Library / Weight and metabolic

Retatrutide

Also known as LY3437943

Human trials

Investigational triple agonist peptide acting on GIP, GLP-1 and glucagon receptors, studied for obesity, type 2 diabetes and related conditions. Phase 2 and phase 3 results have been published but it is not approved anywhere.

Not an approved medicine. Retatrutide is not an approved medicine for this use in Australia or the United States. This page summarises published research for information only. It is not a recommendation to use Retatrutide, and VialOwl does not sell or supply it.

Background

Retatrutide (LY3437943) is an investigational once weekly injectable peptide developed by Eli Lilly. Phase 2 results were published in 2023 and phase 3 TRIUMPH and TRANSCEND trials reported from 2026. It is not approved anywhere.

How it is thought to work

It activates three hormone receptors at once: GLP-1 and GIP, which reduce appetite and improve insulin release, and glucagon, which is thought to increase energy use and liver fat burning.

What is claimed, and the evidence

Claim	Evidence	Notes
Weight loss in adults with obesity	Human trials	Phase 2: up to 24 per cent at 48 weeks. Phase 3 TRIUMPH-1 (company topline, May 2026): 17.6 to 25.0 per cent at 80 weeks versus 3.9 per cent with placebo (treatment regimen estimand).
Blood sugar control in type 2 diabetes	Human trials	Phase 3 TRANSCEND-T2D-1 (537 people): HbA1c fell 1.7 to 1.9 percentage points versus 0.8 with placebo over 40 weeks.
Reduced liver fat	Human trials	Phase 2a randomised trial in fatty liver disease.
Muscle preserving or 'better than tirzepatide' for everyone	No published evidence	Online claims; no head to head trial results published.

Side effects

Effect	How often	Notes
Nausea	Common	29 to 42 per cent versus 15 per cent with placebo in TRIUMPH-1, rising with dose.
Diarrhoea	Common	25 to 34 per cent versus 14 per cent with placebo in TRIUMPH-1.
Constipation	Common	24 to 26 per cent versus 11 per cent with placebo in TRIUMPH-1.
Vomiting	Common	11 to 25 per cent versus 5 per cent with placebo in TRIUMPH-1.
Dysaesthesia (abnormal skin sensations such as tingling or burning)	Common	5 to 12.5 per cent versus 0.9 per cent with placebo in TRIUMPH-1; mostly mild to moderate. A new signal not typical of the class.
Urinary tract infection	Common	7.5 to 8.8 per cent versus 5.3 per cent with placebo in TRIUMPH-1.
Increased heart rate	Common	Dose dependent rises peaking at 24 weeks in the phase 2 trial.
Stopping treatment because of side effects	Common	4.1 to 11.3 per cent versus 4.9 per cent with placebo in TRIUMPH-1.
Severe vomiting with oesophageal tear from a counterfeit product Serious	Case reports	TGA investigated a hospitalised case; the vial contained about 8 times the approved semaglutide level and no retatrutide.

Warnings

Not approved by the TGA or any comparable regulator; the TGA says any retatrutide outside a clinical trial could be counterfeit.
TGA testing in September 2026 found a product sold as retatrutide contained undeclared high dose semaglutide and no retatrutide, after a consumer was hospitalised.
Full phase 3 safety data are not yet published; dysaesthesia is an emerging signal.
Unapproved products are not assessed by the TGA for quality, safety or efficacy and may be contaminated, non-sterile, underdosed, overdosed or contain a different substance altogether.

Phase 3 trials

TRANSCEND-T2D-1

Phase 3, Completed, published. 537 participants, 40 weeks. NCT06354660

Population: Adults with type 2 diabetes managed with diet and exercise alone, HbA1c 7.0 to 9.5%.
Arms: Retatrutide 4 mg weekly; Retatrutide 9 mg weekly; Retatrutide 12 mg weekly; Placebo
Primary outcome: Change in HbA1c.
Results: HbA1c fell 0.88, 1.04 and 1.12 percentage points more than placebo. Body weight changed by minus 11.5%, 13.9% and 15.3% against minus 2.6% on placebo.
Safety: Stopped because of adverse events: 2 to 5% on retatrutide against 0% on placebo.
Source: Bajaj HS et al. Lancet 2026;407:2402-13.

TRIUMPH-1

Phase 3, Completed, not yet published. 2,335 participants, 80 weeks. NCT05929066

Population: Adults with BMI of 30 or more, or 27 or more with a related condition, without diabetes.
Arms: Retatrutide 4 mg weekly; Retatrutide 9 mg weekly; Retatrutide 12 mg weekly; Placebo
Primary outcome: Percentage change in body weight at week 80.
Results: Body weight changed by minus 17.6%, 23.7% and 25.0% against minus 3.9% on placebo (treatment-regimen estimand, counting everyone randomised).
Safety: Nausea 28.6%, 38.4% and 42.4% against 14.8%. Dysaesthesia 5.1%, 12.3% and 12.5% against 0.9%. Stopped because of adverse events: 4.1%, 6.9% and 11.3% against 4.9%.
Source: Eli Lilly press release, 21 May 2026. Company announcement, not peer reviewed.

TRIUMPH-4

Phase 3, Completed, not yet published. 445 participants, 68 weeks, starting at 2 mg and stepping up every 4 weeks. NCT05931367

Population: Adults with BMI of 27 or more and knee osteoarthritis.
Arms: Retatrutide 9 mg weekly; Retatrutide 12 mg weekly; Placebo
Primary outcome: WOMAC knee pain score and percentage change in body weight at week 68.
Results: Body weight changed by minus 20.0% and 23.7% against minus 4.6% on placebo (treatment-regimen estimand, counting everyone randomised). WOMAC pain fell 4.5 and 4.4 points against 2.4.
Safety: Nausea 38.1% and 43.2% against 10.7%. Dysaesthesia 8.8% and 20.9% against 0.7%. Stopped because of adverse events: 12.2% and 18.2% against 4.0%.
Source: Eli Lilly press release, 11 December 2025. Company announcement, not peer reviewed.

Doses in these trials were given under medical supervision.

Regulatory status

Australia: Not approved and not on the ARTG. TGA states it is undergoing clinical trials and has not been approved by the TGA or any comparable regulator; in September 2026 TGA testing found a product sold as retatrutide contained undeclared semaglutide and no retatrutide.
United States: Not FDA approved; investigational (phase 3 TRIUMPH programme). FDA states it cannot be used in compounding. Sponsor has indicated a planned submission in 2027 (Not verified with FDA).

Sources

Also in Weight and metabolic

From the VialOwl library, reviewed 26 September 2026. For education and general information. It is not medical advice and not a recommendation to use Retatrutide.