What is retatrutide? The peptide, the trials and where it stands

What is retatrutide? It's an experimental drug from Eli Lilly for obesity and type 2 diabetes, given as an injection once a week. It is a single peptide that switches on three hormone receptors at once: GLP-1, GIP and glucagon. Retatrutide is not approved by the FDA or any other regulator, and Lilly says it will file for FDA approval in the first three months of 2027 [Lilly, Sep 29, 2026]. In the peer-reviewed phase 3 trials, people on the top dose lost about 15% to 25% of their body weight on average, depending on the trial.

That's the short answer. Below, you'll find what kind of drug it is and what it's being tested for. Then come the trial results, with who reported each number, the main side effects and the only legal ways to get it today. Every claim carries a source tag and a date.

What kind of drug is retatrutide?

Retatrutide is a peptide: a short chain of amino acids, built in a lab. Lilly describes it as an investigational once-weekly triple hormone receptor agonist and a single molecule [Lilly, Oct 2026]. An agonist is a molecule that switches a receptor on. Retatrutide switches on three:

• GLP-1, the receptor semaglutide (Wegovy, Ozempic) acts on.
• GIP, the second receptor tirzepatide (Zepbound, Mounjaro) acts on.
• Glucagon, the new part. Neither semaglutide nor tirzepatide targets it.

In Lilly's lab work, the GLP-1 and GIP effects cut how much the animals ate, and the glucagon effect added a rise in the energy they burned [Cell Metabolism, 2022]. Glucagon on its own makes the liver release sugar [Lilly Medical, Oct 6, 2025], which is one reason researchers watch it closely.

So is retatrutide a GLP-1? Partly. It acts on the GLP-1 receptor, but it is not a GLP-1 drug in the way semaglutide is, because two other receptors are in play. That's why Lilly calls it a "triple agonist." For a plain-words walk through each receptor, see how retatrutide works.

In trials, people inject it under the skin once a week [Lilly, Oct 2026]. The weekly schedule comes from early testing: in a 12-week phase 1 study, the drug's half-life was about 6 days, which the authors said suits once-weekly dosing [Lancet, 2022].

Is it a drug or a biologic? That is disputed. Lilly plans to file a Biologics License Application, the form used for biologics [Lilly, Sep 29, 2026]. But in March 2024 the FDA classified retatrutide as a drug, not a biological product. The argument is over size. FDA rules define a protein as greater than 40 amino acids in size. Retatrutide's main chain has 39 amino acids, and the side chain that holds its fatty acid adds two more. Lilly counts 41. The FDA counts 40, because one side-chain unit is not an alpha amino acid, the type its rule counts [7th Cir. brief, Apr 1, 2026]. On Oct 2, 2026, the FDA again rejected Lilly's argument, and Lilly's appeal was undecided when we checked on Nov 12, 2026 [7th Cir. filing, Oct 8, 2026].

Who makes retatrutide, and does it have a brand name?

Eli Lilly and Company is developing it [Lilly, Oct 2026], and Lilly scientists published its discovery in 2022 [Cell Metabolism, 2022]. In papers and trial records it often appears under its development code, LY3437943 [NEJM, 2023]. The first people received it in a Lilly safety study that started in March 2019 [ClinicalTrials.gov, NCT03841630].

There is no brand name yet. As of Nov 12, 2026, Lilly's retatrutide pages and releases use only the generic name [Lilly, Oct 2026]. So a brand name printed on a product sold online is not Lilly's.

Is retatrutide a "GLP-3"?

No. "GLP-3" is a nickname that spread on social media because the drug hits three receptors. There is no GLP-3 hormone and no GLP-3 drug class. Lilly's own answer is blunt: the term is a scientifically inaccurate label used informally in the media, and triple agonist is the accurate description [Lilly, Oct 2026].

What is retatrutide used for? What it's being tested for

It isn't used for anything yet outside research, because it isn't approved. What it is being tested for is a long list. Lilly's phase 3 trials cover obesity, type 2 diabetes and knee osteoarthritis pain. They also cover moderate to severe obstructive sleep apnea, chronic low back pain, heart and kidney outcomes, and fatty liver disease (MASLD) [Lilly, Oct 2026]. The trial registry backs that up:

Sources: [ClinicalTrials.gov, Nov 12, 2026] [ClinicalTrials.gov, NCT06383390] [ClinicalTrials.gov, NCT07165028]. On Nov 12, 2026, ClinicalTrials.gov listed 33 studies of retatrutide: 21 completed, 9 active but no longer recruiting, 2 recruiting and 1 expanded access program [ClinicalTrials.gov, Nov 12, 2026].

What have the trials found?

The first phase 3 result arrived in December 2025, from TRIUMPH-4 [Lilly, Dec 11, 2025]. Six phase 3 trials have now reported. The headline is weight loss. Across the trials, people taking the highest dose tested, 12 mg once a week, lost far more weight than people on placebo. Here are the main results, each with the trial's own population and length. The FDA filing guide covers the retatrutide clinical trials still under way, and the results guide sets out retatrutide weight loss results trial by trial.

The first table gives the figure each phase 3 paper leads with, which counts everyone assigned to the drug, including people who stopped:

Three more results exist only as Lilly's "efficacy estimand," explained below. That figure assumes everyone stayed on the drug, so it runs higher, and it can't be set beside the table above:

Why do you see two different numbers for the same trial? For TRIUMPH-1, Lilly's May press release said 28.3%, compared with 2.2% on placebo [Lilly, May 21, 2026]. The NEJM paper reports 25.0% compared with 3.9% [NEJM, Sep 29, 2026]. Both are correct. The 28.3% estimates what would have happened if everyone had stayed on the drug. The 25.0% counts everyone who was assigned to it, including people who stopped. The second number is closer to what a group of real patients might see, which is why this page leads with it.

Two patterns stand out. People with type 2 diabetes lost less weight: 18.8% on 12 mg in TRIUMPH-2 [Lancet, Sep 29, 2026], against 25.0% in TRIUMPH-1, both over 80 weeks [NEJM, Sep 29, 2026]. And the extra conditions moved too: knee pain scores fell more than on placebo [NEJM, Nov 8, 2026], and so did breathing pauses during sleep in people with sleep apnea [NEJM, Oct 21, 2026]. Blood sugar fell as well. In TRANSCEND-T2D-1, adults with type 2 diabetes on 12 mg saw their A1C drop by 1.94 percentage points over 40 weeks, compared with 0.81 on placebo [Lancet, Jun 6, 2026]. A1C is the standard measure of average blood sugar over the past few months. The benefits guide grades each of these retatrutide benefits by the strength of its evidence.

What are the main side effects?

The common ones are digestive: nausea, diarrhea, vomiting and constipation, mostly mild to moderate and more frequent at higher doses [NEJM, 2023]. In TRIUMPH-1, nausea affected 42.4% of people on 12 mg, compared with 14.8% on placebo. Side effects led 11.3% of people on 12 mg to stop the drug, compared with 4.9% on placebo [Lilly, May 21, 2026].

Two signals get extra attention. One is dysesthesia, an odd or painful sensitivity of the skin, reported in 12.5% of people on 12 mg in TRIUMPH-1, compared with 0.9% on placebo [Lilly, May 21, 2026]. The other is heart rate: in the phase 2 trial, heart rate rose with the dose, peaked at 24 weeks and then came down [NEJM, 2023]. The full list, trial by trial, is in retatrutide side effects, and the doses each trial tested are in retatrutide dosage in trials.

Is retatrutide approved, and when could it be?

No. Retatrutide is not approved by the FDA or any regulator in the world [Lilly, Oct 28, 2026]. It hasn't even been filed yet. In September, Lilly put its plan in one sentence: Lilly plans to submit a Biologics License Application (BLA) in Q1 2027 to the U.S. Food and Drug Administration. [Lilly, Sep 29, 2026]

Even the type of application is disputed. The FDA has classified retatrutide as a drug, not a biologic, and Lilly's appeal was undecided on Nov 12, 2026, as explained above [7th Cir. filing, Oct 8, 2026]. Lilly told the court it needs to know which application pathway (biological product or drug) to pursue when filing [7th Cir. filing, Oct 8, 2026].

Before filing, Lilly said in July, it is completing the chemistry, manufacturing and controls data package the application needs [Lilly, Jul 23, 2026]. It also said the data support submissions around the world for obesity, knee osteoarthritis pain and obstructive sleep apnea [Lilly, Jul 23, 2026].

A filing starts an FDA review. It is not an approval, and no FDA decision date can exist before Lilly files. That's why the status rail near the top of this page shows "no date" for the FDA decision and for pharmacies. The retatrutide FDA approval timeline explains how long a review usually takes and what could change it. No price exists either; the retatrutide cost guide covers what is known about money and insurance.

How can you get retatrutide legally today?

Only two ways.

A clinical trial. Trial drugs are given to enrolled volunteers under a study protocol. On Nov 12, 2026, one Lilly retatrutide trial was recruiting: a phase 3 study in fatty liver disease. People in it are assigned by chance to retatrutide, tirzepatide or placebo [ClinicalTrials.gov, NCT07165028]. The only other "recruiting" record is a study at one site in China whose sponsor isn't Lilly. We couldn't verify it, so we don't count it as a route [ClinicalTrials.gov, Nov 12, 2026]. Most other trials have finished enrolling.

Lilly's expanded access program. This is for a narrow group. Adults qualify only if their BMI stays at 35 or above even on the top available dose of a weight-management medicine. They also need two or more serious complications of obesity, and no way into a trial [ClinicalTrials.gov, NCT07629401]. You can't apply yourself. The treating doctor contacts Lilly, and the registry record tells patients to go through their doctor [ClinicalTrials.gov, NCT07629401].

There is no third route. Retatrutide is not on the market, and no one can legally sell it for human use [Lilly, Oct 2026]. The FDA says retatrutide cannot be used in compounding under federal law [FDA, Oct 1, 2026]. The FDA has also warned companies selling it as a product for research purposes or not for human consumption [FDA, Oct 1, 2026]. Lilly's position is plain: Anyone purporting to sell retatrutide to consumers is breaking the law. [Lilly, Oct 28, 2026] The buying guide, retatrutide for sale, explains what those online listings are and what tests of them found.

How does retatrutide compare with drugs you can get now?

The two drugs retatrutide is most often compared with are already approved for chronic weight management. Semaglutide, sold as Wegovy, is a GLP-1 receptor agonist [Wegovy label, Jun 2026] and was approved for weight in June 2021 [FDA, Jun 2021]. Tirzepatide, sold as Zepbound, acts on GLP-1 and GIP and was approved for weight in November 2023 [FDA, Nov 8, 2023]. Retatrutide adds the glucagon receptor on top of those two.

Retatrutide's trial numbers look larger than the older drugs' trial numbers, but those trials enrolled different people, at different times, for different lengths. They can't be lined up as a contest. One head-to-head trial has reported: in TRANSCEND-T2D-2, adults with type 2 diabetes lost 20.9% on retatrutide 12 mg, compared with 9.4% on semaglutide 2 mg, over 80 weeks. That figure comes from Lilly's press release and is not yet peer reviewed [Lilly, Oct 28, 2026]. A head-to-head trial against tirzepatide, TRIUMPH-5, is still running, with no results posted [ClinicalTrials.gov, NCT06662383]. For side-by-side detail, see retatrutide vs tirzepatide and retatrutide vs semaglutide.

The practical difference is simpler. Wegovy and Zepbound can be prescribed today. Retatrutide can't, and won't be until a regulator approves it.