TLDR
Retatrutide has now produced significant improvements in both glycemic control and body weight in peer-reviewed Phase 2 and Phase 3 trials involving people with type 2 diabetes. The 2023 Phase 2 study found A1C reductions of about 2 percentage points at the higher doses and weight loss approaching 17% after 36 weeks.
The evidence became considerably stronger in June 2026, when the first Phase 3 diabetes trial, TRANSCEND-T2D-1, was published in The Lancet. Among 537 adults with relatively early type 2 diabetes controlled inadequately with diet and exercise alone, retatrutide reduced A1C by 1.69% to 1.94% under the treatment-regimen analysis and produced 11.5% to 15.3% average weight reduction at 40 weeks. No severe hypoglycemia was reported.
Those results are promising, but they apply to specific study populations. TRANSCEND-T2D-1 primarily studied people with relatively recent diabetes who were not taking glucose-lowering medication. Separate Phase 3 trials are studying retatrutide against semaglutide in people taking metformin and in patients using basal insulin with moderate or severe kidney impairment.
Retatrutide remains investigational and is not currently FDA approved.
Why Type 2 Diabetes Is an Important Test of Retatrutide’s Triple-Agonist Design
Retatrutide is designed to activate three receptors with one peptide:
GIPR, the glucose-dependent insulinotropic polypeptide receptor;
GLP-1R, the glucagon-like peptide-1 receptor;
and GCGR, the glucagon receptor.
The first two make intuitive sense in a diabetes drug.
GIP and GLP-1 are incretin hormones. Their receptors participate in glucose-dependent insulin secretion and other metabolic responses following nutrient intake.
The glucagon receptor is more complicated.
Glucagon can increase hepatic glucose production. At first glance, deliberately activating the glucagon receptor in someone with type 2 diabetes seems counterproductive.
Retatrutide was engineered around the idea that the combined effects of all three receptors could produce a different net result. The original development work found strong GIP receptor activity together with GLP-1 and glucagon receptor agonism. In preclinical models, the complete molecule improved glycemic control while also producing substantial weight reduction.
Human trials now show that the net effect can include substantial A1C lowering.
But that does not mean glucagon’s glucose-raising biology has somehow disappeared. It means the balance among the three receptor systems matters.
What Is A1C and Why Is It Used in Retatrutide Trials?
Hemoglobin A1C, usually written HbA1c or A1C, is a measure of glycated hemoglobin.
Because red blood cells circulate for months, A1C provides an estimate of average blood glucose exposure over a longer period than one fasting glucose measurement.
A person can therefore have a normal-looking glucose value on one particular morning while still having an elevated A1C.
That makes A1C one of the central endpoints in diabetes drug trials.
A change from:
8.0% to 6.0%
represents a reduction of 2 percentage points.
It should not be described as a 25% reduction in A1C. Clinical diabetes trials usually report the absolute percentage-point change.
The First Dedicated Type 2 Diabetes Trial Enrolled 281 Participants
The first major dedicated retatrutide diabetes study was published in The Lancet in 2023.
It was a Phase 2 randomized, double-blind trial conducted at 42 sites in the United States.
Researchers enrolled 281 adults with type 2 diabetes.
Participants had:
A1C between 7.0% and 10.5%,
BMI between 25 and 50,
and diabetes managed either with diet and exercise or a stable dose of metformin.
Their average age was approximately 56 years, and the mean duration of diabetes was about 8.1 years.
That duration matters when comparing Phase 2 with the later Phase 3 trial.
Phase 3 TRANSCEND-T2D-1 studied people with much earlier diabetes, averaging only about 2.5 years since diagnosis.
Phase 2 Included an Active Diabetes-Drug Comparator
The study was more informative than a simple retatrutide-versus-placebo trial.
Participants received either placebo, the established GLP-1 receptor agonist dulaglutide 1.5 mg, or one of several retatrutide regimens.
Retatrutide maintenance doses included:
| Retatrutide regimen | Dose-escalation approach |
|---|---|
| 0.5 mg | 0.5 mg |
| 4 mg | Started at 2 mg then escalated |
| 4 mg | No escalation |
| 8 mg | Started at 2 mg |
| 8 mg | Started at 4 mg |
| 12 mg | Started at 2 mg |
The primary endpoint was change in A1C at week 24.
Participants continued through week 36 so researchers could also study longer-term glycemic control and weight change.
Higher-Dose Retatrutide Reduced A1C by About Two Percentage Points
At week 24, average A1C changes were approximately:
| Group | A1C Change |
|---|---|
| Retatrutide 0.5 mg | -0.43% |
| Retatrutide 4 mg, escalation | -1.39% |
| Retatrutide 4 mg, no escalation | -1.30% |
| Retatrutide 8 mg, slower escalation | -1.99% |
| Retatrutide 8 mg, faster escalation | -1.88% |
| Retatrutide 12 mg | -2.02% |
| Dulaglutide 1.5 mg | -1.41% |
| Placebo | -0.01% |
The 8 mg slower-escalation and 12 mg groups produced statistically greater A1C reductions than dulaglutide 1.5 mg under the study’s prespecified analyses.
This provided an early indication that triple-receptor agonism could produce strong glucose lowering despite including glucagon receptor activation.
Why Isn’t the Glucagon Component Raising A1C?
This is probably the most interesting biological question in retatrutide diabetes research.
Glucagon receptor activation can stimulate hepatic glucose output.
The original retatrutide experiments confirmed that the molecule could activate glucagon receptors in human hepatocyte models and stimulate glucose output. So the glucagon component is pharmacologically real.
But retatrutide simultaneously activates GIP and GLP-1 receptors.
Both can support glucose-dependent insulin responses. GLP-1 pharmacology can also influence glucagon secretion and several other components of glucose regulation.
Then there is weight loss.
Reducing excess adiposity can improve insulin sensitivity and reduce the metabolic burden contributing to hyperglycemia.
The clinical result is therefore the sum of several processes occurring simultaneously.
The Phase 2 and Phase 3 data show that, at the receptor balance engineered into retatrutide, the net clinical effect is glucose lowering rather than worsening hyperglycemia.
That does not establish that any amount of glucagon receptor activation would have the same result. Relative receptor potency is part of the drug design.
Weight Loss Was Also Large in Phase 2
The diabetes trial produced substantial weight changes.
At 36 weeks, mean body-weight reductions were approximately:
| Group | Average Weight Change |
|---|---|
| Retatrutide 0.5 mg | -3.19% |
| Retatrutide 4 mg, escalation | -7.92% |
| Retatrutide 4 mg, no escalation | -10.37% |
| Retatrutide 8 mg, slower escalation | -16.81% |
| Retatrutide 8 mg, faster escalation | -16.34% |
| Retatrutide 12 mg | -16.94% |
| Dulaglutide 1.5 mg | -2.02% |
| Placebo | -3.00% |
For retatrutide doses of 4 mg or greater, weight loss was statistically greater than both placebo and dulaglutide.
The highest-dose groups were still losing weight at the end of the 36-week trial.
That helped motivate longer Phase 3 studies.
Phase 2 Also Showed Why Dose Escalation Matters
Gastrointestinal adverse events were the main tolerability issue.
Across retatrutide groups, these included nausea, diarrhea, vomiting, and constipation.
The faster 8 mg escalation regimen produced gastrointestinal events in about half of participants, compared with substantially lower rates in some more gradual regimens.
That helped shape the Phase 3 titration strategy.
Rather than starting participants directly at 9 or 12 mg, later trials began with a low dose and increased it gradually.
This is a useful example of what Phase 2 development is supposed to accomplish. Researchers were not only trying to establish whether retatrutide worked. They were learning how to administer the molecule more tolerably in subsequent experiments.
Phase 3 TRANSCEND-T2D-1 Was Published in 2026
The next major step was TRANSCEND-T2D-1.
Unlike some of the newer retatrutide obesity data that still exist mainly as topline announcements, TRANSCEND-T2D-1 is now fully peer reviewed.
The Phase 3 study was published in The Lancet in June 2026.
It included 537 adults at 48 sites in the United States, Mexico, and India.
Participants were randomized to:
retatrutide 4 mg,
retatrutide 9 mg,
retatrutide 12 mg,
or placebo.
Treatment lasted 40 weeks.
The Phase 3 Population Was Different From Phase 2
This detail is critical.
Participants entered TRANSCEND-T2D-1 with a mean:
A1C of 7.9%
BMI of 35.8
age of 48.8 years
and diabetes duration of only:
2.5 years.
The study was essentially testing retatrutide as monotherapy in adults whose type 2 diabetes was inadequately controlled by diet and exercise.
They were not a population with long-standing diabetes already taking several glucose-lowering medications.
That makes the study scientifically clean.
But it also limits how far its results can be generalized.
Phase 3 Confirmed Large A1C Reductions
The peer-reviewed paper’s treatment-regimen estimand reported average A1C reductions of:
| Group | A1C Change at 40 Weeks |
|---|---|
| Retatrutide 4 mg | -1.69% |
| Retatrutide 9 mg | -1.86% |
| Retatrutide 12 mg | -1.94% |
| Placebo | -0.81% |
All three doses were statistically superior to placebo.
The placebo group’s 0.81-percentage-point reduction is worth noticing.
Participants were in a clinical trial involving diet and exercise, and trial participation itself can alter behavior and metabolic outcomes.
This is precisely why randomized placebo-controlled studies are more informative than simply observing A1C before and after taking a drug.
The relevant comparison is not just:
1.94% reduction with 12 mg.
It is also the estimated difference versus placebo, which was approximately 1.12 additional percentage points under the treatment-regimen analysis.
The Highest A1C Effect Was Not Perfectly Dose Linear
Under Lilly’s efficacy estimand, reported A1C reductions were:
4 mg: -1.7%
9 mg: -2.0%
12 mg: -1.9%
versus placebo at -0.8%.
So the 12 mg dose did not produce clearly greater glucose lowering than 9 mg.
That is an important nuance.
Retatrutide’s weight-loss response continued increasing with dose, but its A1C effect appears closer to a plateau at the higher doses in this population.
This makes biological sense. Once glucose approaches a near-normal range, there is less room for further reduction.
More drug does not necessarily produce proportionally lower A1C forever.
Many Participants Reached Near-Normal Glycemic Levels
Additional analyses presented with the Phase 3 results showed that up to 85% of retatrutide-treated participants achieved A1C of 6.5% or lower, while up to 46% reached A1C below 5.7%.
Up to 68% simultaneously achieved A1C of 6.5% or lower and at least 10% body-weight reduction.
An A1C below 5.7% is commonly considered within the normal laboratory range.
But reaching that number during active pharmacological treatment should not be confused with proving that type 2 diabetes has permanently disappeared.
The trial measured glycemic control while participants remained on treatment.
Phase 3 Weight Loss Reached About 15% to 17%
Under the treatment-regimen analysis published in The Lancet, average body-weight changes at week 40 were:
4 mg: -11.5%
9 mg: -13.9%
12 mg: -15.3%
placebo: -2.6%.
Under the efficacy estimand, the 12 mg result was somewhat larger at 16.8%, or about 36.6 pounds from a baseline weight of roughly 213.6 pounds.
The weight curve had not clearly plateaued by week 40.
That is relevant because the Phase 3 diabetes trial was shorter than the 80-week Phase 3 obesity studies.
It is therefore possible that longer treatment could produce further average weight reduction.
Why People With Type 2 Diabetes Tend to Lose Less Weight
Retatrutide follows a pattern seen across modern incretin therapies.
Participants without type 2 diabetes tend to show larger average weight reductions than participants with diabetes.
For example, the Phase 3 TRIUMPH-1 obesity population without diabetes reported an efficacy-estimand weight reduction of 28.3% at 80 weeks with 12 mg.
TRANSCEND-T2D-1 reported 16.8% at only 40 weeks, while the longer TRIUMPH-2 study in obesity plus type 2 diabetes later reported about 20.8% at 80 weeks.
Some of the difference is study duration.
But diabetes itself appears to matter as well.
Possible contributors include differences in insulin physiology, glucose loss, medications, disease duration, adipose biology, and the underlying metabolic state.
The precise explanation is still being studied.
Severe Hypoglycemia Was Not Seen in the Phase 3 Monotherapy Trial
One concern with any strong glucose-lowering treatment is hypoglycemia.
In TRANSCEND-T2D-1, no severe hypoglycemia was reported.
That is consistent with an important feature of incretin pharmacology: insulin secretion through GIP and GLP-1 pathways is substantially glucose dependent.
But this result should not be generalized to every future clinical situation.
TRANSCEND-T2D-1 did not combine retatrutide with basal insulin or sulfonylureas.
Hypoglycemia risk may be different when a new glucose-lowering treatment is added to insulin.
That question is being studied separately.
Gastrointestinal Effects Remained the Main Adverse Events
At 40 weeks, common adverse events included nausea, diarrhea, and vomiting.
Lilly reported nausea in:
16.4% at 4 mg,
19.5% at 9 mg,
26.5% at 12 mg,
versus 3.7% with placebo.
Diarrhea occurred in 18.7%, 26.3%, and 22.8% across increasing retatrutide doses, compared with 4.5% with placebo.
Vomiting occurred in 15.7%, 15.0%, and 17.6%, compared with 2.2% with placebo. These events occurred mainly during escalation.
Adverse-event discontinuation remained comparatively low at 2.2%, 4.5%, and 5.1% across the three doses.
Dysesthesia Appeared but at Lower Rates Than in Some Obesity Trials
Retatrutide Phase 3 studies have also identified dysesthesia, meaning abnormal or altered sensation.
In TRANSCEND-T2D-1, dysesthesia occurred in:
4.5% at 4 mg,
2.3% at 9 mg,
4.4% at 12 mg,
and 0% with placebo.
The events were generally described as mild, and most resolved during treatment.
Those rates were lower than those reported in some of the obesity Phase 3 studies.
That variation is another reason complete safety evaluation needs data from the entire development program rather than one trial.
Retatrutide Also Improved Several Cardiometabolic Risk Markers
TRANSCEND-T2D-1 reported improvements in:
non-HDL cholesterol,
triglycerides,
and systolic blood pressure.
Those changes are biologically plausible because substantial weight loss and improved insulin sensitivity can influence several cardiovascular risk factors at once.
But improving risk markers is not the same as demonstrating fewer cardiovascular events.
Retatrutide’s cardiovascular and renal outcome research is being conducted separately.
TRANSCEND-T2D-1 Does Not Answer Every Diabetes Question
The Phase 3 result is strong, but the population was deliberately narrow.
It does not yet tell us exactly how retatrutide compares with current therapies in people already taking common diabetes medications.
That is why the TRANSCEND program contains additional studies.
TRANSCEND-T2D-2 Is Directly Comparing Retatrutide With Semaglutide
TRANSCEND-T2D-2 is especially important.
It is a Phase 3 randomized study involving about 1,250 adults whose diabetes remains inadequately controlled on metformin, with or without an SGLT2 inhibitor.
Rather than placebo, the comparator is semaglutide.
As of September 2026, the trial is active but no longer recruiting, with primary completion listed for August 2026 and overall completion expected in early 2027. No results are currently posted on ClinicalTrials.gov.
That study will answer a much more clinically relevant comparative question:
How does triple GIP/GLP-1/glucagon receptor agonism compare directly with established GLP-1 receptor agonism in people already receiving standard diabetes treatment?
TRANSCEND-T2D-3 Is Studying a Much Higher-Risk Population
TRANSCEND-T2D-3 addresses another gap.
It includes people with:
type 2 diabetes,
moderate or severe kidney impairment,
and inadequate glycemic control despite basal insulin, with or without metformin and/or an SGLT2 inhibitor.
The trial has enrolled roughly 320 participants.
This population is substantially different from the relatively early, medication-naïve diabetes population in TRANSCEND-T2D-1.
It should provide useful information about:
hypoglycemia,
insulin requirements,
renal impairment,
and efficacy in more advanced diabetes.
What the Type 2 Diabetes Evidence Supports Today
Retatrutide now has a credible diabetes evidence base extending from mechanistic research through Phase 2 and Phase 3 randomized trials.
The evidence supports several conclusions.
Retatrutide can substantially lower A1C.
It can simultaneously produce large reductions in body weight.
The strongest A1C effects appear close to a plateau at higher doses, while weight reduction continues to increase with dose.
And in a Phase 3 monotherapy population, strong glucose lowering occurred without severe hypoglycemia.
What remains unresolved is just as important.
We do not yet have published head-to-head Phase 3 data against semaglutide.
We do not yet have completed Phase 3 evidence in insulin-treated patients with significant kidney disease.
Long-term microvascular and cardiovascular outcomes remain under study.
And retatrutide is still investigational.
The Phase 3 diabetes results are therefore a major step forward, but they are not the end of the research program.
FAQs
How much did retatrutide lower A1C?
In the peer-reviewed Phase 3 TRANSCEND-T2D-1 trial, treatment-regimen average A1C reductions ranged from 1.69 to 1.94 percentage points at 40 weeks, compared with 0.81 points with placebo.
Did any participants reach normal A1C?
Yes. Additional Phase 3 reporting indicated that up to 46% reached A1C below 5.7% during treatment. That does not prove permanent diabetes remission after medication is stopped.
How much weight did people with type 2 diabetes lose?
In TRANSCEND-T2D-1, average treatment-regimen weight reductions ranged from 11.5% to 15.3% at 40 weeks. The efficacy-estimand result at 12 mg was 16.8%.
Did retatrutide cause severe hypoglycemia?
No severe hypoglycemia was reported in TRANSCEND-T2D-1. The trial primarily studied retatrutide as monotherapy rather than in combination with insulin.
Has retatrutide been compared directly with semaglutide for diabetes?
A Phase 3 study, TRANSCEND-T2D-2, is directly comparing the two in people taking metformin with or without an SGLT2 inhibitor. As of September 2026, results are not yet posted.
Is retatrutide approved for type 2 diabetes?
No. Retatrutide remains investigational.
References
Coskun T, Urva S, Roell WC, et al. LY3437943, a novel triple glucagon, GIP, and GLP-1 receptor agonist for glycemic control and weight loss: From discovery to clinical proof of concept. Cell Metabolism. 2022. Read the original retatrutide discovery paper
Rosenstock J, Frias J, Jastreboff AM, et al. Retatrutide, a GIP, GLP-1 and glucagon receptor agonist, for people with type 2 diabetes: a randomised, double-blind, placebo and active-controlled, phase 2 trial. The Lancet. 2023. Read the Phase 2 diabetes study
Bajaj HS, Welch M, Shah P, et al. Efficacy and safety of retatrutide in people with type 2 diabetes and inadequate glycaemic control with diet and exercise (TRANSCEND-T2D-1). The Lancet. 2026. Read the Phase 3 TRANSCEND-T2D-1 study
ClinicalTrials.gov. TRANSCEND-T2D-1, NCT06354660. View the Phase 3 trial record
ClinicalTrials.gov. TRANSCEND-T2D-2: Retatrutide Compared With Semaglutide. View the retatrutide vs. semaglutide trial
ClinicalTrials.gov. TRANSCEND-T2D-3: Retatrutide in Type 2 Diabetes With Renal Impairment and Basal Insulin. View the TRANSCEND-T2D-3 trial