TLDR
Retatrutide has produced some of the largest average weight reductions reported in randomized obesity-drug trials, but the evidence needs to be read carefully.
The strongest peer-reviewed obesity result remains the 2023 Phase 2 trial: participants receiving 12 mg lost an average 24.2% of baseline body weight at 48 weeks. Higher doses produced greater weight loss, and the weight curves had not clearly plateaued when treatment ended.
Phase 3 has since reported larger results. In TRIUMPH-1, Lilly reported 28.3% average weight loss at 80 weeks with 12 mg using the trial’s efficacy estimand. The more pragmatic treatment-regimen analysis, which includes the effects of treatment discontinuation and prohibited weight-management treatments, showed 25.0% weight loss at the same dose.
A selected extension group with higher baseline BMI continued to 104 weeks and reached 30.3% average weight reduction with the 12 mg treatment assignment. That figure should not be interpreted as the result for the entire randomized TRIUMPH-1 population.
Retatrutide has also produced substantial weight loss in Phase 3 populations with type 2 diabetes, cardiovascular disease, and knee osteoarthritis. But as of September 2026, several of those Phase 3 findings remain company-reported or conference-presented results rather than complete peer-reviewed publications. Lilly says detailed TRIUMPH-2 and TRIUMPH-3 results are still intended for publication.
Retatrutide remains investigational and is not FDA approved.
What Is Retatrutide?
Retatrutide, previously called LY3437943, is an investigational peptide developed by Eli Lilly.
It activates three receptors involved in metabolic regulation:
GIPR, the glucose-dependent insulinotropic polypeptide receptor;
GLP-1R, the glucagon-like peptide-1 receptor;
and GCGR, the glucagon receptor.
That makes retatrutide a triple receptor agonist.
Its GLP-1 and GIP activity overlaps partly with pathways used by current incretin-based obesity medicines. The glucagon-receptor component is the major additional feature.
Preclinical research suggested that the combination could influence both food intake and energy expenditure, although the exact contribution of each receptor to human weight loss remains unresolved. The clinical trials demonstrate the effect of the complete retatrutide molecule, not the percentage of weight loss attributable independently to GIP, GLP-1, or glucagon signaling.
The First Major Weight-Loss Evidence Came From Phase 2
The pivotal early obesity study was published in the New England Journal of Medicine in 2023.
It was a randomized, double-blind, placebo-controlled Phase 2 trial involving 338 adults.
Participants had either a BMI of at least 30, or a BMI between 27 and 30 plus at least one weight-related condition. People with diabetes were not the population studied in this obesity trial.
Participants received once-weekly injections for 48 weeks.
The trial tested multiple retatrutide regimens:
1 mg,
4 mg,
8 mg,
and 12 mg.
Different starting-dose schedules were also tested at 4 mg and 8 mg because investigators were evaluating whether slower escalation could improve tolerability.
Phase 2 Showed a Strong Dose-Response Relationship
The results at 24 weeks already showed a clear relationship between retatrutide dose and weight reduction.
Average weight changes were:
| Group | 24 Weeks | 48 Weeks |
|---|---|---|
| Retatrutide 1 mg | -7.2% | -8.7% |
| Retatrutide 4 mg | -12.9% | -17.1% |
| Retatrutide 8 mg | -17.3% | -22.8% |
| Retatrutide 12 mg | -17.5% | -24.2% |
| Placebo | -1.6% | -2.1% |
The 4 mg and 8 mg figures combine the different dose-escalation groups.
Importantly, increasing the dose had a much larger effect at 48 weeks than simply looking at the early 24-week values might suggest. The higher-dose participants continued losing weight through the second half of the study.
The 24.2% Number Is the Main Peer-Reviewed Retatrutide Benchmark
The most frequently cited peer-reviewed retatrutide result is:
24.2% average weight reduction after 48 weeks with 12 mg.
That came from the randomized Phase 2 trial.
This number is scientifically important because, when published in 2023, the investigators noted that randomized pharmacotherapy trials of one year or less had not previously reported that magnitude of mean weight reduction.
But several qualifications matter.
The study was Phase 2.
The entire trial contained 338 participants across multiple treatment groups.
The 12 mg result therefore came from a much smaller group than today’s Phase 3 populations.
And 48 weeks is not enough time to establish long-term maintenance, cardiovascular outcomes, or uncommon safety events.
Phase 2 established a very strong efficacy signal.
Phase 3 was designed to test whether that signal persisted in thousands rather than hundreds of participants.
Many Participants Reached Large Weight-Loss Thresholds
The Phase 2 investigators also looked at how many participants crossed specific weight-loss thresholds.
At 48 weeks, among participants receiving 12 mg:
100% reached at least 5% weight reduction.
93% reached at least 10%.
83% reached at least 15%.
For the 8 mg group, the corresponding figures were 100%, 91%, and 75%.
The placebo group was substantially different: 27%, 9%, and 2% reached those thresholds.
These threshold analyses are useful because an average can hide substantial individual variation.
But they still do not mean every participant lost 24.2%.
That number is a group mean.
Some participants lost considerably more and others less.
Weight Loss Had Not Clearly Plateaued at 48 Weeks
One of the most interesting findings from Phase 2 was the shape of the weight curve.
Participants receiving the higher doses were still losing weight as the 48-week trial ended.
The investigators specifically noted that a plateau had not clearly been reached.
That raised an obvious question:
What happens if treatment continues longer?
TRIUMPH-1 later provided part of the answer.
TRIUMPH-1 Moved Retatrutide Into a Much Larger Phase 3 Population
TRIUMPH-1 was a randomized, double-blind, placebo-controlled Phase 3 study in adults with obesity or overweight plus a weight-related comorbidity, without type 2 diabetes.
The study compared:
4 mg retatrutide,
9 mg,
12 mg,
and placebo.
Participants began retatrutide at 2 mg and increased the dose every four weeks until reaching their target.
The trial enrolled roughly 2,300 participants, making it several times larger than the original Phase 2 study.
The primary weight analysis occurred at 80 weeks.
TRIUMPH-1 Reported 28.3% Weight Loss at 80 Weeks
Lilly reported the following results using the efficacy estimand:
| Group | Average Weight Change at 80 Weeks |
|---|---|
| Retatrutide 4 mg | -19.0% |
| Retatrutide 9 mg | -25.9% |
| Retatrutide 12 mg | -28.3% |
| Placebo | -2.2% |
The average baseline weight was approximately 248.5 pounds, with an average BMI around 40.
Participants receiving 12 mg lost an average of about 70.3 pounds under this analysis.
Those are very large mean changes.
But understanding the word estimand is essential before comparing the 28.3% figure with results from other trials.
What Does the 28.3% “Efficacy Estimand” Mean?
Modern obesity trials often present more than one statistical estimate of treatment effect.
TRIUMPH-1’s efficacy estimand estimated what the average effect would be if randomized participants remained on their study intervention, allowing certain dose modifications, and did not initiate prohibited weight-management treatment.
This is useful because it asks:
How effective was the medication when treatment was continued as intended?
But it is not the only clinically relevant question.
TRIUMPH-1 also reported a treatment-regimen estimand, which estimates average outcomes regardless of adherence to the assigned study intervention or initiation of prohibited weight-management therapy.
Under that analysis, average weight changes were:
4 mg: -17.6%
9 mg: -23.7%
12 mg: -25.0%
placebo: -3.9%.
Both analyses are legitimate.
They answer different questions.
The 28.3% figure is therefore not wrong, but presenting it without explaining the estimand makes the evidence look simpler than it is.
A Remarkable Number of Participants Lost 25% or More
TRIUMPH-1 also reported responder thresholds.
Under the efficacy analysis, among participants assigned to 12 mg:
62.5% reached at least 25% weight reduction.
45.3% reached at least 30%.
27.2% reached at least 35%.
Those percentages illustrate just how far the average weight-loss distribution moved at the highest dose.
Still, these remain trial-level results.
They do not allow prediction of exactly how any individual person would respond.
What Does the 30.3% Weight-Loss Result at 104 Weeks Mean?
The 30.3% figure requires even more context.
TRIUMPH-1 included a prespecified extension.
The extension did not simply continue every randomized participant to 104 weeks.
It enrolled 532 participants who had a baseline BMI of at least 35, completed the main study on study drug under the extension’s eligibility criteria, and entered an additional 24 weeks of treatment.
Participants were titrated toward their maximum tolerated dose where applicable.
Within the original 12 mg assignment, average weight reduction reached 30.3% at 104 weeks using the efficacy analysis.
This is scientifically interesting because it suggests that substantial additional weight reduction can continue beyond 80 weeks in some participants.
But it should not be summarized as:
“Retatrutide causes 30.3% average weight loss after two years.”
That wording suggests the entire randomized Phase 3 population produced the result.
It did not.
It came from a selected continuation cohort with higher baseline BMI and demonstrated ability to continue treatment.
The 104-Week Treatment-Regimen Estimate Was Slightly Lower
In the same extension, the treatment-regimen estimand for the original 12 mg assignment was 29.9% rather than 30.3%.
The difference is small in this selected extension population, but again it illustrates why reading the statistical definition behind an obesity-trial percentage matters.
TRIUMPH-4 Produced Another Very Large Phase 3 Result
TRIUMPH-4 studied a somewhat different group.
Participants had obesity or overweight plus knee osteoarthritis, and they did not have diabetes.
The trial included 445 participants and tested 9 mg, 12 mg, or placebo for 68 weeks.
About 84% of participants had a baseline BMI of at least 35.
Using the efficacy estimand:
9 mg produced 26.4% average weight reduction.
12 mg produced 28.7%.
Placebo produced 2.1%.
The treatment-regimen results were lower:
20.0% with 9 mg,
23.7% with 12 mg,
and 4.6% with placebo.
Once again, which number is being quoted matters.
Why TRIUMPH-4 Should Not Simply Be Combined With TRIUMPH-1
TRIUMPH-4 included people with symptomatic knee osteoarthritis and generally high BMI.
TRIUMPH-1 studied a broader obesity population.
The trials had different durations.
They had different populations.
And their treatment discontinuation patterns differed.
That means the 28.7% from TRIUMPH-4 and 28.3% from TRIUMPH-1 should not be treated as two identical repetitions of the same experiment.
They are separate Phase 3 studies that happen to produce similarly large efficacy-estimand weight reductions at the highest dose.
That replication across populations is encouraging.
But the study context remains important.
People With Type 2 Diabetes Lost Less Weight
This pattern first appeared in Phase 2.
In the dedicated Phase 2 type 2 diabetes study, the 12 mg retatrutide group lost approximately 16.9% at 36 weeks.
That was a major weight reduction, but smaller than the 24.2% observed at 48 weeks in the obesity trial without diabetes. The studies differed in both duration and population, so that is not a direct comparison.
Phase 3 showed the same general pattern.
In TRIUMPH-2, adults with obesity or overweight and type 2 diabetes receiving 12 mg lost an average 20.8% at 80 weeks under the efficacy estimand.
The 9 mg group lost 19.1%, while 4 mg produced 12.7%.
Placebo produced 4.0%.
Reduced weight-loss magnitude in people with type 2 diabetes has also been observed with other incretin-based obesity drugs.
The exact biological reasons are not fully resolved.
Retatrutide Also Produced Large Weight Loss in People With Cardiovascular Disease
TRIUMPH-3 studied adults with severe obesity and established cardiovascular disease, with or without type 2 diabetes.
The average baseline BMI was approximately 40.4.
At 80 weeks, Lilly reported:
9 mg: -21.6%
12 mg: -22.6%
placebo: -3.2%
using the efficacy estimand.
These results show that substantial weight reduction occurred even in a population with advanced cardiometabolic disease.
But TRIUMPH-3 was not large enough to establish whether retatrutide reduces heart attacks, strokes, or cardiovascular mortality.
The cardiovascular event confidence intervals were wide and included both possible benefit and no benefit.
A dedicated outcomes trial is needed for that question.
Greater Weight Loss Comes With a Tolerability Tradeoff
The weight-loss data cannot be separated from treatment tolerability.
In TRIUMPH-1, common adverse events increased with dose.
At 12 mg, Lilly reported:
nausea in 42.4%,
diarrhea in 32.0%,
constipation in 26.1%,
and vomiting in 25.3%.
Discontinuation due to adverse events occurred in 11.3% of the 12 mg group versus 4.9% with placebo.
An additional event that became more noticeable in Phase 3 was dysesthesia, an altered skin sensation.
TRIUMPH-1 reported dysesthesia in 12.5% of participants at 12 mg versus 0.9% with placebo.
TRIUMPH-4 reported an even higher 20.9% incidence at 12 mg in that study population. Most reported cases were described as mild to moderate.
Full peer-reviewed Phase 3 publications will be important for examining these safety signals in more detail.
Weight Loss Is Not the Same as Fat Loss
Body weight contains:
fat,
lean tissue,
bone,
water,
and other components.
Large weight reductions therefore do not represent pure fat loss.
A dedicated retatrutide body-composition study has found reductions in both fat mass and lean mass.
That study reported that the proportion of lean-mass loss relative to overall weight reduction was broadly similar to that seen with other obesity treatments rather than showing uniquely severe lean-tissue loss.
This deserves its own analysis because DXA “lean mass” is also not identical to skeletal muscle.
The important point here is simpler:
A 28% reduction in body weight is not a 28% reduction in body fat.
Are the Phase 3 Results Peer Reviewed Yet?
This is one of the most important distinctions in the current retatrutide literature.
The original Phase 2 obesity trial is fully peer reviewed and published in the New England Journal of Medicine.
The Phase 3 TRIUMPH program has progressed rapidly, and TRIUMPH-1 data were presented at the 2026 American Diabetes Association meeting.
But as of September 2026, Lilly still states that detailed TRIUMPH-2 and TRIUMPH-3 results will be presented and published later.
That does not make the Phase 3 numbers meaningless.
Large randomized Phase 3 trials produce valuable data before journal publication.
But a corporate topline announcement is not the same evidentiary object as a complete peer-reviewed manuscript with full methodology, supplementary analyses, adverse-event tables, and independent editorial review.
For now, articles should label the evidence accordingly.
What Does the Evidence Support Right Now?
The evidence strongly supports several conclusions.
Retatrutide produces large, dose-dependent reductions in body weight in randomized trials.
The effect has now been observed across Phase 2 and Phase 3 studies.
Higher-dose retatrutide has produced mean efficacy-estimand weight reductions in the mid-to-high 20% range in several non-diabetic obesity populations.
Participants with type 2 diabetes still experience substantial weight loss, but the average magnitude has been smaller.
And higher doses generally produce more adverse effects and more treatment discontinuation.
What has not yet been established is equally important.
Retatrutide is not FDA approved.
Long-term weight maintenance after stopping the drug has not been established.
Long-term cardiovascular and kidney outcomes remain under study.
The complete Phase 3 safety database has not yet appeared in peer-reviewed form.
And there is still no completed direct trial showing that retatrutide produces more weight loss than tirzepatide.
Why the Final Question Is Not Simply “How Much Weight Can Retatrutide Make Someone Lose?”
The clinical-trial percentages are useful.
But a mature assessment of an obesity treatment ultimately needs more than the biggest number on the scale.
Researchers need to understand:
the durability of treatment;
the amount and composition of weight lost;
tolerability;
long-term adherence;
cardiovascular outcomes;
kidney outcomes;
metabolic effects;
and uncommon adverse events.
Retatrutide’s weight-loss signal is already difficult to dispute.
The larger scientific question is now whether that very large reduction translates into an overall long-term risk-benefit profile that is better than existing treatments.
That question will require more than another weight-loss percentage.
FAQs
How much weight did people lose with retatrutide in the peer-reviewed obesity trial?
The highest-dose group in the 2023 Phase 2 trial lost an average of 24.2% of baseline body weight after 48 weeks.
Where does the 28.3% retatrutide figure come from?
It comes from the efficacy estimand in the Phase 3 TRIUMPH-1 trial at 80 weeks for the 12 mg dose. Lilly announced the result in May 2026.
Did every participant lose 28.3%?
No. That figure is a group average under a particular statistical estimand. Individual responses varied.
Did retatrutide really produce 30% weight loss?
A selected TRIUMPH-1 extension cohort with baseline BMI of at least 35 continued to 104 weeks. The original 12 mg assignment reached 30.3% average weight reduction under the efficacy estimand. It was not the result for the entire randomized TRIUMPH-1 population.
Does retatrutide work better in people without diabetes?
Average weight reductions have been larger in the obesity trials without type 2 diabetes than in trials involving type 2 diabetes. Similar patterns occur with other incretin-based therapies. Cross-trial comparisons cannot isolate the exact reason.
Has retatrutide weight loss plateaued?
The Phase 2 high-dose curves had not clearly plateaued at 48 weeks. TRIUMPH-1 showed additional weight reduction through 80 weeks and, in a selected extension population, through 104 weeks.
Is retatrutide FDA approved?
No. As of September 2026 it remains investigational.
References
Jastreboff AM, Kaplan LM, Frías JP, et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity: A Phase 2 Trial. New England Journal of Medicine. 2023. PubMed record
Jastreboff AM, Kaplan LM, Frías JP, et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity. New England Journal of Medicine. NEJM article
Eli Lilly and Company. TRIUMPH-1 Phase 3 obesity results. May 21, 2026. Lilly TRIUMPH-1 results
Eli Lilly and Company. TRIUMPH-2 and TRIUMPH-3 Phase 3 topline results. July 23, 2026. Lilly TRIUMPH-2 and TRIUMPH-3 results
Eli Lilly and Company. TRIUMPH-4 Phase 3 results. December 11, 2025. Lilly TRIUMPH-4 results
ClinicalTrials.gov. TRIUMPH-1, NCT05929066. ClinicalTrials.gov trial record
Rosenstock J, Frías JP, Jastreboff AM, et al. Retatrutide, a GIP, GLP-1 and glucagon receptor agonist, for people with type 2 diabetes. The Lancet. 2023. PubMed record