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Retatrutide: Triple-Agonist Results and Timeline

2026-10-21
A stack of printed clinical study pages with a line graph, a fountain pen and an olive sprig on cream linen.

Retatrutide is the next big name in weight-loss medicine. It is an investigational once-weekly injection from Eli Lilly, the maker of Zepbound. In 2026, its phase 3 trials showed average weight loss of about a quarter of body weight on the top dose. Two full trial reports were published on the same day in September: one in the New England Journal of Medicine and one in The Lancet.

It is also not approved. As of October 21, 2026, Lilly has not yet filed for FDA approval. The company says it plans to submit in the first quarter of 2027. Meanwhile, "retatrutide" is being sold online and by some clinics as a "research peptide." Lilly has sued sellers, and the FDA says it cannot legally be compounded.

Here is what the drug does, what the trials actually showed, what the side effects look like, and why we tell patients to wait for the real thing.

Table of Contents

  1. What retatrutide is and how a triple agonist works
  2. The phase 2 results that started the buzz
  3. What the TRIUMPH phase 3 trials showed
  4. Side effects and open safety questions
  5. Retatrutide vs tirzepatide
  6. Where FDA review stands
  7. Why gray-market retatrutide is risky
  8. Frequently asked questions
  9. The bottom line

What retatrutide is and how a triple agonist works

Today's main weight-loss injections work on one or two gut-hormone receptors:

  • Semaglutide (Wegovy) activates the GLP-1 receptor.
  • Tirzepatide (Zepbound) activates the GIP and GLP-1 receptors.
  • Retatrutide activates GIP, GLP-1 and glucagon receptors. That's why it's called a "triple agonist."

GLP-1 and GIP act on appetite, fullness and blood sugar control. Glucagon is the newer piece. It is usually thought of as the hormone that raises blood sugar, so adding it to a diabetes-friendly drug sounds odd. Researchers are interested in glucagon receptor activity because it may increase energy use and affect fat in the liver. The drug is designed so the GLP-1 and GIP effects keep blood sugar in check.

That's the theory. The trial results are what matter, and so far they are strong.

If you want a refresher on the older drugs first, see our explainer on how GLP-1 medications work and our comparison of semaglutide vs tirzepatide.

The phase 2 results that started the buzz

In 2023, the New England Journal of Medicine published a 48-week phase 2 trial of retatrutide in 338 adults with obesity or overweight. At 48 weeks, the average weight change was:

  • −24.2% on the 12 mg dose
  • −22.8% on the combined 8 mg groups
  • −2.1% on placebo

On 12 mg, 83% of participants lost at least 15% of their body weight, compared with 2% on placebo. Side effects were mostly gastrointestinal, dose-related, and mostly mild to moderate. Heart rate rose in a dose-dependent way, peaked at 24 weeks, and then came down.

Phase 2 trials are small and short. The question was whether the results would hold up in larger phase 3 trials. They largely did.

What the TRIUMPH phase 3 trials showed

Lilly's phase 3 program is called TRIUMPH. Four trials have reported results so far. A note on numbers before we start: drug makers often headline an "efficacy estimand," which estimates the effect if everyone stayed on the drug as assigned. Journals usually lead with the "treatment-regimen estimand," which counts everyone randomized, including people who stopped. The second number is lower and closer to real life. We give both where we have them.

TRIUMPH-1: obesity without diabetes

This was the main obesity trial. It enrolled 2,339 adults and ran 80 weeks. It was published in the New England Journal of Medicine on September 29, 2026.

DoseWeight change (journal, treatment-regimen)Weight change (Lilly, efficacy estimand)
4 mg−17.6%−19.0%
9 mg−23.7%−25.9%
12 mg−25.0%−28.3%
Placebo−3.9%−2.2%

Lilly reported that 45.3% of people on 12 mg lost at least 30% of their body weight. People with a starting BMI of 35 or higher who stayed on through a 104-week extension averaged about 30% weight loss on the 12 mg dose. The trial also included people with knee osteoarthritis and sleep apnea. Both groups saw significant improvements in pain scores and breathing events during sleep compared with placebo.

TRIUMPH-2: obesity with type 2 diabetes

People with type 2 diabetes usually lose less weight on these drugs, and that held true here. This 80-week trial enrolled 1,152 adults and was published in The Lancet. The treatment-regimen results were:

  • −11.9% on 4 mg
  • −16.8% on 9 mg
  • −18.8% on 12 mg
  • −5.1% on placebo

Lilly's efficacy-estimand figure for 12 mg was −20.8%. A1C, a measure of average blood sugar, fell by up to an average of 1.6 percentage points.

TRIUMPH-3: severe obesity with heart disease

In July 2026, Lilly reported topline results in adults with severe obesity and established cardiovascular disease. They averaged −21.6% on 9 mg and −22.6% on 12 mg, versus −3.2% on placebo, at 80 weeks (efficacy estimand). Major cardiovascular events were less common than expected in both groups. The confidence intervals were wide, so the trial could not show whether retatrutide reduces heart attacks or strokes. A dedicated outcomes trial, TRIUMPH-Outcomes, is still running.

TRIUMPH-4: knee osteoarthritis

In December 2025, Lilly reported that people with obesity and knee osteoarthritis on 12 mg lost an average of 28.7% of body weight at 68 weeks. Knee-pain scores fell by up to an average of 75.8%. These are topline company figures; a full peer-reviewed paper was not available to us as of this writing.

Side effects and open safety questions

The side effects look like those of the other incretin drugs, with a few extras worth watching.

Stomach and gut effects

In TRIUMPH-1, nausea occurred in 28.6% to 42.4% of people on retatrutide (by dose), versus 14.8% on placebo. Vomiting occurred in 10.6% to 25.3%, versus 4.8%. Diarrhea and constipation were also common. As with other drugs in this class, these effects tend to be dose-related. Our guide to managing GLP-1 side effects covers what usually helps.

Dysesthesia

This is the unusual one. Dysesthesia means abnormal skin sensations, such as tingling, burning or sensitivity to touch. In TRIUMPH-1, it was reported in 5.1%, 12.3% and 12.5% of people on 4, 9 and 12 mg, versus 0.9% on placebo. Lilly says most cases were mild to moderate and resolved during treatment. It is still something regulators will look at closely.

Stopping because of side effects

In TRIUMPH-1, 11.3% of people on 12 mg stopped because of adverse events, compared with 4.9% on placebo. In TRIUMPH-2, permanent discontinuation due to adverse events or death was 12% on 9 mg and 8% on 12 mg, versus 5% on placebo. Low blood pressure and dysesthesia were also more frequent with retatrutide.

Heart rate and muscle

Heart-rate increases were seen in phase 2. As with any drug that causes large, fast weight loss, protecting muscle matters too. Our article on muscle preservation during weight loss explains why.

The longer-term questions, such as cardiovascular outcomes, kidney outcomes and safety over many years, will not be answered until trials like TRIUMPH-Outcomes finish. ClinicalTrials.gov lists its estimated primary completion as February 2029.

Retatrutide vs tirzepatide

This is the most common question we hear, and the honest answer is that no one knows yet.

The retatrutide numbers look bigger than the numbers from tirzepatide's own trials. But comparing results across separate trials is unreliable. Populations, durations, dosing schedules and statistical methods all differ.

A direct comparison is underway. TRIUMPH-5 is a head-to-head trial of retatrutide versus tirzepatide in adults with obesity. ClinicalTrials.gov lists it as active and not recruiting, with an estimated primary completion of November 2026. Until those results are out, any claim that one drug is definitively better is a guess.

Also remember that tirzepatide and semaglutide are FDA-approved, available now, and backed by years of use. A drug that works a bit better on paper isn't better for you if you can't get it legally and safely.

Where FDA review stands

As of October 21, 2026:

  • Not approved anywhere. Lilly states that no medicine containing retatrutide has been approved for human use by any regulator in the world.
  • Not yet submitted. In its September 29, 2026 release, Lilly said it plans to submit a Biologics License Application to the FDA in the first quarter of 2027. Earlier, it said it was completing the chemistry, manufacturing and controls package needed for that submission.
  • Approval timing is unknown. FDA review takes months after a submission is accepted, and the outcome is not guaranteed. Anyone promising an approval date is speculating.
  • The only legal access is a trial. Lilly states retatrutide is legally available only to participants in its clinical trials.

Why gray-market retatrutide is risky

Because retatrutide made headlines before it was approved, a black market filled the gap. It sells vials labeled "for research purposes" or "not for human consumption."

The FDA's position

The FDA states that retatrutide cannot be used in compounding under federal law. It is not part of any FDA-approved drug, so no pharmacy can legally make a "compounded retatrutide." The FDA has issued warning letters to telehealth companies, ingredient distributors and outsourcing facilities over retatrutide. In a December 2024 warning letter to one seller, the FDA said that despite "RESEARCH USE ONLY" labels, evidence from the seller's websites established that the products were "intended to be drugs for human use."

Lilly's lawsuits

On August 12, 2026, Lilly announced six lawsuits against U.S. businesses it says sold illegal retatrutide. The defendants include compounding pharmacies, medical spas and online sellers. Lilly said it had referred more than 200 individuals and entities to regulators and law enforcement. It also said it had reported more than 14,000 websites, ads, posts and listings marketing retatrutide in over 100 countries. These are allegations; the cases have not been decided.

What can go wrong

You don't know what's in the vial. You don't know the dose, the purity, whether it's sterile, or whether it contains retatrutide at all. A 2026 review in Missouri Medicine named retatrutide among the unapproved injectables sold without prescriptions. It lists contamination, manufacturing impurities and inaccurate dosing among the risks. With a drug this potent and side effects this dose-related, guessing at the dose is a real hazard.

Frequently asked questions

When will retatrutide be approved?

There is no approval date. Lilly plans to submit to the FDA in the first quarter of 2027. The FDA's review and decision would come after that, and approval is not guaranteed.

How much weight do people lose on retatrutide?

In TRIUMPH-1, adults without diabetes on the 12 mg dose lost an average of 25.0% of body weight at 80 weeks, counting everyone who started. Lilly's efficacy estimate was 28.3%. People with type 2 diabetes lost less. These are group averages, not predictions for any one person.

Is retatrutide better than tirzepatide?

We don't know yet. The head-to-head trial, TRIUMPH-5, has not reported. Comparing numbers from separate trials can mislead.

Can a compounding pharmacy make retatrutide?

No. The FDA states that retatrutide cannot be used in compounding under federal law. Any "compounded retatrutide" is not a legal product.

What can I use now?

FDA-approved options, including semaglutide and tirzepatide, are available today for eligible adults. A licensed provider can review whether medication fits your health history and goals.

The bottom line

Retatrutide's results are real and impressive. Two peer-reviewed phase 3 papers now back up what Lilly's press releases promised. Weight loss averaged about a quarter of body weight on the highest dose in people without diabetes. It's also a drug with notable stomach side effects, a new signal (dysesthesia) to watch, no long-term outcomes data yet, and no FDA submission until 2027.

Our advice is simple: be excited, but don't buy it. Gray-market vials are illegal, unverified and not worth the risk when proven, FDA-approved medications are available right now. If you're considering medical weight management, our physician-supervised care team can walk you through current options and what's on the horizon.

Related reading: Wegovy Pill vs Foundayo: GLP-1 Pills Compared · Compounded GLP-1s in 2026: The FDA 503B Shift · Is BPC-157 Legal? What the FDA Panel Vote Means

Sources

This article is provided for general educational purposes and is not a substitute for individualized medical advice, diagnosis, or treatment. Retatrutide is investigational and not FDA-approved; talk with a licensed provider about approved weight-management options rather than using unapproved products.

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