Retatrutide Trial Results Explained: All Phase 3 Data (2026)

Five Phase 3 trials of retatrutide have reported, and all five met their primary endpoints. Average weight loss on the highest dose ranged from 16.8% to 28.7% depending on who was studied, reaching 30.3% at two years in one extension group.

Retatrutide is still investigational. Eli Lilly plans to submit it to the FDA in the first quarter of 2027. None of these results mean it can be prescribed in Ireland or anywhere else.

This article walks through each trial, then explains how to read the numbers, because headline figures in this area are easy to misread.

Where it started: Phase 2

The Phase 2 trial, published in the New England Journal of Medicine in June 2023, enrolled 338 adults with obesity, or overweight with a weight-related condition. At the highest dose, participants lost up to 24.2% of body weight at 48 weeks.

That result, larger than anything previously seen in this drug class at that stage, is what made retatrutide the most closely watched obesity drug in development.

The Phase 3 results, trial by trial

TRIUMPH-4: obesity and knee osteoarthritis (December 2025)

The first Phase 3 readout. Participants on the highest dose lost an average of 28.7% of body weight at 68 weeks, with meaningful reductions in knee pain.

This trial also produced the first clear signal of dysesthesia, meaning altered skin sensation, tingling or tenderness to touch, reported by roughly one in five participants on the top dose.

TRANSCEND-T2D-1: type 2 diabetes (March 2026)

The first dedicated diabetes trial, in 537 people with type 2 diabetes. On the highest dose over 40 weeks, HbA1c fell by 2.0 percentage points and body weight by 16.8%.

TRIUMPH-1: the pivotal obesity trial (May 2026)

This is the trial the regulatory submission will rest on, because it studies the population the drug would mainly be prescribed to. It randomised 2,339 adults with obesity or overweight and at least one weight-related condition, without diabetes, and followed them for 80 weeks. The placebo group lost 3.9%.

Coverage of the top-dose result shows two different figures, 28.3% in Lilly's announcement and 25.0% elsewhere. The section below on estimands explains why.

Around 45% of participants on the highest dose lost at least 30% of their body weight, a level historically associated with bariatric surgery. In an extension for participants starting with a BMI of 35 or above, average weight loss reached 30.3% at 104 weeks. A sleep apnoea substudy, presented in June 2026, reported a 60.6% reduction in breathing interruptions from a severe starting point.

Discontinuation due to side effects rose with dose, reaching roughly 11% on the highest dose against about 5% on placebo.

TRIUMPH-2: obesity with type 2 diabetes (July 2026)

1,152 adults with type 2 diabetes and obesity or overweight, over 80 weeks. Average weight loss was 12.7%, 19.1% and 20.8% across the three doses, against 4.0% on placebo. Blood sugar control improved alongside.

TRIUMPH-3: obesity with established heart disease (July 2026)

1,949 adults with a BMI of 35 or above and established cardiovascular disease, such as a previous heart attack or stroke. Average weight loss reached 22.6% at 80 weeks.

On the highest dose, triglycerides fell by 37%, non-HDL cholesterol by 16.5%, systolic blood pressure by 9.3 mmHg, waist circumference by about 19 cm, and hsCRP, an inflammation marker, by 51%.

Major cardiovascular events such as heart attacks and strokes occurred less often than expected in both the treatment and placebo groups. That sounds positive, but it means the trial had too few events to show whether retatrutide actually reduces them.

How to read these numbers

Why results range from 16.8% to 30%

The differences come from who was studied and for how long, not from inconsistency in the drug. People with type 2 diabetes lose less weight on every medicine in this class, which is why TRIUMPH-2 and TRANSCEND-T2D-1 sit at the lower end. Trial length matters too: 40 weeks, 80 weeks and 104 weeks are not comparable figures.

Why the same trial can report two numbers

Obesity trials usually report results two ways, known as estimands.

The efficacy estimand estimates what happens if people take the drug as intended. The treatment-regimen estimand counts everyone who was randomised, including those who stopped or missed doses.

Company announcements tend to lead with the efficacy figure, which is always higher. The treatment-regimen figure is closer to what happens in ordinary practice, where some people stop. The gap between 28.3% and 25.0% in TRIUMPH-1 coverage is consistent with that difference.

When comparing any two weight-loss drugs, check that both numbers use the same estimand. Mixed comparisons are common and misleading.

Topline results are not the full paper

Most of these results were first released as topline announcements, written by the sponsor, choosing which figures to highlight. Full data, including detailed side effect tables, arrive with peer-reviewed publication and conference presentation, and only some have been published in full so far. Topline results are rarely wrong, but they are never the complete picture.

Weight loss is a surrogate measure

Falling cholesterol, blood pressure and inflammation markers are encouraging, but they are not the same as fewer heart attacks, strokes or deaths. That question belongs to TRIUMPH-Outcomes, a cardiovascular and kidney trial of roughly 10,000 participants, which will not report before 2027.

Trial doses are not a guide

The 4 mg, 9 mg and 12 mg figures describe trial arms, not anything a reader should act on. Participants escalated gradually over weeks, under medical supervision, with regular monitoring and the option to stop.

What is still to come

TRIUMPH-5 compares retatrutide head to head with tirzepatide, which will answer the question most people actually have: is it better than what is already available? TRIUMPH-Outcomes will address cardiovascular and kidney events. Both are multi-year studies with earliest readouts from 2027.

Lilly plans to file with the FDA in Q1 2027. European authorisation typically follows US approval by six to twelve months, so availability on prescription in Ireland before 2028 is unlikely.

If you are considering retatrutide

For the approval timeline, regulatory position and why buying it online is a bad idea, see our guide to retatrutide's current status.

Approved alternatives exist now. Semaglutide, tirzepatide and oral orforglipron are all prescription medicines available through a GP, with known manufacturing, known dosing and medical supervision.

About this article

This is a summary of published and announced trial results. It is not medical advice and it is not legal advice. Nothing here is a recommendation to use retatrutide or any other medicine.

Results will be refined as full papers are published. Check current sources before relying on any figure above.