Retatrutide: The Complete 2026 Guide

How retatrutide works

Retatrutide is described as a triple agonist because it activates three separate hormone receptors at once: GIP, GLP-1 and glucagon. GLP-1 (glucagon-like peptide-1) is the receptor targeted by already-approved drugs like semaglutide, and it is the pathway most associated with appetite suppression and slower stomach emptying. GIP (glucose-dependent insulinotropic polypeptide) is the second receptor, also targeted by tirzepatide’s dual-agonist mechanism, and it works alongside GLP-1 to affect insulin response and appetite. Glucagon is retatrutide’s third and distinguishing target: it is the structural reason retatrutide is classed as a triple agonist rather than a dual agonist, and researchers point to it as the leading mechanistic explanation for the drug’s larger trial result.

Retatrutide, Eli Lilly’s compound LY3437943, is dosed once weekly by subcutaneous injection, the same route and frequency used by approved GLP-1 medications such as semaglutide and tirzepatide. Semaglutide, by comparison, activates only the GLP-1 receptor. Tirzepatide activates GLP-1 and GIP, the same two pathways retatrutide shares, but stops there. Retatrutide’s addition of a third receptor, glucagon, is the specific structural difference between “dual agonist” and “triple agonist,” and it is the reason retatrutide is discussed as a possible next step beyond the GLP-1 medications already on the market.

In the phase 2 trial that produced its published results, researchers led by Ania M. Jastreboff of Yale, with co-authors including Lee M. Kaplan of Harvard and Juan P. Frías, randomized 338 adults (51.8% men) to one of six active-dose arms or placebo, registered as ClinicalTrials.gov NCT04881760. That trial design, double-blind and placebo-controlled, is what lets the weight-loss figures below be attributed to the drug itself rather than to who happened to enroll in which arm: neither participants nor the researchers assessing their weight change knew who received retatrutide and who received a matching placebo injection, which keeps expectation, on either side, from quietly inflating the reported result.

What the trials found

Retatrutide produced dose-dependent weight loss in its phase 2 trial: the higher the target dose, the larger the mean reduction in body weight at 48 weeks. The table below reports the least-squares mean percentage change in body weight for every dose the trial measured, placebo included, all from Jastreboff AM, Kaplan LM, Frías JP, et al., New England Journal of Medicine, 2023;389(6):514-526 (DOI: 10.1056/NEJMoa2301972, PMID 37366315).

Reaching a weight-loss threshold, not just the average, is what most people researching retatrutide actually care about. The table below reports the share of participants at each dose who reached three thresholds, 5%, 10% and 15% or more body weight lost, at 48 weeks.

At 48 weeks, 100% of participants on the 8 mg or 12 mg dose had lost at least 5% of body weight, compared with 27% on placebo. The two top doses were almost identical at that 5% bar and separated only at the hardest threshold: at 15% or more weight loss, 75% of the 8 mg group and 83% of the 12 mg group met it, against just 2% on placebo, so the gap between the two widened from zero at the 5% bar to eight points at the 15% bar. Even the 4 mg group, the lowest dose in this responder analysis, separated clearly from placebo: 92% of it lost at least 5% of body weight, more than three times the placebo rate at that same bar. Read alongside the mean weight-loss table above, the two data sets tell the same story from different angles: retatrutide’s effect was not a handful of high responders pulling up an average, it was consistent across most of the trial population at every dose the analysis covered.

Side effects

Retatrutide’s most common side effects in the phase 2 trial were gastrointestinal: nausea, diarrhea, vomiting and constipation, and they were dose-related. Most were reported as mild to moderate, and the trial’s own safety findings show they were partially mitigated by starting participants at a 2 mg dose rather than 4 mg. Heart rate was the other dose-dependent measure the trial tracked closely: increases peaked at 24 weeks and then declined, rather than continuing to climb for the rest of the study. Because the trial was blinded, participants did not know their dose while reporting symptoms, which is what makes the dose-relatedness finding itself, more side effects at higher doses, trustworthy rather than a reporting artifact. For the full breakdown by dose, and how the trial’s safety data should be read, see our retatrutide side effects guide.

Dosing

In the phase 2 trial, retatrutide was titrated upward every 4 weeks toward one of four target doses: 1, 4, 8 or 12 mg once weekly. Six active-dose arms were tested in total. Three of those six arms started at a 2 mg dose before stepping up, and four of the six began below their eventual target dose altogether, rather than starting directly at the target. That slower on-ramp is the trial’s own explanation for why the higher-dose arms were tolerable enough for participants to complete the full 48 weeks. For the complete arm-by-arm escalation chart and the reconstitution math behind “how much BAC water” searches, see our retatrutide dosage guide, or use the retatrutide dosage calculator to run the concentration and unit math for any vial size.

The reconstitution arithmetic behind that calculator is straightforward once written out: concentration in milligrams per milliliter equals the total milligrams in the vial divided by the milliliters of bacteriostatic water added, and the volume to draw equals the desired dose divided by that concentration. A 10 mg vial mixed with 1 mL of bacteriostatic water works out to 10 mg/mL, so a 2 mg amount at that concentration is 0.2 mL, which reads as 20 units on a U-100 insulin syringe. That is trial-protocol and pharmacy arithmetic, not a recommendation to prepare or administer anything.

Can you get it?

No. Retatrutide cannot legally be bought or prescribed for weight loss anywhere. It has no FDA approval and no marketing authorization in any country, so there is no pharmacy, telehealth provider or clinic that can legally dispense it, and no official dosing label for a clinician to prescribe from. Products sold online under the name “retatrutide” are labeled for laboratory research only, meaning they were never manufactured, tested or approved for a person to inject, with no purity or dose-accuracy guarantee behind the label. A phase 2 trial result, however striking the number, is a scientific milestone, not a regulatory one, and retatrutide has since moved into phase 3 trials, the stage that comes before any FDA review can even begin. For the full explanation of why no legal channel exists, and what that research-only label actually means, see where to buy retatrutide.

Retatrutide vs the approved drugs

Retatrutide’s 24.2% trial figure is larger than tirzepatide is typically credited with, but the two numbers come from separate trials, not a head-to-head study, so “which one is stronger” is not a settled clinical claim. The more decisive difference is access: tirzepatide is FDA-approved, marketed as Mounjaro for type 2 diabetes and Zepbound for weight loss, and a licensed clinician can prescribe it this week. Retatrutide has no approved label, no pharmacy supply chain and no clinician supervision available outside a clinical trial. Separating those two questions matters: one is scientific (which molecule produced more weight loss in the trial that measured it), the other is practical (which drug can a person actually start this month, with a clinician who can adjust the dose and monitor for side effects). On the practical question, only tirzepatide currently has an answer. For the full factor-by-factor comparison, including the mechanism difference between retatrutide’s triple-agonist design and tirzepatide’s dual-agonist design, see retatrutide vs tirzepatide.

What you can be prescribed today

Two GLP-1-class medications solve the practical problem behind most retatrutide searches, and both are FDA-approved and prescribable this week. Tirzepatide is marketed as Mounjaro for type 2 diabetes and Zepbound for weight loss. Semaglutide is marketed as Ozempic for type 2 diabetes and Wegovy for weight loss. Both come from a real pharmacy supply chain, with an approved dosing label and a licensed clinician to evaluate whether either one is appropriate.

The providers above evaluate eligibility and prescribe tirzepatide or semaglutide, the two GLP-1-class medications currently available by prescription. Neither offers retatrutide, because no licensed provider can while it remains investigational, still moving through the clinical-trial process that produced the numbers on this page.

Starting tirzepatide or semaglutide follows the same shape of process the retatrutide trial itself relied on for its 338 participants: an initial evaluation, a low starting dose, and a schedule that steps the dose up gradually while a clinician checks in on how it is tolerated. The difference is that the approved version of that process is available to the public today, on a real label, while retatrutide’s version currently exists only inside the trial infrastructure still generating its evidence base.

Sources

Jastreboff AM, Kaplan LM, Frías JP, et al. “Triple-Hormone-Receptor Agonist Retatrutide for Obesity, A Phase 2 Trial.” New England Journal of Medicine, 2023;389(6):514-526. DOI: 10.1056/NEJMoa2301972. PMID 37366315. Registered as ClinicalTrials.gov NCT04881760. This is the primary source for every retatrutide figure on this page.

Rosenstock J, Frías J, et al. reported retatrutide’s effect in people with type 2 diabetes in a separate trial published in The Lancet in 2023. DOI: 10.1016/S0140-6736(23)01053-X.

Bajaj HS, et al. published newer retatrutide trial data in The Lancet on 13 June 2026. DOI: 10.1016/S0140-6736(26)00967-0, more recent than the sources most pages on this topic currently cite.