Retatrutide TRIUMPH-1 Phase 3: What the 25% Data Means

This article is for research and educational purposes only. Not medical advice.

What Is Retatrutide?

Retatrutide (LY3437943, Eli Lilly) is a once-weekly injectable peptide that simultaneously activates three hormone receptors: the glucagon-like peptide-1 (GLP-1) receptor, the glucose-dependent insulinotropic polypeptide (GIP) receptor, and the glucagon (GCG) receptor. This triple agonism is what distinguishes it from all currently approved weight loss medications.

• GLP-1 receptor activation suppresses appetite, slows gastric emptying, and improves insulin secretion, the same mechanism as semaglutide (Ozempic/Wegovy).
• GIP receptor activation adds to appetite reduction and may improve the sensitivity of GLP-1 receptor signalling, the same component tirzepatide (Mounjaro) adds to GLP-1 monotherapy.
• Glucagon receptor activation is the new element. Glucagon is primarily known for raising blood glucose, but at the receptor level it also increases energy expenditure and promotes fat oxidation in the liver. Activating this receptor in combination with GLP-1 and GIP appears to drive deeper fat loss than either dual or single agonism achieves.

The addition of glucagon agonism is the hypothesis behind why retatrutide produces weight loss numbers that appear to exceed tirzepatide in both Phase 2 and Phase 3 data.

The TRIUMPH-1 Trial: Design and Population

TRIUMPH-1 is a Phase 3, randomised, double-blind, placebo-controlled trial sponsored by Eli Lilly. Its topline results were announced on 21 May 2026.

• Participants: 2,339 adults with obesity (BMI ≥30) or overweight (BMI ≥27) with at least one weight-related comorbidity, with no type 2 diabetes diagnosis
• Treatment arms: retatrutide 4 mg, 9 mg, or 12 mg weekly (all titrated upward from 2 mg over four-week intervals) versus placebo
• Primary endpoint: mean percent change in body weight from baseline at 80 weeks
• Study duration: 104 weeks total (80-week primary endpoint; 104-week secondary analyses)

The dose titration protocol (starting at 2 mg and escalating every four weeks) mirrors the approach used in Phase 2 and is designed to reduce the gastrointestinal side effects that are common to this class of drugs during the early weeks of treatment.

Headline Results: Dose-Response Weight Loss at 80 Weeks

At the 80-week primary endpoint, all three doses of retatrutide produced statistically significant and clinically substantial weight reductions compared to placebo:

Treatment	Mean weight loss at 80 weeks
Retatrutide 4 mg	17.6%
Retatrutide 9 mg	23.7%
Retatrutide 12 mg	25.0%
Placebo	3.9%

A 25% mean weight reduction at the highest dose is, by any measure, a landmark result. For context, a person starting at 120 kg would, on average, reach approximately 90 kg (a loss of around 30 kg) at the 12 mg dose. The dose-response relationship is strong and consistent: each higher dose produced meaningfully greater weight loss, with the difference between 9 mg and 12 mg smaller than the jump from 4 mg to 9 mg, suggesting the curve may be approaching a plateau at 12 mg without having fully reached it.

The Higher-BMI Subgroup: Up to ~30% at 104 Weeks

A pre-specified subgroup analysis of participants with higher baseline BMI (approximately n=532) who were escalated to the maximum tolerated dose showed weight reductions of up to approximately 30% of body weight by week 104. This is the figure that has attracted comparisons to bariatric surgery outcomes in reporting on TRIUMPH-1.

Several caveats apply to interpreting this number:

1. This is a subgroup result, not the primary endpoint. Subgroup analyses carry higher statistical uncertainty than trial-level primary endpoints.
2. "Maximum tolerated dose" means these participants were on the highest dose they could individually sustain, which may not be 12 mg for all. The subgroup definition selects for participants with both higher baseline BMI and adequate tolerability.
3. Week 104 is the full trial duration, not the primary 80-week endpoint. Weight loss at 104 weeks reflects continued treatment for an additional 24 weeks beyond the primary read.
4. The full peer-reviewed publication has not been released as of the time of writing. These are topline results from a press announcement. The peer-reviewed paper will provide full pre-specification details, confidence intervals, and subgroup definitions.

That said, the ~30% figure in this subgroup is consistent with what the Phase 2 data trajectory suggested was possible with longer treatment duration, the Phase 2 weight-loss curves had not plateaued at 48 weeks.

How This Compares to Existing GLP-1 Drugs

Cross-trial comparisons in obesity pharmacology are inherently observational. Trials differ in population, duration, baseline BMI, dose titration protocol, lifestyle co-intervention, and statistical methodology. With that important caveat stated:

Drug	Trial	Mean weight loss	Duration
Semaglutide 2.4 mg	STEP 1	14.9%	68 weeks
Tirzepatide 15 mg	SURMOUNT-1	20.9%	72 weeks
Retatrutide 12 mg (Phase 2)	NEJM 2023	24.2%	48 weeks
Retatrutide 12 mg (TRIUMPH-1)	Phase 3	25.0%	80 weeks

These cross-trial figures suggest a stepped improvement: GLP-1 monotherapy (semaglutide) to dual GLP-1/GIP agonism (tirzepatide) to triple GLP-1/GIP/GCG agonism (retatrutide). The approximately 5 percentage point gap between tirzepatide and retatrutide at Phase 3 is consistent with what the Phase 2 data suggested and with the hypothesis that glucagon receptor co-agonism drives additive fat loss.

These are not head-to-head comparisons and should not be read as such. Population differences, trial designs, and measurement approaches all differ.

Side Effects and the Discontinuation Trade-off

Adverse event-driven discontinuation rates in TRIUMPH-1 followed a clear dose-response pattern:

Dose	Discontinuation due to adverse events
Placebo	4.9%
Retatrutide 4 mg	4.1%
Retatrutide 9 mg	6.9%
Retatrutide 12 mg	11.3%

The most commonly reported adverse events at the highest dose were gastrointestinal: nausea (42.4%), diarrhoea (32.0%), constipation (26.1%), and vomiting (25.3%). These are consistent with the class-level profile of GLP-1 receptor agonists and were predominantly mild to moderate in severity.

The discontinuation rate of 11.3% at 12 mg is higher than what was seen with tirzepatide at 15 mg in SURMOUNT-1 (approximately 7%) and reflects the additional physiological load of glucagon receptor activation. The net question for clinical development is whether the additional weight loss at the highest doses justifies the higher discontinuation rate, or whether the 9 mg dose, which produced 23.7% weight loss with a more moderate 6.9% discontinuation rate, represents a better benefit-risk balance for most patients.

The Phase 2 NEJM data confirmed that the lower starting dose of 2 mg (rather than 4 mg) was an effective strategy for reducing early gastrointestinal burden, which is why TRIUMPH-1 adopted the 2 mg titration start.

What the Phase 2 Data Already Told Us

Retatrutide's Phase 2 trial, published in the New England Journal of Medicine on 26 June 2023, enrolled 338 adults and ran for 48 weeks. At the 12 mg dose, mean weight loss reached 24.2% at 48 weeks, and the weight loss curves had not plateaued, suggesting continued loss with longer treatment. That trajectory is consistent with the 25.0% observed at 80 weeks in the larger Phase 3 cohort, where the slower titration protocol (starting at 2 mg) likely explains why the Phase 3 number is only modestly higher despite a much longer treatment duration.

The Phase 2 paper noted: "Data are lacking from randomised clinical trials of 1 year or less in duration reporting this degree of weight reduction with antiobesity pharmacotherapeutics."

What Is Still Unknown

Weight regain on discontinuation. SURMOUNT-4 (tirzepatide) and STEP-4 (semaglutide) both showed that substantial weight regain follows discontinuation of GLP-1 class drugs. There is no reason to expect retatrutide to behave differently. The remaining TRIUMPH trials will need to examine this directly.

Long-term cardiovascular outcomes. Semaglutide demonstrated a 20% reduction in major adverse cardiovascular events in the SELECT trial. Tirzepatide cardiovascular outcomes data is still emerging. Retatrutide has no published cardiovascular outcomes data. The cardiovascular implications of glucagon receptor co-agonism (which includes modest increases in heart rate) need dedicated outcomes trial evaluation.

The full peer-reviewed publication. Topline press release data does not include the full statistical analysis, confidence intervals, pre-specification details, or subgroup definitions that the peer-reviewed publication will provide.

Remaining TRIUMPH trials. TRIUMPH-1 is the first Phase 3 readout. The TRIUMPH programme includes trials in patients with type 2 diabetes and other populations. The full scope of retatrutide's Phase 3 evidence base is not yet available.

Regulatory timeline. Eli Lilly has not announced a specific regulatory submission timeline for retatrutide as of the time of writing.

Summary

TRIUMPH-1 is the most impressive Phase 3 weight loss trial data published to date. A mean weight loss of 25.0% at 80 weeks at the 12 mg dose (in a trial of 2,339 participants without type 2 diabetes) validates the triple agonist hypothesis and extends the Phase 2 trajectory into a properly powered, long-duration study. The subgroup result of up to ~30% in higher-BMI participants at 104 weeks warrants interpretation with appropriate statistical caution but is directionally consistent with the biology. The dose-response discontinuation trade-off, particularly the 11.3% adverse event-driven dropout at 12 mg, will shape benefit-risk discussions in regulatory review. Retatrutide does not yet have an approved indication anywhere in the world; the path from these Phase 3 results to clinical availability involves full peer-review publication, additional TRIUMPH trial data, regulatory submission, and approval.

For background on how GLP-1 and dual GLP-1/GIP agonism work mechanistically (the foundation retatrutide builds on) see our explanation of how GLP-1 agonists work and the SURMOUNT data summary for tirzepatide. For the newly approved oral GLP-1 option, see the orforglipron (Foundayo) explainer.

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Sources:

1. Eli Lilly and Company. "Lilly Announces Positive Phase 3 TRIUMPH-1 Results for Retatrutide." Investor Relations press release, 21 May 2026. https://investor.lilly.com

2. Pharmaceutical Journal. "Phase III retatrutide study demonstrates 30% weight loss." May 2026. https://pharmaceutical-journal.com/article/news/phase-iii-retatrutide-study-demonstrates-30-weight-loss

3. Jastreboff AM, Aronne LJ, Ahmad NN, et al. "Triple–Hormone-Receptor Agonist Retatrutide for Obesity, A Phase 2 Trial." New England Journal of Medicine 2023; 389:514–526. Published 26 June 2023. https://pubmed.ncbi.nlm.nih.gov/37366315/