Compounds / Retatrutide
Retatrutide
LY3437943
Retatrutide is not approved. It's an investigational once-weekly injection from Eli Lilly that acts on three hormone receptors: GIP, GLP-1 and glucagon1,3. Lilly says it is legally available only to people in its clinical trials, so vials sold online as "research" retatrutide are not the trial drug2.
- Status
- Not approved anywhere
- Taken in trials
- Once a week, injection
- Half-life
- About 6 days
- Type
- GIP, GLP-1 and glucagon agonist
What is retatrutide and how does it work?
Retatrutide is an experimental weight-loss injection from Eli Lilly, given once a week in trials. It's a single peptide that switches on three hormone receptors: GIP, GLP-1 and glucagon3,6. During development it was called LY34379436.
Its half-life was about 6 days in an early 12-week study in adults with type 2 diabetes, and the authors said that suits a once-a-week dose6. It's being studied for obesity, type 2 diabetes, knee osteoarthritis and sleep apnea4.
Is retatrutide approved?
No. Retatrutide isn't approved in any country. Lilly says it can't be legally sold or marketed for human use, and plans to apply to FDA in the first quarter of 20271. Approval would only come after FDA has reviewed it.
| Country | Status |
|---|---|
| US | Not approved. FDA says it can't be used in compounding and isn't part of any approved drug7 |
| UK | Not authorised. The MHRA says products sold as retatrutide outside clinical trials are supplied illegally8 |
| Canada | Not authorised. Health Canada lists it among unauthorized injectable peptides it has seized9, and no retatrutide product is in its drug database10 |
What doses of retatrutide were used in trials?
The phase 3 TRIUMPH trials gave adults 4 mg, 9 mg or 12 mg of retatrutide once a week for 80 weeks3–4. There's no approved dose. These are the amounts given to trial volunteers under medical supervision, not a schedule to follow.
| Trial | Who took part | Doses | Length |
|---|---|---|---|
| Phase 1b6 | 72 adults with type 2 diabetes | 0.5 mg up to 12 mg, the top doses reached in steps | 12 weeks |
| Phase 2, diabetes11 | 281 adults with type 2 diabetes | 0.5 mg to 12 mg, higher doses reached in steps from 2 mg or 4 mg | 36 weeks |
| Phase 2, obesity5 | 338 adults with obesity or overweight | 1 mg, 4 mg, 8 mg or 12 mg, higher doses started at 2 mg or 4 mg | 48 weeks |
| TRIUMPH-1 and TRIUMPH-23–4 | 3,491 adults with obesity or overweight, with or without type 2 diabetes | 4 mg, 9 mg or 12 mg | 80 weeks |
Trial doses, not a recommendation. In TRIUMPH-1, Lilly said the 4 mg dose was reached with a single step up2.
Because there's no label, there are no official rules for missed doses, storage or who shouldn't take it. In TRIUMPH-2, people injected the study drug themselves4. This page has no retatrutide calculator on purpose.
What are the side effects of retatrutide?
The most common side effects of retatrutide in trials were nausea, diarrhea, constipation and vomiting, and they were more common at higher doses. In TRIUMPH-1, Lilly reported nausea in 42.4% of people on 12 mg, compared with 14.8% on placebo2.
| Side effect | Placebo | 4 mg | 9 mg | 12 mg |
|---|---|---|---|---|
| Nausea | 14.8% | 28.6% | 38.4% | 42.4% |
| Diarrhea | 13.5% | 25.2% | 34.1% | 32.0% |
| Constipation | 10.9% | 23.8% | 25.9% | 26.1% |
| Vomiting | 4.8% | 10.6% | 22.8% | 25.3% |
| Dysesthesia (odd skin sensations) | 0.9% | 5.1% | 12.3% | 12.5% |
| Urinary tract infection | 5.3% | 7.5% | 8.8% | 8.4% |
Company statement, not yet a full peer-reviewed safety table. Lilly said dysesthesia and urinary tract infections were mostly mild to moderate and most resolved during treatment.
Other signals have come up. In phase 2, heart rate rose with the dose, peaked at 24 weeks, then came down5. In TRIUMPH-2, low blood pressure (up to 6% vs under 1%) and dysesthesia (up to 7% vs 1%) were more common than on placebo. Seven people died across all groups in that trial, and investigators judged none of the deaths related to the study drug4. How these rates compare with the approved medicines is in our GLP-1 side effects guide.
How much weight did people lose on retatrutide?
In TRIUMPH-1, adults with obesity and without diabetes lost an average of 25% of their body weight on 12 mg at 80 weeks, compared with 3.9% on placebo3. Weight loss was smaller in people with type 2 diabetes4.
| Trial | Placebo | 1 mg | 4 mg | 8 mg | 9 mg | 12 mg |
|---|---|---|---|---|---|---|
| Phase 2, obesity, 48 weeks5 | −2.1% | −8.7% | −17.1% | −22.8% | Not tested | −24.2% |
| TRIUMPH-1, no diabetes, 80 weeks3 | −3.9% | Not tested | −17.6% | Not tested | −23.7% | −25.0% |
| TRIUMPH-2, type 2 diabetes, 80 weeks4 | −5.1% | Not tested | −11.9% | Not tested | −16.8% | −18.8% |
TRIUMPH results use the treatment-regimen estimand, which counts everyone randomized whether or not they kept taking the drug. Lilly's efficacy estimand, which estimates the result if everyone had stayed on treatment, gave 28.3% at 12 mg in TRIUMPH-12.
TRIUMPH-1 also included people with knee osteoarthritis and sleep apnea. On 12 mg, knee pain scores fell by 3.6 points vs 1.9 on placebo, and breathing pauses (apnea-hypopnea events) fell by 31.7 an hour vs 9.93. Every phase 2 and phase 3 result, including two TRIUMPH trials known only from press releases, is in our retatrutide trial results guide.
How does retatrutide compare with tirzepatide and semaglutide?
Nobody can say yet. The published retatrutide trials compared it with placebo, and two early diabetes trials also included dulaglutide (Trulicity)6,11. None of them tested it against tirzepatide or semaglutide, and weight loss from separate trials can't be compared directly because the people and methods differ.
Lilly, which runs the retatrutide trials, also makes tirzepatide (Zepbound)2. The tirzepatide page covers its approvals, doses and trial results.
Retatrutide fact sheet
- Also called
- LY3437943 (Lilly's development code)6
- What isn't known yet
- With no approved label, there's no official list of warnings, interactions or who shouldn't use it
- Type
- Triple hormone receptor agonist (GIP, GLP-1 and glucagon receptors). Injected under the skin once a week in trials.3
- Status (US)
- Not approved. Lilly calls it an investigational molecule that cannot be legally sold or marketed for human use, and plans to submit it to FDA in the first quarter of 2027 (statement of September 29, 2026)1.
- Compounding
- FDA says retatrutide cannot be used in compounding under federal law, and it is not part of any FDA-approved drug7.
- Doses used in studies
- Phase 3 (TRIUMPH-1, TRIUMPH-2): 4 mg, 9 mg or 12 mg once weekly for 80 weeks3–4. Phase 2: 1 mg to 12 mg once weekly for 48 weeks; the 4 mg to 12 mg groups started at 2 mg or 4 mg5. These are trial doses in adults, not a recommendation.
- Weight change in trials
- TRIUMPH-1 (obesity, no diabetes): average loss of 25% of body weight at 12 mg vs 3.9% with placebo at 80 weeks3. TRIUMPH-2 (obesity and type 2 diabetes): 18.8% vs 5.1%4.
- Half-life
- About 6 days, measured in a 12-week phase 1b study in adults with type 2 diabetes.6
- Common side effects
- Mostly stomach and gut effects such as nausea and diarrhea. In TRIUMPH-2, low blood pressure and dysesthesia (odd skin sensations) were also more common than with placebo4.
Retatrutide guides
Also useful: Dose schedule planner · Missed dose checker · Units to dose
Questions
Is retatrutide FDA approved?
No. As of Lilly's statement of September 29, 2026, retatrutide is an investigational molecule that cannot be legally sold or marketed for human use. Lilly plans to submit it to the FDA in the first quarter of 2027. Approval would come only after FDA review.
Is research-grade retatrutide the same as the trial drug?
No. Lilly says retatrutide is legally available only to people in its clinical trials, so vials sold online are not from those trials. FDA has warned companies that sell drugs labeled 'for research purposes' or 'not for human consumption' to people.
What doses of retatrutide were used in trials?
The phase 3 TRIUMPH trials used 4 mg, 9 mg or 12 mg once a week for 80 weeks. The 2023 phase 2 trial used 1 mg to 12 mg once a week for 48 weeks, with lower starting doses that were stepped up. These are doses given to adults in clinical trials, not a dose to follow.
How much weight did people lose on retatrutide?
In TRIUMPH-1, adults with obesity and without diabetes lost an average of 17.6% (4 mg), 23.7% (9 mg) and 25% (12 mg) of body weight at 80 weeks, compared with 3.9% on placebo. In the phase 2 trial, the 12 mg group lost 24.2% at 48 weeks, compared with 2.1% on placebo.
Can a compounding pharmacy make retatrutide?
No. FDA says retatrutide cannot be used in compounding under federal law. It is not part of any FDA-approved drug.
How long does retatrutide last in the body?
Its half-life was about 6 days in an early study in adults with type 2 diabetes, and the study authors said this suits once-a-week dosing.
Sources
- 1Eli Lilly press release: retatrutide delivered substantial weight loss and A1C reduction (TRIUMPH-2), September 29, 2026. regulatory plans and investigational status
- 2Eli Lilly press release: retatrutide delivered powerful weight loss in pivotal Phase 3 obesity trial (TRIUMPH-1), May 21, 2026. "legally available only to participants in Lilly's clinical trials"; company statement on adverse events and the efficacy estimand
- 3Jastreboff AM et al. Retatrutide, a triple hormone receptor agonist, for treatment of obesity (TRIUMPH-1). N Engl J Med 2026;395:1781-1793. phase 3; treatment-regimen estimand
- 4Bellido V et al. Retatrutide in adults with obesity and type 2 diabetes (TRIUMPH-2). Lancet 2026;408:1455-1472. phase 3; treatment-regimen estimand
- 5Jastreboff AM et al. Triple-hormone-receptor agonist retatrutide for obesity: a phase 2 trial. N Engl J Med 2023;389:514-526
- 6Urva S et al. LY3437943, a novel triple GIP, GLP-1, and glucagon receptor agonist in people with type 2 diabetes: a phase 1b trial. Lancet 2022;400:1869-1881. pharmacokinetics (half-life)
- 7FDA: FDA's concerns with unapproved GLP-1 drugs used for weight loss. content current as of October 1, 2026
- 8MHRA: Counterfeit weight loss pens worth £2 million seized by MHRA and West Midlands Police. published October 15, 2026
- 9Health Canada: Think twice before injecting peptides bought online (public advisory RA-81874). published April 9, 2026
- 10Health Canada Drug Product Database, active ingredient search for retatrutide. no products found, searched November 9, 2026
- 11Rosenstock J et al. Retatrutide, a GIP, GLP-1 and glucagon receptor agonist, for people with type 2 diabetes: a phase 2 trial. Lancet 2023;402:529-544
This page explains what labels and published studies say. It isn't medical advice and doesn't replace your prescriber or pharmacist.
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