Retatrutide studies: what does the research say?

Retatrutide (LY3437943) has become one of the most studied metabolic peptides in just a few years. This overview lists the published studies, from the first phase 1 trials to the phase 3 results Eli Lilly announced in 2025 and 2026.

Discovery and phase 1

Retatrutide was designed by researchers at Eli Lilly and described in Cell Metabolism in 2022 (Coskun et al.). The same year, The Lancet published a phase 1b study in people with type 2 diabetes (Urva et al.) that confirmed the long half-life of about six days and laid the basis for weekly dosing in later studies.

Phase 2 in obesity (NEJM, 2023)

In the phase 2 study by Jastreboff et al., 338 adults with obesity or overweight received retatrutide or placebo for 48 weeks. Weight loss increased with dose and had not yet plateaued at 48 weeks.

  • 1 mg: −8.7%
  • 4 mg: −17.1%
  • 8 mg: −22.8%
  • 12 mg: −24.2%
  • Placebo: −2.1%

Phase 2 in type 2 diabetes (The Lancet, 2023)

Rosenstock et al. followed 281 participants with type 2 diabetes for 36 weeks. With 12 mg, HbA1c fell by a mean 2.02 percentage points (placebo: 0.01) and body weight by 16.9%. A sub-study in Nature Medicine (Sanyal et al., 2024) also showed that liver fat fell by more than 80% after 24 weeks at the higher doses.

Phase 3: the TRIUMPH and TRANSCEND programmes

After phase 2, Eli Lilly started a large phase 3 programme with thousands of participants: TRIUMPH for obesity and related conditions, TRANSCEND-T2D for type 2 diabetes. The first results have been published since late 2025:

  • TRIUMPH-4 (December 2025) – obesity with knee osteoarthritis: 28.7% weight loss after 68 weeks with 12 mg, and knee pain (WOMAC) fell by a mean 75.8%
  • TRANSCEND-T2D-1 – type 2 diabetes: HbA1c up to 2.0 percentage points lower and 16.8% weight loss after 40 weeks
  • TRIUMPH-1 (May 2026) – 2,339 adults with obesity or overweight: 19.0% (4 mg), 25.9% (9 mg) and 28.3% (12 mg) weight loss after 80 weeks; participants with a BMI of 35 or higher reached 30.3% after 104 weeks
  • TRIUMPH-3 (July 2026) – obesity with cardiovascular disease: up to 22.6% weight loss after 80 weeks, versus 3.2% on placebo
  • TRIUMPH-2 (September 2026, The Lancet) – obesity with type 2 diabetes, 1,152 participants: 20.8% weight loss and up to 1.6 percentage points lower HbA1c after 80 weeks

Side effects

As with other incretin peptides, gastrointestinal complaints were the most common side effects, usually mild to moderate and mostly during dose escalation. In TRIUMPH-1, 42.4% of participants on 12 mg reported nausea (placebo: 14.8%) and 11.3% stopped because of side effects (placebo: 4.9%). Dysaesthesia (altered skin sensation) was also seen more often than with placebo.

When will retatrutide reach the market?

Retatrutide is not yet approved as a medicine anywhere. According to Eli Lilly, the phase 3 package for obesity, sleep apnoea and knee osteoarthritis is complete and the US submission is planned for the first quarter of 2027, so approval is not expected before 2027. Further phase 3 studies are running, including on back pain, fatty liver disease (MASLD) and heart and kidney outcomes. Until then, retatrutide is only available in clinical trials and as a research reagent for the lab.

Retatrutide for laboratory research

Virexa Labs supplies retatrutide as a lyophilized research peptide in 15 mg and 20 mg vials. Every batch is tested by HPLC for ≥99% purity and by mass spectrometry for identity; the COA is online. Our retatrutide is intended solely for in-vitro research – not for human or veterinary use.

Retatrutide and related research peptides

Frequently asked questions

How much weight loss did retatrutide give in studies?

In the phase 3 TRIUMPH-1 study, participants on 12 mg lost a mean 28.3% of body weight after 80 weeks; in the phase 2 study it was 24.2% after 48 weeks.

What phase is retatrutide in?

Phase 3. Since December 2025, results from TRIUMPH-4, TRANSCEND-T2D-1, TRIUMPH-1, TRIUMPH-3 and TRIUMPH-2 have been released; more phase 3 studies are running.

When will retatrutide be approved?

Eli Lilly plans the US submission for the first quarter of 2027. Approval is not expected before 2027.

What side effects did retatrutide have in studies?

Mainly gastrointestinal: nausea, diarrhoea, constipation and vomiting, usually mild to moderate and mostly during dose escalation. Dysaesthesia and, in phase 2, a temporary rise in heart rate were also reported.

Who develops retatrutide?

Eli Lilly, under the development code LY3437943. Lilly also developed tirzepatide.

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