Retatrutide

Retatrutide is an investigational drug developed by Eli Lilly: a single engineered peptide that activates three hormone receptors, those for GIP, GLP-1 and glucagon, and is injected once a week. In phase 3 trials it has produced mean weight losses of up to about a quarter of body weight in adults with obesity, along with lower blood glucose in people with type 2 diabetes. As of November 2026 it is approved nowhere, and the trials that would show whether it prevents heart attacks, kidney failure or death are still running.

§How it works

Retatrutide extends the incretin approach behind the GLP-1 receptor agonists. GLP-1 agonists such as semaglutide act on one receptor; tirzepatide acts on two, GIP and GLP-1; retatrutide adds a third, the receptor for glucagon. Lilly's discovery paper describes it as a peptide with balanced glucagon and GLP-1 receptor activity in cell assays and stronger activity at the GIP receptor.¹ Its half-life in people with type 2 diabetes is about six days, which is what allows weekly dosing.²

The glucagon arm is the novel part, and it is counterintuitive because glucagon raises blood glucose. The rationale comes from animal work. In obese mice the drug's weight loss was larger than appetite suppression alone explained, and the authors attributed the extra loss to glucagon-driven increases in energy expenditure.¹ That is a mouse result. In people, the clearest sign of a distinct effect is in the liver: in a phase 2a substudy of 98 adults with fatty liver disease, the 12 mg dose cut liver fat by a mean 82.4% at 24 weeks, and 86% of that group reached normal liver fat levels, against none on placebo.³ The authors found the reductions tracked weight loss and metabolic improvements, so the trial does not separate a glucagon effect from the effect of losing weight.

§Development history

1. 2022
   Discovery and first human data
   Lilly scientists publish the molecule's design, mouse results and a single-dose study, followed by a 12-week phase 1b trial in 72 people with type 2 diabetes.
2. 2023
   Phase 2 results
   Trials in obesity and type 2 diabetes report in the New England Journal of Medicine and The Lancet; the TRIUMPH phase 3 obesity programme begins.
3. 2024
   Liver data and outcome trial
   A phase 2a substudy reports large falls in liver fat, and the event-driven TRIUMPH-Outcomes trial starts enrolling.
4. 2025
   First phase 3 readout
   In December Lilly announces topline TRIUMPH-4 results in knee osteoarthritis, reporting weight loss of up to 28.7% at 68 weeks and dysaesthesia in up to 20.9% on the top dose.
5. 2026
   Phase 3 results and papers
   Five more phase 3 trials report; peer-reviewed papers appear in The Lancet and the New England Journal of Medicine, and Lilly sets a US filing target of early 2027.

The two phase 2 trials set the expectations. In 338 adults with obesity, the 12 mg dose produced a mean weight loss of 24.2% at 48 weeks, against 2.1% on placebo.⁴ In 281 people with type 2 diabetes, the higher doses lowered HbA1c by about 2 percentage points at 24 weeks and cut body weight by about 17% at 36 weeks; the 12 mg dose outperformed the GLP-1 agonist dulaglutide on both measures.⁵

§Current state

By November 2026, Lilly had reported topline results from six phase 3 trials, in two programmes: TRIUMPH for obesity and its complications, and TRANSCEND for type 2 diabetes. Several have since appeared in peer-reviewed journals; others exist only as company announcements.

Figures from the papers use the intention-to-treat estimand; figures from press releases use the efficacy estimand, which runs higher (see the note below).

§Obesity and its complications

TRIUMPH-1 is the pivotal obesity trial. Its paper reports mean weight losses of 17.6%, 23.7% and 25.0% on 4, 9 and 12 mg over 80 weeks, against 3.9% on placebo. Nested inside it were 574 participants with knee osteoarthritis, whose pain scores improved more than on placebo, and 243 with obstructive sleep apnoea.⁶ A companion paper reported the sleep apnoea groups from TRIUMPH-1 and TRIUMPH-2. The apnoea-hypopnoea index, the hourly count of breathing pauses and shallow-breathing episodes, averaged 56-59 at baseline; at 80 weeks the 9 mg dose had lowered it by 34.3 in people without diabetes and by 24.7 in people with diabetes, against about 10 on placebo in both (hybrid estimand).⁷ Sleep apnoea's links to health are covered under Sleep and longevity.

A prespecified extension of TRIUMPH-1 continued 532 participants to 104 weeks. Lilly reports that those who stayed on 12 mg lost 30.3% of body weight on average (efficacy estimand). The extension admitted only people with a starting BMI of at least 35 who had completed 80 weeks on the drug without stopping or permanently reducing the dose, so it describes people who tolerated the drug well, not everyone who started it.⁸ Lilly's announcement described the 12 mg result as a level of weight loss long associated with bariatric surgery.

TRIUMPH-4, in knee osteoarthritis, reported weight losses of up to 28.7% at 68 weeks and a mean fall of 4.4-4.5 points on a 10-point pain scale. Pain also fell by 2.4 points on placebo, a large placebo response.⁹

§Type 2 diabetes

In TRANSCEND-T2D-1, people with diabetes diagnosed for a mean of 2.5 years and not taking glucose-lowering drugs saw HbA1c fall by 1.69-1.94 percentage points at 40 weeks, against 0.81 on placebo, with weight losses of 11.5-15.3%.¹⁰ TRIUMPH-2, in people with both obesity and diabetes, found smaller weight losses than TRIUMPH-1 did in people without diabetes: 18.8% on 12 mg at 80 weeks against 5.1% on placebo.¹¹

TRANSCEND-T2D-2 is the first phase 3 trial comparing retatrutide with another incretin. In 1,298 adults already taking metformin, both retatrutide doses lowered HbA1c by 2.2 points at 80 weeks, against 1.8 points on semaglutide 2 mg, the top dose of Ozempic, and produced weight losses of 19.5% and 20.9% against 9.4%. The trial was open-label, and as of November 2026 the results exist only as a Lilly announcement.¹² A second head-to-head trial, TRIUMPH-5, compares retatrutide with tirzepatide.¹²

§Heart, kidney and liver

TRIUMPH-3 enrolled people with severe obesity and established cardiovascular disease, but its primary endpoint was weight. Major cardiovascular events were rarer than expected in both arms, and the results, pooled across the 9 and 12 mg doses, run in both directions: a hazard ratio of 0.82 (95% CI 0.55 to 1.22) for a five-part composite including heart failure and revascularisation, and 1.12 (0.64 to 1.96) for the conventional three-part composite of cardiovascular death, heart attack and stroke.¹³ Neither excludes harm or benefit. The 12 mg dose lowered average high-sensitivity C-reactive protein, a marker of the low-grade inflammation discussed under Inflammaging, by 51.2%.¹³ The trial designed to answer the question is TRIUMPH-Outcomes, an event-driven study of an estimated 10,000 people with cardiovascular or chronic kidney disease, with primary completion estimated for February 2029.¹⁴

The kidney evidence is mechanistic. The phase 2b TRANSCEND-CKD trial randomized 146 adults with overweight or obesity and an estimated filtration rate of 25-75 to retatrutide or placebo for 24 weeks.¹⁵ Its primary endpoint, filtration rate measured by iohexol clearance, did not differ between groups, according to a poster at the American Society of Nephrology's Kidney Week in October 2026. In the roughly 30% of participants with raised urine albumin, the albumin-to-creatinine ratio fell 53.7% on retatrutide and 8.8% on placebo, a placebo-adjusted reduction of 49.2%. Weight fell by 14 kg, systolic blood pressure by 14 mmHg against 1 mmHg on placebo, and pulse rose by 9 beats a minute.¹⁶ Albuminuria is a surrogate marker, not a kidney outcome.

Two estimands, two numbers

Lilly's announcements lead with the "efficacy estimand", which estimates the effect had every participant stayed on the drug. The journals lead with the "treatment-regimen estimand", which counts everyone randomized, whether or not they kept taking it. For 12 mg in TRIUMPH-1 the first gives 28.3% weight loss and the second 25.0%.⁸⁶ Neither is wrong, but headline figures quoted without the label usually come from the higher one.

§Regulatory status

No regulator anywhere had approved retatrutide as of late October 2026, according to Lilly.¹² In July 2026 the company said its phase 3 data supported global submissions for obesity, knee osteoarthritis pain and sleep apnoea, and that it planned to file a Biologics License Application with the US Food and Drug Administration in the first quarter of 2027, once its chemistry, manufacturing and controls data package was complete.¹³ Outside trials, the only lawful route is a pre-approval expanded access programme for single patients, registered by Lilly in June 2026, for adults with a BMI of at least 35, two or more serious obesity-related complications, no response to the best approved therapy, and no access to a trial.¹⁷

§Limitations

Nearly everything known about retatrutide comes from trials Lilly funded, designed and analysed, and several of the largest results exist so far only as press releases, which report the company's preferred estimand and give only selected safety figures. The longest placebo- or comparator-controlled exposure reported is 80 weeks; the 104-week figure comes from a selected subgroup.

The endpoints are mostly surrogates. Weight, HbA1c, liver fat, albuminuria and blood pressure all predict disease, but no completed trial shows that retatrutide reduces heart attacks, kidney failure or deaths. Lilly's earlier drug tirzepatide received a US approval to reduce cardiovascular risk in adults with type 2 diabetes in August 2026;¹⁸ retatrutide has no comparable evidence yet. Nor has any trial used biological aging as an endpoint. Claims that large weight loss slows aging belong to the Geroscience hypothesis and remain untested. A narrower claim, that treating obesity could compress the years spent ill, is a version of Compression of morbidity rather than an extension of Maximum human lifespan.

§Risks

The commonest side effects are gastrointestinal and dose-related, as with other incretin drugs.⁴ Three other findings draw attention.

Dysaesthesia. Abnormal skin sensations appeared in phase 3 at rates well above placebo: 8.8% and 20.9% on 9 and 12 mg in TRIUMPH-4, against 0.7%. Lilly described the TRIUMPH-4 events as generally mild and rarely leading to discontinuation;⁹ for TRIUMPH-1 it called them mild to moderate and mostly resolving during treatment.⁸ A pharmacovigilance study found reports of dysaesthesia, including burning skin, with semaglutide and of hyperaesthesia with tirzepatide, and suggested the effect appears dose-dependent and more frequent with more potent agents.¹⁹

Heart rate and blood pressure. In phase 2, heart rate rose with dose, peaked at 24 weeks and then declined.⁴ In TRIUMPH-2, hypotension was reported in up to 6% of participants on retatrutide against under 1% on placebo.¹¹

Discontinuation. In TRIUMPH-4, 18.2% of participants on 12 mg stopped treatment because of adverse events, against 4.0% on placebo. Lilly said the rate was highly correlated with baseline BMI and included people who stopped over perceived excessive weight loss.⁹ Rates on 12 mg were lower in other trials, 11.3% in TRIUMPH-1 and 13.5% in TRIUMPH-3.⁸¹³

Loss of muscle alongside fat is a concern for any drug producing this much weight loss, and especially for older people at risk of Sarcopenia. The published body-composition data come from a phase 2 substudy in type 2 diabetes in which 103 people completed both scans; its authors found the share of weight lost as lean mass similar to other obesity treatments.²⁰

What is not known

Long-term safety beyond two years is unknown, and the cardiovascular question is open: TRIUMPH-3's hazard ratio for heart attack, stroke and cardiovascular death was 1.12 with a confidence interval from 0.64 to 1.96.¹³ Whether the pulse increase seen in phase 2 and again in the kidney trial matters clinically is a question only longer trials can answer. The main open questions rest on TRIUMPH-Outcomes, whose primary completion is estimated for February 2029.¹⁴

§The unapproved market

A grey market in unapproved retatrutide has grown alongside the trial programme. The FDA states that retatrutide cannot be used in compounding under federal law, has warned telehealth companies for marketing it, active-ingredient distributors for selling it to compounders, and outsourcing facilities for repackaging it, and has warned sellers of products falsely labelled "for research purposes" or "not for human consumption" that are sold to consumers with dosing instructions.²¹ In August 2026 Lilly announced six new US lawsuits against sellers, four peptide vendors, an aesthetics clinic and a pharmacy, and said it had reported more than 14,000 listings and posts in over 100 countries.²²

What people actually inject in this market is largely unverified. A study of Reddit posts identified 13,589 users describing their own use and found increased appetite, fatigue, increased energy, nausea, food craving, insomnia and raised heart rate among the most reported effects, a pattern unlike the trial results. The authors note that product identity, purity and dose could not be verified, so the data can generate hypotheses but cannot establish incidence or cause.²³ The trade sits beside the self-experimentation described under Biohacking and grinders and the weakly regulated Dietary supplements market, with the difference that the compound has a large clinical programme behind it whose results do not transfer to unverified vials.

§Outlook

Any US decision depends on the filing Lilly plans for early 2027. TRIUMPH-5, the head-to-head trial against tirzepatide, would give a direct comparison of efficacy and tolerability with Lilly's current drug. With no approval, the drug has no price, so where retatrutide would sit in the pattern described under Access and inequality is not yet known. Use by people seeking weight loss beyond medical need would reopen the treatment-enhancement boundary argued over in enhancement ethics.

The larger claim made for drugs of this kind, that they extend Healthspan, is not tested by any registered trial. TRIUMPH-Outcomes can show whether retatrutide prevents heart and kidney events in high-risk people. It cannot show whether a population taking it ages more slowly.

§See also

• GLP-1 receptor agonists
• Sarcopenia
• Geroscience hypothesis
• Compression of morbidity
• Healthspan
• Access and inequality
• Biohacking and grinders

§References