Retatrutide: The Triple Agonist Explained

What is retatrutide?

Retatrutide is an experimental injectable peptide that activates three receptors at the same time: the GIP receptor, the GLP-1 receptor and the glucagon receptor. It is given once a week, its phase 3 trials have reported larger mean weight reductions than semaglutide or tirzepatide reported in theirs, and it is not approved anywhere. The molecule, developed under the code LY3437943, is a 39-amino-acid analog with two aminoisobutyric acid residues, an alpha-methyl-leucine, a C-terminal amide and a C20 fatty diacid attached through a linker to lysine 17, which binds albumin and stretches the half-life to roughly a week. Its molar mass is 4,731 g/mol and its PubChem record is titled Triple G.

The company running the trials is referred to here as the sponsor, following the convention of the retatrutide compound page, where the chemistry card and every trial row below are kept with their sources. Two of those sources are sponsor investor releases, because the TRIUMPH-4 results and the full escalation ladder have so far been published only that way.

Receptors

GIP, GLP-1, glucagon

One more than tirzepatide, two more than semaglutide.

Structure

39 residues, 4,731 g/mol

C20 fatty diacid on lysine 17; half-life about a week.

Best reported result

28.7% at 68 weeks

TRIUMPH-4, 12 mg, topline; 25.0% at 80 weeks in TRIUMPH-1 by intention to treat.

Status

Investigational

No label, no approved dose, no missed-dose rule.

What do the three receptors do differently?

The incretin receptors are the familiar part. GLP-1 receptor activation, the mechanism of semaglutide, slows gastric emptying, increases glucose-dependent insulin release and reduces appetite. GIP is the second incretin hormone, and adding its receptor is what distinguishes tirzepatide; the two are compared in the semaglutide versus tirzepatide guide. The glucagon receptor is the addition that makes retatrutide a triple agonist. Glucagon raises blood glucose, which is why pairing it with two incretins is the point: the incretin activity is meant to offset that effect while glucagon receptor signaling raises energy expenditure, a rationale drawn from animal pharmacology rather than from a demonstrated human mechanism.

One observation from the trials is often attributed to the glucagon component. The phase 2 obesity trial recorded a dose-dependent rise in heart rate that peaked at 24 weeks and then declined. Whether the glucagon activity also explains why the TRIUMPH-1 weight curve was still falling at week 104 is not something the trials were designed to separate.

What did the phase 2 trials report?

The obesity trial, published in 2023, randomized 338 adults with obesity, or overweight plus a weight-related condition, without diabetes, to 1, 4, 8 or 12 mg weekly or placebo for 48 weeks. Mean weight fell 8.7%, 17.1%, 22.8% and 24.2% in the four dose groups against 2.1% on placebo. Gastrointestinal events were dose related and were partly mitigated by starting at 2 mg rather than 4 mg, which is the origin of the 2 mg start used in every phase 3 trial. The companion type 2 diabetes trial gave 281 adults 0.5, 4, 8 or 12 mg for 36 weeks against dulaglutide 1.5 mg and placebo. HbA1c fell by about 2 percentage points and body weight by up to 16.9% on 12 mg, mild to moderate gastrointestinal events occurred in 35% of retatrutide participants, and no severe hypoglycemia was reported.

What did the phase 3 TRIUMPH trials find?

The first phase 3 readout was TRIUMPH-4, in 445 adults with obesity or overweight and knee osteoarthritis, reported by the sponsor in a 2025 investor release: topline weight loss of 26.4% at 9 mg and 28.7% at 12 mg over 68 weeks, with knee pain scores on the WOMAC scale falling by up to 4.5 points. The pivotal trial, TRIUMPH-1, randomized 2,339 adults without diabetes to 4, 9 or 12 mg or placebo for 80 weeks, with a 24-week extension for participants with a BMI of 35 or more. Mean weight change at 80 weeks was 17.6%, 23.7% and 25.0% against 3.9% on placebo by intention to treat, and 19.0%, 25.9% and 28.3% among those who stayed on treatment. The registry record lists the trial as started in July 2023 and completed in April 2026. Weight was still falling at week 104 in the extension. Participants received sponsor-supplied drug under medical supervision, and up to two permanent dose reductions were allowed, most often for weight loss judged excessive or a BMI of 22 or below.

How were doses escalated in the trials?

The TRIUMPH program used a fixed ladder with four weeks per step. Every arm started at 2 mg. The 4 mg arm stepped up once; the 9 mg arm went through 4 and 6 mg; the 12 mg arm went through 4, 6 and 9 mg, so reaching the top dose took 16 weeks. The sponsor's 2026 release describes the ladder; 6 mg was an escalation step only and was never studied as a maintenance dose. The phase 2 trial used a different ladder, 2, 4, 8 and 12 mg, which is why charts online disagree with each other. Community guides stretch the climb further, to 0.5, 1, 2, 4, 6 and 8 mg over 20 weeks or more, on the reasoning that slower is gentler on the gut. That start has never been compared with the 2 mg trial start; the phase 2 trial only showed that 2 mg was better tolerated than 4 mg.

1. 1
   Weeks 1-4: 2 mg

   Every TRIUMPH participant started here. Gastrointestinal effects cluster in this phase.

2. 2
   Weeks 5-8: 4 mg

   The lowest maintenance dose in TRIUMPH-1, 17.6% at 80 weeks, with the fewest discontinuations among the active arms.

3. 3
   Weeks 9-12: 6 mg

   A step toward 9 and 12 mg only; no trial has held participants here.

4. 4
   Weeks 13-16: 9 mg

   A maintenance dose in TRIUMPH-1 and TRIUMPH-4, and the last step before 12 mg.

5. 5
   Week 17 onward: 12 mg

   The highest dose any published trial has used. Nothing above it has been tested.

How many participants stopped, and why?

Nausea, diarrhea, constipation and vomiting were the most common adverse events in TRIUMPH-1, as in the rest of the class. Discontinuation for adverse events rose with dose, from 4.1% at 4 mg to 11.1% at 12 mg, against 4.6% on placebo; the sponsor's topline release put the 9 mg arm at 6.9%. The phase 2 obesity trial recorded the same dose relationship and the heart-rate rise described above. The diabetes trial reported no severe hypoglycemia. What none of the trials reports is what happens after stopping: no withdrawal study has been published, no taper has been tested, and the community guides that describe tapering over 4 to 6 weeks cite weight regain after other drugs in the class, not data on this one.

Is retatrutide approved anywhere?

No. Retatrutide has no marketing authorization in any country, no label, no approved dose and no published missed-dose rule. The TRIUMPH ladder is a trial protocol, not a dosing recommendation, and 12 mg once weekly is the highest dose any published study has used. Product sold online as retatrutide is unregulated and untested; because there is no label, there is also no reference concentration, so any syringe-unit figure for such a vial depends entirely on what the seller claims is in it. The calculator can convert a stated concentration into a volume, and nothing more.

What this guide does not establish

The trials establish that retatrutide produced large mean weight reductions against placebo under trial conditions. They do not establish how it compares with approved drugs head to head, what happens after it is stopped, how it behaves over years, or anything about vials sold under its name. The approved alternatives and their label schedules are compared in the semaglutide versus tirzepatide guide, the class context is in the metabolic research guide, and the full trial and community tables, each row linked to its source, are on the retatrutide compound page.