What is retatrutide?
Short answer
Retatrutide is an investigational once-weekly injection made by Eli Lilly that activates three hormone receptors: GIP, GLP-1 and glucagon.[1] In a phase 3 trial, adults on the highest dose lost an average of 28.3% of their body weight over 80 weeks.[1] As of October 2026 it is not FDA-approved, and Lilly plans to apply for U.S. approval in the first quarter of 2027.[4][3]
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Retatrutide is not FDA-approved and has not been found safe and effective for any condition.[4] FDA has approved tirzepatide (Zepbound) for chronic weight management in adults with obesity, or overweight with at least one weight-related condition.[9]
TRIUMPH-1 excluded people with diabetes.[6] A separate trial in adults with type 2 diabetes, TRIUMPH-2, found up to 20.8% average weight loss at 80 weeks.[2]
TRIUMPH-1 excluded people who had had pancreatitis or had a personal or family history of medullary thyroid carcinoma or MEN-2.[6] A provider will weigh this history.
TRIUMPH-1 excluded people who had taken weight-loss drugs, including over-the-counter ones, within 90 days before screening.[6] A provider will review any medicine you take now.
In TRIUMPH-1, nausea, diarrhea, constipation and vomiting were the most common side effects, and 11.3% on the highest dose stopped because of side effects.[1] In phase 2, stomach effects rose with dose and were partly reduced by a lower starting dose.[5]
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Key facts
- What it is
- A single molecule that activates the receptors for GIP, GLP-1 and glucagon, known as a triple hormone receptor agonist.[2]
- How it is taken
- An injection under the skin once a week. In trials, the dose started at 2 mg and rose in steps every 4 weeks.[1][6]
- Approval status
- Not approved. FDA says it has not been found safe and effective for any condition.[4]
- Phase 3 result
- 28.3% average weight loss at 80 weeks on the 12 mg dose, compared with 2.2% on placebo.[1]
- Most common side effects
- Nausea, diarrhea, constipation and vomiting.[1]
Development milestones
| Date | Milestone |
|---|---|
| June 2023 | Phase 2 obesity trial published in NEJM. Adults on 12 mg lost an average of 24.2% of body weight at 48 weeks.[5] |
| July 2023 | TRIUMPH-1, the pivotal phase 3 obesity trial, begins.[6] |
| November 2024 | TRIUMPH-5 begins, comparing retatrutide directly with tirzepatide.[8] |
| December 2025 | First phase 3 results (TRIUMPH-4, knee osteoarthritis): 28.7% average weight loss on 12 mg.[7] |
| May 2026 | TRIUMPH-1 topline results: 28.3% average weight loss at 80 weeks on 12 mg.[1] |
| July 2026 | TRIUMPH-2 and TRIUMPH-3 topline results: up to 20.8% and 22.6% average weight loss at 80 weeks.[2] |
| August 2026 | Lilly says its clinical data package is complete and plans to submit a Biologics License Application to FDA.[3] |
| First quarter of 2027 (planned) | Lilly's planned FDA submission.[3] |
How it works
- Three receptors
- Retatrutide is one molecule that activates receptors for three hormones: GIP, GLP-1 and glucagon.[2]
- Compared with tirzepatide
- Tirzepatide activates GLP-1 and GIP receptors to reduce appetite and food intake.[9] Retatrutide also activates the glucagon receptor.[2]
- Once a week
- Its half-life is about 6 days, which allows weekly dosing.[5]
- Gradual dose increases
- In TRIUMPH-1, everyone on retatrutide started at 2 mg a week. The dose went up every 4 weeks until it reached 4, 9 or 12 mg.[1]
What the trials showed
- Phase 2 (48 weeks)
- Among 338 adults with obesity, average weight loss was 24.2% on 12 mg, compared with 2.1% on placebo.[5]
- TRIUMPH-1 (80 weeks)
- In adults without diabetes, average weight loss was 19.0% on 4 mg, 25.9% on 9 mg and 28.3% on 12 mg, compared with 2.2% on placebo.[1] Counting everyone, including people who stopped treatment, the 12 mg result was 25.0%.[1]
- TRIUMPH-2 (type 2 diabetes)
- Average weight loss at 80 weeks was up to 20.8% on 12 mg, compared with 4.0% on placebo.[2]
- TRIUMPH-3 (heart disease)
- Adults with severe obesity and established cardiovascular disease lost up to an average of 22.6% at 80 weeks.[2] There were 27 heart attacks, strokes or cardiovascular deaths among people on retatrutide and 23 on placebo, and both groups had fewer major heart events than expected.[2]
- Head to head with tirzepatide
- TRIUMPH-5 compares the two directly. Its primary completion is estimated for November 2026.[8]
Side effects
- Stomach symptoms
- In TRIUMPH-1, nausea affected 28.6% to 42.4% of people on retatrutide, depending on dose, compared with 14.8% on placebo.[1] Diarrhea, constipation and vomiting were also common.[1]
- Linked to dose
- In phase 2, stomach side effects rose with dose, were mostly mild to moderate, and were partly reduced by starting at a lower dose.[5]
- Dysesthesia
- Reported in 5.1% to 12.5% of people on retatrutide, compared with 0.9% on placebo. Most cases were mild to moderate and resolved during treatment.[1]
- Heart rate
- In phase 2, heart rate rose with dose, peaked at 24 weeks and then declined.[5]
- Stopping because of side effects
- 4.1% on 4 mg, 6.9% on 9 mg and 11.3% on 12 mg, compared with 4.9% on placebo.[1]
Safety: unapproved products
- Not legal to sell
- Lilly says retatrutide cannot be legally sold or marketed for human use.[2] No medicine containing it has been approved by any regulator in the world.[11]
- No compounding
- Under federal law, retatrutide cannot be used in compounding.[4]
- "Research use only" labels
- FDA has warned companies selling retatrutide falsely labeled "for research purposes" or "not for human consumption." It urges consumers not to buy these products, which are of unknown quality and may be harmful.[4]
- Warning letters
- In August 2026, FDA told one online seller that its "GLP-3 Reta (Retatrutide)" product was an unapproved new drug, despite its "research use only" labeling.[10] FDA has also warned telehealth companies for marketing unapproved drugs such as retatrutide, including directly to consumers.[4]
- Reported deaths
- CBS News reported in October 2026 that FDA received reports of two deaths after use of bootleg retatrutide. The reports are unverified and do not prove the products caused the deaths.[12]
- Lawsuits
- In August 2026, Lilly filed six lawsuits against U.S. sellers of black-market retatrutide.[11]
Common questions
Is retatrutide FDA approved?
No. FDA says retatrutide is not a component of any FDA-approved drug and has not been found safe and effective for any condition.[4] Lilly plans to submit it to FDA in the first quarter of 2027.[3]
How is retatrutide different from tirzepatide?
Tirzepatide, sold as Zepbound for weight management, is FDA-approved and activates GLP-1 and GIP receptors.[9] Retatrutide also activates the glucagon receptor and is still investigational.[2] A head-to-head trial, TRIUMPH-5, is underway, with primary completion estimated for November 2026.[8]
How much weight do people lose on retatrutide?
In TRIUMPH-1, average weight loss at 80 weeks was 19.0%, 25.9% and 28.3% on the 4, 9 and 12 mg doses, compared with 2.2% on placebo.[1] In adults with type 2 diabetes, it was up to 20.8%.[2] These are averages across trial participants, not a promise for any one person.[1]
When will retatrutide be available?
Lilly plans to submit an application to FDA in the first quarter of 2027, and approval is not guaranteed.[3][1] In May 2026, Lilly said it was legally available only to people in its clinical trials.[1] In August 2026, Lilly confirmed a limited early-access pathway, requested by health care providers, for some patients who meet defined criteria and cannot join a trial.[13]
What are the side effects of retatrutide?
In TRIUMPH-1, the most common were nausea, diarrhea, constipation and vomiting.[1] Some people also reported dysesthesia, which was mostly mild to moderate.[1] Up to 11.3% on the highest dose stopped because of side effects.[1]
Is "reta" the same thing as retatrutide?
"Reta" is a nickname for retatrutide, and it appears in product names FDA has cited, such as "GLP-3 Reta (Retatrutide)."[10][12] Lilly calls black-market retatrutide entirely unverified and unapproved.[11] FDA says products sold for "research purposes" are of unknown quality and may be harmful.[4]
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Sources
- Lilly's triple agonist, retatrutide, delivered powerful weight loss in pivotal Phase 3 obesity trial. Eli Lilly and Company. May 21, 2026.
- Lilly's triple agonist, retatrutide, successful in two additional Phase 3 obesity trials, delivering significant improvements in weight and A1C. Eli Lilly and Company. July 23, 2026.
- Second-quarter 2026 sales and earnings release (Form 8-K exhibit). Eli Lilly and Company, via U.S. Securities and Exchange Commission. August 5, 2026.
- FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight Loss. U.S. Food and Drug Administration. Updated October 1, 2026.
- Jastreboff AM, et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity: A Phase 2 Trial. New England Journal of Medicine. Published online June 26, 2023.
- A Study of Retatrutide (LY3437943) in Participants Who Have Obesity or Overweight (TRIUMPH-1), NCT05929066. ClinicalTrials.gov, U.S. National Library of Medicine. Last updated August 21, 2026.
- Lilly's triple agonist, retatrutide, delivered weight loss of up to an average of 71.2 lbs along with substantial relief from osteoarthritis pain in first successful Phase 3 trial. Eli Lilly and Company. December 11, 2025.
- A Study of Retatrutide (LY3437943) Compared to Tirzepatide (LY3298176) in Adults Who Have Obesity (TRIUMPH-5), NCT06662383. ClinicalTrials.gov, U.S. National Library of Medicine. Last updated July 20, 2026.
- FDA Approves New Medication for Chronic Weight Management. U.S. Food and Drug Administration. November 8, 2023.
- Warning letter: Peptide Partners LLC, 735063. U.S. Food and Drug Administration. August 24, 2026.
- Lilly calls on online platforms, payment companies and regulators to shut down the illegal retatrutide black market. Eli Lilly and Company. August 12, 2026.
- Two deaths were reported to the FDA. Is the bootleg weight-loss drug "Reta" to blame?. CBS News. October 2026.
- Lilly Expands Retatrutide Access Amid Doctor Pushback. AJMC. August 5, 2026.