: a randomised, double-blind, placebo and active-controlled, parallel-group, phase 2 trial conducted in the USA. Lancet. 2023;402(10401):529-544. Paywalled. pubmed.ncbi.nlm.nih.gov/37385280

3. Sanyal AJ, Kaplan LM, Frias JP, et al. Triple hormone receptor agonist retatrutide for metabolic dysfunction-associated steatotic liver disease: a randomized phase 2a trial. Nat Med. 2024;30(7):2037-2048. Open access. pubmed.ncbi.nlm.nih.gov/38858523

4. Bajaj HS, Welch M, Shah P, et al. Efficacy and safety of retatrutide, a GIP, GLP-1, and glucagon receptor agonist, in people with type 2 diabetes and inadequate glycaemic control with diet and exercise (TRANSCEND-T2D-1): a double-blind, randomised, phase 3 trial. Lancet. 2026;407(10546):2402-2413. Paywalled. pubmed.ncbi.nlm.nih.gov/42250575

5. Coskun T, Wu Q, Schloot NC, et al. Effects of retatrutide on body composition in people with type 2 diabetes: a substudy of a phase 2, double-blind, parallel-group, placebo-controlled, randomised trial. Lancet Diabetes Endocrinol. 2025;13(8):674-684. Paywalled. pubmed.ncbi.nlm.nih.gov/40609566

6. Coskun T, Urva S, Roell WC, et al. LY3437943, a novel triple glucagon, GIP, and GLP-1 receptor agonist for glycemic control and weight loss: From discovery to clinical proof of concept. Cell Metab. 2022;34(9):1234-1247.e9. Paywalled. pubmed.ncbi.nlm.nih.gov/35985340

7. Li W, et al. Structural insights into the triple agonism at GLP-1R, GIPR and GCGR manifested by retatrutide. Cell Discov. 2024;10(1):77. Open access. pubmed.ncbi.nlm.nih.gov/39019866

8. Urva S, Coskun T, Neff LM, et al. LY3437943, a novel triple GIP, GLP-1, and glucagon receptor agonist in people with type 2 diabetes: a phase 1b, multicentre, double-blind, placebo-controlled, randomised, multiple-ascending dose trial. Lancet. 2022;400(10366):1869-1881. Paywalled. pubmed.ncbi.nlm.nih.gov/36354040

9. Giblin K, Kaplan LM, Somers VK, et al. Retatrutide for the treatment of obesity, obstructive sleep apnea and knee osteoarthritis: Rationale and design of the TRIUMPH registrational clinical trials. Diabetes Obes Metab. 2026;28(1):83-93. Design paper; reports no outcome data. Open access. pubmed.ncbi.nlm.nih.gov/41090431

10. Sinha B, Ghosal S. Efficacy and Safety of GLP-1 Receptor Agonists, Dual Agonists, and Retatrutide for Weight Loss in Adults With Overweight or Obesity: A Bayesian Network Meta-Analysis. Obesity (Silver Spring). 2025;33(11):2046-2054. Indirect comparison, not a head-to-head trial. Paywalled. pubmed.ncbi.nlm.nih.gov/40685589

11. Eli Lilly and Company. Lilly's triple agonist, retatrutide, delivered weight loss of up to an average of 71.2 lbs along with substantial relief from osteoarthritis pain in first successful Phase 3 trial. Press release, 11 December 2025. Company topline; not peer reviewed. biospace.com

12. Eli Lilly and Company. Lilly's triple agonist, retatrutide, delivered powerful weight loss in pivotal Phase 3 obesity trial. Press release, 21 May 2026. Company topline; presented at ADA 2026 but not peer reviewed. biospace.com

13. Eli Lilly and Company. Lilly's triple agonist, retatrutide, successful in two additional Phase 3 obesity trials, delivering significant improvements in weight and A1C. Press release, 23 July 2026. Company topline; not peer reviewed. Contains the stated Q1 2027 BLA submission plan. biospace.com

14. US Food and Drug Administration. FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight Loss. Drug Alerts and Statements. Content current as of 1 September 2026. fda.gov

15. ClinicalTrials.gov. NCT06383390, TRIUMPH-Outcomes: a study of retatrutide in participants with obesity and cardiovascular disease. Event-driven cardiovascular and renal outcomes trial; ongoing. Record checked 5 September 2026. clinicaltrials.gov/study/NCT06383390

Not Medical Advice

Retatrutide is not approved for human use in any jurisdiction and cannot lawfully be sold to consumers. A large trial result is not a recommendation, and an effect observed in a trial population may not transfer to any individual. Consult a qualified healthcare provider before acting on anything here.

Literature and regulatory sources verified 5 September 2026. Phase 3 figures attributed to press releases were unpublished at that date and should be re-checked against the peer-reviewed papers once they appear. Evidence counts shown above were last retrieved 2026-07-23 from PubMed and ClinicalTrials.gov.