Retatrutide: what the trials show so far

What retatrutide is

Retatrutide is a single synthetic peptide that activates three hormone receptors at once: GIP, GLP-1 and glucagon.

It is injected under the skin once a week. Its half-life is about 6 days.

Across Lilly's completed trials registered on ClinicalTrials.gov, about 7,900 people have been enrolled, with roughly three quarters randomized to retatrutide (an estimate, as allocations vary by study). Every published clinical trial of retatrutide was run and funded by Eli Lilly. No independent randomized controlled trial exists. It is not approved in any country as of November 2026.

Phase 2: up to 24% weight loss at 48 weeks

The first large obesity trial randomized 338 adults to weekly doses of 1 to 12 mg or placebo for 48 weeks. The 12 mg group lost 24.2% of body weight versus 2.1% on placebo.

A separate phase 2 trial in 281 adults with type 2 diabetes tested retatrutide against placebo and dulaglutide over 36 weeks. HbA1c fell by up to 2.0 percentage points at 24 weeks on retatrutide, against almost no change on placebo, and results held at 36 weeks. A body composition substudy enrolled 189 participants, 103 of whom had DXA scans at both the start and 36 weeks. Fat mass fell 15 to 26% on 4 mg and higher, against 4.5% on placebo.

TRIUMPH-1: 25% weight loss in people without diabetes

This is the largest trial so far. It randomized 2,339 adults with obesity but not diabetes to 4, 9 or 12 mg weekly or placebo for 80 weeks. Weight loss, counting everyone who started (including dropouts), was 17.6% on 4 mg, 23.7% on 9 mg and 25.0% on 12 mg versus 3.9% on placebo.

TRIUMPH-2: smaller effect with type 2 diabetes

In 1,152 adults who had both obesity and type 2 diabetes, the same doses over 80 weeks produced 11.9%, 16.8% and 18.8% weight loss versus 5.1% on placebo. That is less weight loss than in TRIUMPH-1, though the two trials enrolled different people.

Breathing pauses dropped in both sleep apnea sub-trials

Prespecified sub-trials within TRIUMPH-1 and TRIUMPH-2 measured sleep apnea. In 243 people in the TRIUMPH-1 sub-trial, apnea events fell by about 23 to 34 per hour on retatrutide (depending on dose) versus about 10 per hour on placebo. In a separate group of 448 people from TRIUMPH-2 who had type 2 diabetes and sleep apnea, the reduction was about 19 to 25 events per hour versus about 10 on placebo.

Blood sugar fell in TRANSCEND-T2D-1

This phase 3 trial enrolled 537 adults with type 2 diabetes for 40 weeks. HbA1c dropped 1.69 to 1.94 percentage points on retatrutide versus 0.81 on placebo. Body weight fell 11.5 to 15.3% versus 2.6%.

Liver fat fell sharply in a phase 2 substudy

A phase 2a imaging substudy looked at 98 people with fatty liver disease. At 24 weeks, liver fat fell 81.4% on 8 mg and 82.4% on 12 mg, compared with a 0.3% rise on placebo. Between 79% and 86% of people on those doses reached normal liver fat levels.

This is imaging data from a phase 2 substudy with 98 people. No liver biopsy outcomes have been published.

What we do not know yet

Long-term safety is the biggest gap. Published data covers 80 weeks at most. A cardiovascular and kidney outcomes trial in about 10,000 people is running and expected to report around 2029.

Heart rate increased during the phase 2 obesity trial, peaking at about 24 weeks and then declining. Whether that matters over years is unanswered.

No head-to-head comparison with tirzepatide has reported results. A trial called TRIUMPH-5 is comparing the two in about 800 adults, with a listed primary completion date of November 2026. Comparing weight loss percentages across separate trials is unreliable, so any "retatrutide beats tirzepatide" claim has no direct data behind it.

It is not approved anywhere

Retatrutide is not approved in any country as of November 2026.

An expanded-access program exists for individual patients whose doctor applies on their behalf. It covers adults with a BMI of 35 or more who are already on the highest dose of an approved weight-loss treatment, have at least two serious weight-related conditions, and cannot join a trial.

Retatrutide is prohibited at all times by the World Anti-Doping Agency under category S0 (unapproved substances).

Anything sold online as "retatrutide" is not produced by Lilly. We found no published independent lab analysis of gray-market vials. One case report describes a person with type 1 diabetes who developed near-diabetic ketoacidosis after injecting online-bought "retatrutide." The authors could not confirm whether the product caused it. They note retatrutide has no established role in type 1 diabetes.

Doses used in the studies

Phase 3 trials used 4, 9 or 12 mg injected under the skin once a week for up to 80 weeks. Phase 2 tested 1 to 12 mg, with starting doses of 2 or 4 mg. Starting at 2 mg instead of 4 mg partly reduced gut side effects.

Side effects in the trials

Gut problems are the most common. In TRIUMPH-2, diarrhea affected 27 to 34% on retatrutide versus 13% on placebo, and nausea 14 to 28% versus 8%.

Skin tingling or burning (called dysesthesia) became more common at higher doses. In TRIUMPH-2 it affected 4 to 7% on retatrutide versus 1% on placebo.

More people quit the higher doses because of side effects. In TRIUMPH-2, 4% on 4 mg, 12% on 9 mg and 8% on 12 mg stopped because of side effects or death, versus 5% on placebo.

Where this stands

The weight loss is real: about 25% at 80 weeks on 12 mg, confirmed in a phase 3 trial with over 2,300 people. The blood sugar and sleep apnea results come from phase 3 trials. The liver fat result comes from a 98-person phase 2 imaging substudy. Retatrutide is still unapproved, only Lilly has studied it, nothing beyond 80 weeks is published, and up to 12% on the higher doses in TRIUMPH-2 stopped because of side effects. If you see it sold online, it is not the same product used in these trials.

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