Retatrutide: The Triple Agonist Behind Up to 71 lbs Average Weight Loss in Trials

Retatrutide is an investigational weight-loss medicine that has produced some of the largest average weight losses seen in obesity drug trials so far. In one Phase 3 clinical trial (TRIUMPH-4), Lilly reported average weight loss of up to 28.7% (71.2 lbs, or 32.3 kg) over 68 weeks.

As of October 2026 it is not approved or available outside clinical trials. But understanding what it is, how it works, and what the data shows matters, because it could change treatment options if it is approved.

What is retatrutide?

Retatrutide (development code LY3437943) is a once-weekly subcutaneous injection developed by Eli Lilly. It is sometimes called the "triple agonist" or "triple G" because it simultaneously activates three distinct hormone receptors:

• GIP receptor (glucose-dependent insulinotropic polypeptide): stimulates insulin release, regulates fat metabolism and fat cell function
• GLP-1 receptor (glucagon-like peptide-1): suppresses appetite powerfully, slows gastric emptying, stimulates glucose-dependent insulin release
• Glucagon receptor: increases energy expenditure, promotes fat breakdown in the liver, reduces liver fat

Every approved GLP-1 medication activates a subset of these receptors:

• Ozempic/Wegovy (semaglutide): GLP-1 only
• Mounjaro/Zepbound (tirzepatide): GLP-1 + GIP
• Retatrutide: GLP-1 + GIP + glucagon

The addition of the glucagon receptor component is the key differentiator.

The glucagon paradox: why adding a hunger-stimulating hormone helps

Glucagon is known primarily as a hormone that raises blood sugar — the opposite of insulin. So why does activating glucagon receptors help with weight loss rather than hinder it?

The answer is mechanism and context.

On its own, glucagon raises blood glucose. When activated alongside GLP-1 and GIP — which lower blood sugar and reduce appetite — the glucagon component is thought to add metabolic effects while its blood-sugar-raising effect is balanced out:

Increased energy expenditure: Glucagon receptor activation is thought to raise energy expenditure, so the body may burn more calories. This is an additional weight loss lever that GLP-1 and GIP alone do not provide.

Enhanced fat breakdown: Glucagon promotes lipolysis (breaking down stored fat) and specifically targets liver fat (hepatic steatosis). For patients with fatty liver disease — common with obesity — this is an additional benefit.

Thermogenic effect: In animal studies, glucagon has been linked to activation of brown adipose tissue (brown fat), which burns energy as heat; how much this matters in people is not yet clear.

When these three mechanisms work together — strong appetite suppression (GLP-1 + GIP) plus increased calorie burning (glucagon) — the trial results so far show greater average weight loss than has been seen with single- or dual-mechanism drugs, although no head-to-head results have been reported yet.

Phase 2 results: the first signal

Phase 2 results, published in The New England Journal of Medicine in June 2023, evaluated retatrutide in 338 adults with obesity, or overweight with a weight-related condition, over 48 weeks. People with diabetes were not included.

Results at 48 weeks:

At 24 weeks, the 12 mg group had already lost 17.5% of body weight, compared with 1.6% on placebo. For context, Wegovy produced 14.9% in the 68-week STEP 1 trial, although the trials are not directly comparable.

These Phase 2 numbers immediately set retatrutide apart from every other GLP-1-class drug studied to that point. The question was whether Phase 3 would confirm them.

Phase 3 results: the landmark numbers

Eli Lilly has been running the TRIUMPH Phase 3 programme since 2023. As of October 2026, the main obesity trials have reported.

TRIUMPH-4: Obesity with knee osteoarthritis (December 2025)

This Phase 3 trial enrolled adults with obesity and knee osteoarthritis — a population where weight loss would be expected to have dual benefits: metabolic and joint.

• Duration: 68 weeks
• Dose: 12 mg (top dose)
• Average weight loss: up to 71.2 lbs (32.3 kg), from an average starting weight of 112.7 kg
• Percentage weight loss: up to 28.7% using Lilly's efficacy estimand; using the treatment-regimen estimand, 23.7% on 12 mg and 20.0% on 9 mg, compared with 4.6% on placebo
• Knee pain: up to a 75.8% average reduction in WOMAC pain score (compared with 40.3% on placebo)
• Side effects: stopping treatment because of side effects was 12.2% (9 mg) and 18.2% (12 mg) versus 4.0% on placebo; dysesthesia (unusual skin sensations) was reported by 8.8% and 20.9% versus 0.7%

For context:

Important caveat: These trials enrolled different populations, used different inclusion criteria, and ran at different times. The numbers cannot be directly compared as if it were a head-to-head trial. The TRIUMPH-4 osteoarthritis population may have different baseline characteristics that affect weight loss outcomes.

Still: the size of the weight loss is striking, and it is consistent with what the Phase 2 data predicted.

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

Topline results from the first Phase 3 trial in type 2 diabetes were announced March 19, 2026. Over 40 weeks, retatrutide lowered HbA1c by up to 2.0% and body weight by up to 16.8% (36.6 lbs), using Lilly's efficacy estimand.

TRIUMPH-1: Main obesity trial (May 2026)

The pivotal obesity trial enrolled 2,339 adults with obesity, or overweight with a weight-related condition, without diabetes. Published in The New England Journal of Medicine in 2026, it found average weight loss at 80 weeks of 17.6% (4 mg), 23.7% (9 mg) and 25.0% (12 mg), compared with 3.9% on placebo (treatment-regimen estimand). Lilly's headline figure for 12 mg, using its efficacy estimand, was 28.3%. Retatrutide also reduced knee pain in participants with knee osteoarthritis and breathing interruptions in participants with obstructive sleep apnea.

The osteoarthritis finding: beyond weight

The TRIUMPH-4 knee pain result deserves its own attention.

An average reduction in WOMAC knee pain scores of up to 75.8% is a notable result for people who have both obesity and knee osteoarthritis, although the placebo group also improved (40.3%).

The relationship between obesity and joint disease is well established: excess weight directly increases mechanical load on joints and drives inflammatory processes that accelerate cartilage degradation. Weight loss reduces both.

Whether the pain reduction in TRIUMPH-4 comes only from weight loss or also from other effects of the drug is not yet known.

If retatrutide receives a specific indication for obesity with musculoskeletal complications, it would be the first weight loss drug approved with a joint pain claim — a meaningful addition to how physicians frame treatment decisions.

Liver disease: a likely additional benefit

Retatrutide's glucagon receptor component directly targets liver fat. Fatty liver disease (metabolic dysfunction-associated steatotic liver disease, MASLD) affects about 30% of adults worldwide, according to a 2023 systematic review, and is strongly linked to obesity and insulin resistance.

All GLP-1 drugs reduce liver fat somewhat — primarily through weight loss. Retatrutide's glucagon component adds a direct hepatic effect that may be additive. Early data suggests retatrutide may produce significant reductions in liver fat independent of the weight loss component alone.

Formal NAFLD/MASLD trials for retatrutide are underway. This indication could expand the drug's clinical reach substantially when the data matures.

Side effects

Phase 2 and Phase 3 data show a side effect profile consistent with the GLP-1 medication class:

Most common:

• Nausea (most common, especially during dose escalation)
• Diarrhoea
• Vomiting
• Constipation
• Decreased appetite

Pattern: GI side effects were dose-dependent — more common and more pronounced at the 12 mg dose than at lower doses — but generally mild to moderate in severity and decreased significantly over time as participants adjusted.

Dysesthesia: Phase 3 trials reported dysesthesia (unusual skin sensations such as tingling or burning), more often at higher doses; in TRIUMPH-4 it affected 20.9% of people on 12 mg versus 0.7% on placebo. It was generally mild and rarely led to stopping treatment. This is not typical of other GLP-1 medicines.

Stopping because of side effects: more common at higher doses (in TRIUMPH-1, 11.3% on 12 mg versus 4.9% on placebo).

Heart rate effects warrant watching. In the Phase 2 trial, resting heart rate rose in a dose-related way, peaking at around 24 weeks and declining afterwards. Regulators will review this when Lilly applies for approval.

As with approved GLP-1 medicines, serious problems such as pancreatitis, gallbladder disease and dehydration-related kidney injury are possible. Anyone in a trial who develops severe, persistent stomach pain, vomiting that stops them keeping fluids down, or signs of a severe allergic reaction should seek urgent medical help.

When will retatrutide be available?

As of October 2026: not yet available outside of clinical trials.

In July 2026, Lilly said its clinical data package was complete and that it plans to submit a Biologics License Application to the US FDA in the first quarter of 2027. Approval, if granted, would follow the FDA review.

Do not fall for unapproved products. Retatrutide is not commercially available anywhere. Any product sold as retatrutide — from compounding pharmacies, online sellers, or "research peptide" companies — is unapproved, its contents are unknown, and it is potentially dangerous.

If you want to access retatrutide legally today, the only path is clinical trial participation. Search clinicaltrials.gov for "retatrutide" to see if you meet eligibility criteria for any ongoing study.

How retatrutide fits in the pipeline context

The GLP-1 drug pipeline has moved extraordinarily fast. Placing retatrutide in context:

Status as of October 2026. Figures come from different trials and are not directly comparable.

Based on trial results so far, retatrutide has produced the largest average weight loss of the medicines in this table. Where it would fit in treatment, if approved, will depend on its full safety data, the head-to-head trial against tirzepatide, cost and access.

Final takeaway

Retatrutide is a once-weekly triple agonist (GLP-1 + GIP + glucagon) that produced 25.0% average weight loss at 80 weeks in its main Phase 3 obesity trial, and up to 28.7% in a trial of people with knee osteoarthritis. Phase 3 results in type 2 diabetes have also been positive.

It is not yet approved anywhere, and Lilly plans to apply to the FDA in early 2027.

For now, the options for patients are approved medicines, such as semaglutide (Wegovy) or tirzepatide (Zepbound or Mounjaro), chosen with a prescriber, while the retatrutide development programme concludes.

Related Articles

• New GLP-1 Drugs Pipeline 2026–2027: What's Coming
• CagriSema: Novo Nordisk's 22.7% Weight Loss Drug Explained
• Switching from Ozempic to Mounjaro: What to Know

Consult your healthcare provider before making any decisions about treatment changes based on pipeline data.

Consult your healthcare provider before starting any medication.

Sources

• ClinicalTrials.gov search for retatrutide trials: https://clinicaltrials.gov/search?term=retatrutide
• PubMed record for retatrutide phase 2 obesity results (NEJM 2023): https://pubmed.ncbi.nlm.nih.gov/37366315/
• TRIUMPH-1 publication (NEJM 2026): https://pubmed.ncbi.nlm.nih.gov/42814954/
• Patient Care — TRIUMPH-4 results (December 2025): https://www.patientcareonline.com/view/retatrutide-achieves-up-to-28-7-weight-loss-and-marked-knee-pain-reduction-in-phase-3-triumph-4-trial
• Rheum-Live — TRIUMPH-4 results by estimand (December 2025): https://www.rheum-live.com/view/triumph-4-retatrutide-delivers-weight-loss-knee-osteoarthritis-pain-relief
• Consultant360 — TRANSCEND-T2D-1 topline results (March 2026): https://www.consultantlive.com/view/retatrutide-delivers-a1c-reduction-weight-loss-in-phase-3-transcend-t2d-1-trial
• Eli Lilly Q2 2026 results (SEC Form 8-K, July 2026): https://www.sec.gov/Archives/edgar/data/0000059478/000005947826000077/q226lillysalesandearningsp.htm
• STEP 1 semaglutide trial — PubMed: https://pubmed.ncbi.nlm.nih.gov/33567185/
• SURMOUNT-1 tirzepatide trial — PubMed: https://pubmed.ncbi.nlm.nih.gov/35658024/
• Younossi ZM, et al. The global epidemiology of NAFLD and NASH: a systematic review. Hepatology 2023: https://pubmed.ncbi.nlm.nih.gov/36626630/
• Healio — FDA approves orforglipron (April 2026): https://www.healio.com/news/endocrinology/20260401/fda-approves-orforglipron-an-oral-glp1-for-adults-with-obesity
• Consultant360 — FDA approves Wegovy HD 7.2 mg (March 2026): https://www.consultantlive.com/view/fda-approves-higher-dose-semaglutide-wegovy-hd-injection-7-2-mg-for-obesity