What Is Retatrutide? The Full Explainer

Retatrutide is an experimental once-weekly injection from Eli Lilly and Company for obesity and type 2 diabetes. It's a single peptide that switches on three hormone receptors at once (GIP, GLP-1 and glucagon), which is why it gets called a triple agonist, and online "reta" or "GLP-3". As of October 31, 2026, six Phase 3 trials have reported positive results, and no approved version exists yet anywhere in the world.

The headline numbers come from two of those trials. In TRIUMPH-1, adults without diabetes on the top 12 mg dose lost an average of 28.3% of their body weight over 80 weeks. In TRANSCEND-T2D-2, the first head-to-head trial, people with type 2 diabetes lost 20.9% on 12 mg against 9.4% on Ozempic 2 mg. Lilly has said it plans to file in early 2027, but on October 2 the FDA decided what kind of application it needs, and Lilly's appeal on that question is pending in court.

What is retatrutide?

Retatrutide (trial code LY3437943) is a lab-made peptide: a chain of 39 amino acids, according to the FDA's substance registry, with a fatty acid attached. That fatty acid lets it bind to albumin in the blood, which is what stretches its half-life to about 6 days and makes a weekly shot possible. Lilly's discovery paper in Cell Metabolism calls it a triple agonist peptide at the glucagon, GIP and GLP-1 receptors.

Chemically it's a peptide. For approval, the FDA decided on October 2, 2026 that it's a drug and not a biological product, which changes the paperwork Lilly files (see the status section).

Lilly is testing it for obesity and overweight, type 2 diabetes and the conditions that travel with them. Its trials so far have measured knee osteoarthritis pain, sleep apnea, heart events, blood sugar and liver fat, and Lilly's October 2026 FAQ adds "cardiovascular and renal outcomes" and "existing chronic kidney disease" to the list. The benefits page goes through each one.

You'll see it under several names:

• Reta, reta peptide or reta shot: forum shorthand for the same drug.
• GLP-3: a nickname. Lilly's FAQ calls it a scientifically inaccurate label, and none of the three receptors is called GLP-3. The GLP-3 page explains where it came from.
• Retratrutide and retaglutide: common misspellings of retatrutide.
• No brand name yet (more on the name).

Every trial has given it as an injection under the skin. There's no pill in trials; oral retatrutide covers what the "reta pills" sold online are. For the company behind it and its other obesity drugs, see Eli Lilly and retatrutide.

Retatrutide's status in October 2026

Six Phase 3 trials have reported positive results: TRIUMPH-4 (Dec 11, 2025), TRANSCEND-T2D-1 (Mar 19, 2026), TRIUMPH-1 (May 21, 2026), TRIUMPH-2 and TRIUMPH-3 (Jul 23, 2026), and TRANSCEND-T2D-2 (Oct 28, 2026). Lilly's FAQ puts it plainly: "To date, Lilly has disclosed retatrutide Phase 3 data from six clinical trials." Adding up Lilly's figures for each trial, 7,720 people were randomized across the six.

Four peer-reviewed papers now cover three of those trials plus two sleep apnea substudies. TRIUMPH-1 is in the New England Journal of Medicine, TRIUMPH-2 and TRANSCEND-T2D-1 are in The Lancet, and the TRIUMPH-1 and TRIUMPH-2 sleep apnea results are in NEJM. TRIUMPH-3, TRIUMPH-4 and TRANSCEND-T2D-2 have no paper yet (Europe PMC search to October 31, 2026), so their numbers come from Lilly's releases only.

The drug-or-biologic fight is about which application route and rules apply, and it goes back to a lawsuit Lilly filed in 2024. It isn't a finding about whether retatrutide works. Neither Lilly's October 28 release nor its third-quarter report a day later restated a filing date. There's also no launch date or price: Lilly's FAQ says launch "will depend on the completion of these trials and the regulatory approval process", and that it plans to price retatrutide "to reflect the overall value to the healthcare system". The FDA approval and release date pages follow both.

More trials are still running. These are the main ones, with the estimated dates on ClinicalTrials.gov as of October 31, 2026:

The clinical trials hub has every study and whether you can still join one.

How much weight people lost in the trials

On the top 12 mg dose, average weight loss ran from 16.8% to 28.7% across the six Phase 3 trials. Where a trial lands in that range depends mostly on who was in it and for how long.

Two things explain most of the spread. People with type 2 diabetes lose less on every drug in this class, so TRIUMPH-2 (20.8% at 80 weeks) and both TRANSCEND-T2D trials sit below TRIUMPH-1 and TRIUMPH-4. And TRANSCEND-T2D-1 ran 40 weeks, half as long as most of the others.

The chart uses the efficacy estimand, Lilly's "as if everyone stayed on treatment" number. The stricter count includes everyone randomized, whether they kept taking it or not, and it's lower: 25.0% on 12 mg vs 3.9% on placebo in TRIUMPH-1. That's the primary result in the NEJM paper.

Averages hide a wide spread. In TRIUMPH-1, 62.5% of people on 12 mg, about 6 in 10, lost a quarter or more of their starting weight, and 27.2% lost 35% or more, against 2.2% and 0.3% on placebo. Starting from an average of 248.5 lb, the 12 mg group was down 70.3 lb (efficacy estimand). A 104-week extension then followed 532 people with a starting BMI of 35 or more who'd finished 80 weeks on treatment; those who began on 12 mg were down 30.3% (efficacy estimand). They were a selected group, so that isn't the average for everyone.

The trials measured more than the scale:

A1C, knee and waist figures are efficacy estimand; sleep apnea is the paper's hybrid-treatment estimand. The heart result needs care: heart events were rarer than Lilly expected in both groups, and an interval that crosses 1 means the trial can't say retatrutide lowered them. TRIUMPH-Outcomes, with about 10,000 people, is built to answer that.

Our read: the stricter 25.0% is the safer number to set expectations with, because it counts everyone who started. And 9 mg came close to 12 mg in TRIUMPH-1 (25.9% vs 28.3%), with fewer people stopping because of side effects on 9 mg in five of the six trials, so the top dose isn't automatically the better deal.

For what the loss looks like month by month, see results and the results timeline. Belly fat and muscle loss cover where the weight comes from.

Retatrutide vs Ozempic and tirzepatide

TRANSCEND-T2D-2 is the only head-to-head result so far. Lilly randomized 1,298 adults with type 2 diabetes on metformin, with or without an SGLT2 inhibitor, to retatrutide 9 mg, retatrutide 12 mg or Ozempic 2 mg once a week for 80 weeks. It was open label, so people knew which drug they were on.

Both retatrutide doses lowered A1C more than Ozempic did, and both more than doubled its weight loss. Nausea was about even across the three groups. Diarrhea was a bit more common on 12 mg (32.7% vs 26.4%), constipation was less common (13.1% vs 19.8%), and more people stopped treatment on retatrutide. Two caveats travel with these numbers. All of them are efficacy estimand figures from Lilly's release, and the 9 mg A1C advantage over Ozempic is "nominal" only, meaning it wasn't covered by the trial's control for multiple comparisons. The 12 mg comparisons were.

Against tirzepatide (Zepbound, Mounjaro) there's no direct answer yet. TRIUMPH-5 compares the two in adults with obesity and has an estimated primary completion of November 2026. Until it reports, any retatrutide vs tirzepatide claim compares different trials with different people. Our tirzepatide comparison lays out what can and can't be compared, and the Zepbound and Mounjaro pages cover the two brands. If you're on tirzepatide now, switching from tirzepatide covers what the trials did and didn't test.

How does retatrutide work?

Retatrutide acts like three of the body's own hormones at once. GLP-1 and GIP are gut hormones that help you feel full and keep blood sugar in check. Glucagon, made in the pancreas, mostly acts on the liver. Semaglutide works on one of these receptors, tirzepatide on two, and retatrutide adds the third.

It doesn't hit all three equally. In lab cell tests reported by Lilly Medical Information, retatrutide had 8.9-fold greater potency than human GIP at the GIP receptor, 2.5-fold less potency than human GLP-1 at the GLP-1 receptor, and 2.9-fold less than human glucagon at the glucagon receptor. Nature Medicine's 2024 paper puts the last two at about 0.4 and 0.3 times, which is close to the same picture.

The glucagon part is the new piece. In Lilly's discovery work it raised energy use on top of the reduced eating that GIP and GLP-1 drive, but that finding comes from lab animals and hasn't been shown the same way in people. The clearest human signal is in the liver. In the Phase 2 fatty liver substudy, a fat-burning marker in the blood (beta-hydroxybutyrate) rose 2 to 3 fold at 4 mg and above, and liver fat fell by up to 82.4% at 24 weeks (Nature Medicine, 2024).

Timing explains the weekly shot. The half-life is about 6 days in the Phase 1b trial, and Lilly Medical Information gives a time to peak of 12 to 72 hours after a dose. How it works goes deeper on the receptors, and the half-life page works through how much is left after each week.

Retatrutide side effects in the trials

Stomach side effects are the main story, and they bunch up in the weeks after each dose increase. In TRIUMPH-1, about 4 in 10 people on 12 mg (42.4%) had nausea at some point over 80 weeks, against 14.8% on placebo. Here's how the top dose compared across all six Phase 3 trials, from Lilly's releases:

Nausea ran lower in the diabetes and heart disease trials, and next to Ozempic it was about even. TRIUMPH-4 had the most people stopping, and Lilly said those stops tracked with starting BMI and included people who stopped because they felt they'd lost too much weight.

One side effect is less familiar from older drugs: burning or oversensitive skin (dysesthesia). It affected 12.5% on 12 mg in TRIUMPH-1 vs 0.9% on placebo, and Lilly said most cases were mild to moderate and cleared up while people kept taking it. Heart rate rose with dose in the Phase 2 trial, peaking at 24 weeks and easing after, though the paper's abstract gives no beats-per-minute figure. In the TRIUMPH-2 paper, low blood pressure was reported by 6% on 12 mg vs under 1% on placebo.

Some gaps are still open. Lilly hasn't published serious adverse event rates for any Phase 3 trial in a release or abstract. The deaths reported so far were all judged unrelated to the study drug: 2 in TRANSCEND-T2D-1, both on 4 mg, and 7 in TRIUMPH-2 (2 on 4 mg, 3 on 9 mg, 1 on 12 mg and 1 on placebo). Retatrutide has no label yet. The labels for tirzepatide and semaglutide carry a boxed warning about thyroid C-cell tumors (seen in non-human studies) and warnings on pancreatitis and gallbladder problems. That's class evidence, not retatrutide data.

The side effects hub covers each one, with pages on nausea, how long side effects last and mood. For the overall safety picture, see is retatrutide safe?

How retatrutide was dosed in the trials

Every Phase 3 trial started people on 2 mg once a week and raised the dose every 4 weeks, through 4 and 6 mg where needed, to a target of 4, 9 or 12 mg. TRIUMPH-1, TRIUMPH-2 and TRANSCEND-T2D-1 tested all three targets. TRIUMPH-3, TRIUMPH-4 and TRANSCEND-T2D-2 used 9 and 12 mg.

The slow start has a reason. The Phase 2 trial tested it directly: among people aiming for 8 mg, 60.0% (21 of 35) had nausea when they started at 4 mg, against 17.1% (6 of 35) when they started at 2 mg (ClinicalTrials.gov results). Phase 3 then used the 2 mg start for everyone.

There's no approved dose. These schedules come from trials run with medical monitoring, and they aren't instructions. The dosage hub explains each step, the dosing schedule lays the weeks out, and starting dose covers why 2 mg comes first. The dose calculator turns a dose in mg into syringe units, and the reconstitution calculator does the mixing math. Both are arithmetic, not dosing advice.

Is retatrutide legal to buy?

No. Lilly's October 2026 FAQ says retatrutide "has not been reviewed or approved by the FDA or any regulatory agency in the world, and no one can legally sell it for human use." The FDA adds that it "cannot be used in compounding under federal law", so a compounding pharmacy can't legally make it either.

Vials sell online anyway, under names such as "reta", "GLP-3 Reta" and "R-10". In August 2026 Lilly said it had reported more than 14,000 websites, ads, posts and listings marketing retatrutide in more than 100 countries. None of them is Lilly's product, and testing has turned up real problems. When Australia's TGA tested a vial sold as retatrutide after a hospital case, it held no retatrutide and undeclared semaglutide at about 8 times the level in approved products. Victoria's health department has tied 6 cases of acute liver injury since January 2026 to products labeled retatrutide, possibly from a contaminant.

If you're looking at retatrutide for sale online, that page explains what's in those vials and how to read a test report. Nothing on a vial links it to the drug in Lilly's trials.

The legal routes are narrow. You can join a trial, or a doctor can apply to Lilly's expanded access program for adults who still have a BMI of 35 or more despite the highest available dose of current weight treatment, have two or more serious obesity-related complications and can't join a trial. The doctor contacts Lilly, not the patient. Approved drugs in the same family are the other route. Zepbound (tirzepatide) starts at $299 a month self-pay through LillyDirect for the 2.5 mg KwikPen or vial (checked October 31, 2026).

The law by country is on is retatrutide legal?, the legitimate routes are on how to get retatrutide, local trial sites are on retatrutide near me, and prices are on the price page.

Retatrutide timeline and what comes next

Lilly's first single-dose study in healthy people finished in July 2019. The first Phase 3 result came in December 2025, and five more had followed by the end of October 2026.

What to watch over the next few months:

• TRIUMPH-5 vs tirzepatide. Lilly's gap from primary completion to topline has run 27 to 98 days, which points to roughly late November 2026 to early March 2027 for a readout. That's our arithmetic from registry and release dates, not a Lilly date.
• TRANSCEND-T2D-3, in people with diabetes and kidney impairment. Its estimated primary completion was October 2026, and no result had come out by October 31.
• The Seventh Circuit appeal (No. 26-1301) on whether retatrutide is a drug or a biologic. The last filings we found were the October 8 briefs on whether the court has jurisdiction.
• Lilly's US filing, planned for the first quarter of 2027 as of its September 29 statement.
• More papers. TRIUMPH-3, TRIUMPH-4 and TRANSCEND-T2D-2 have none yet, and Lilly says the head-to-head results will appear at a future meeting and in a journal.

Where to go from here

The pages linked above cover the main questions. A few more that readers ask next:

• The whole schedule on one page: the dosage chart.
• Mixing, injecting and storing: how to use, injection and storage.
• Side effects not covered above: diarrhea and fatigue.
• How it compares with the whole GLP-1 class: retatrutide vs GLP-1 drugs.
• Drinking on it, or pairing it with other peptides: alcohol and GHK-Cu. Neither combination has been tested in people.
• What people say online: Reddit, reviews and before and after. These are user reports, not trial data.
• Fatty liver specifically: fatty liver.