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What is retatrutide? A plain-English explainer

Last reviewed by retainfo editorial team. How we review content.

Retatrutide (code name LY3437943) is an experimental medicine being developed by the American company Eli Lilly for obesity and related conditions. It is given as a once-weekly injection under the skin. It is not approved for use anywhere in the world.

The short version

Your gut and pancreas release hormones after you eat. Some of those hormones tell your brain you are full, slow your stomach down, and help control blood sugar. A class of medicines copies those signals. Wegovy copies one hormone (GLP-1). Mounjaro copies two (GLP-1 and GIP). Retatrutide copies three: GLP-1, GIP and glucagon.[1]

That third hormone, glucagon, is the new part. Glucagon receptor activation appears to increase the energy your body burns and to reduce fat stored in the liver, on top of the appetite effects of the other two. The how it works page goes through each in turn.

How it differs from Wegovy and Mounjaro

Wegovy (semaglutide)Mounjaro (tirzepatide)Retatrutide
Receptors targetedGLP-1GLP-1 + GIPGLP-1 + GIP + glucagon
UK licence for weight managementYes[5]Yes[6]No
Available on the NHSYes, with criteriaYes, with criteria (phased rollout)No
StageMarketedMarketedPhase 3 trials
Reported average weight loss in main obesity trialAbout 15% at 68 weeksAbout 21% at 72 weeks25.0% at 80 weeks (12 mg, treatment-regimen estimand)[4]

The trial figures are not a like-for-like comparison. The trials had different lengths, populations and ways of counting. A direct head-to-head trial against tirzepatide (TRIUMPH-5) is running and has not reported. Until it does, nobody can say by how much, if at all, retatrutide beats Mounjaro in the same people. See how to read a trial.

Who makes it

Eli Lilly discovered retatrutide and runs all the registered trials.[8] Lilly does not sell it anywhere, to anyone, outside a clinical trial. There is no licensed manufacturer of retatrutide. Anything labelled retatrutide that you can order is either made by an unlicensed producer, or is a different substance altogether. The safety section covers what testing has found.

Where it is in development

Retatrutide development timelinePhase 2 results in 2023; TRIUMPH-4 December 2025; TRIUMPH-1 May 2026; TRIUMPH-2 and 3 July 2026; Lilly plans an FDA filing in the first quarter of 2027; the outcomes trial is expected to complete around 2029. No regulator has approved it.Phase 2 (NEJM)2023up to 24.2% at 48 wkTRIUMPH-4Dec 2025knee OATRIUMPH-1May 2026obesity, no T2DTRIUMPH-2 & 3Jul 2026T2D · CVDFDA filingQ1 2027Lilly's stated planTRIUMPH-Outcomes2029CV & kidney, ~10,000Filled = reported. Outlined = planned or pending. No marketing authorisation exists anywhere as of September 2026.
Figure: where retatrutide is in development. Dates from Lilly announcements and ClinicalTrials.gov.
  • 2023: the Phase 2 trial in the New England Journal of Medicine reported up to 24.2% average weight loss at 48 weeks.[1]
  • December 2025: TRIUMPH-4, in people with knee osteoarthritis, was the first Phase 3 trial to report.[9]
  • May 2026: TRIUMPH-1, the main obesity trial, reported.[4]
  • July 2026: TRIUMPH-2 (type 2 diabetes) and TRIUMPH-3 (cardiovascular disease) reported. Lilly said it plans to submit a US marketing application in the first quarter of 2027.[10][11]
  • Still running: the head-to-head against tirzepatide, a 10,000-person heart and kidney outcomes trial, and studies in sleep apnoea, liver disease and back pain. Full tracker.

When could it be available in the UK?

Nobody knows. Lilly has only given a US filing target. If the US review takes a typical year, the earliest realistic US decision is late 2027 or 2028. The MHRA runs its own process. For tirzepatide, UK authorisation came within about a year of the US decision. NHS access then needs a NICE appraisal and a rollout plan, which for Mounjaro is taking years. A reasonable expectation is UK authorisation no earlier than 2027–2028, and NHS availability later. Treat any website giving you a firm date as guessing.

What retatrutide is not

  • Not a licensed medicine. No regulator has reviewed the full evidence yet.
  • Not "the same as Mounjaro but stronger". It is a different molecule with a different third target and a partly different side-effect profile, including a new skin-sensation effect not seen with the others. See side effects.
  • Not a supplement or a "research peptide". It is a prescription-strength drug in development. Labels saying "research use only" do not change what it does to a person who injects it, or the law on selling it. See is it legal in the UK?

References

  1. Jastreboff AM, Kaplan LM, Frías JP, et al. Triple–Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial. New England Journal of Medicine 2023;389:514-526, 26 June 2023. Primary source
  2. All registered retatrutide (LY3437943) studies. ClinicalTrials.gov search, 1 September 2026. Primary source
  3. MHRA smashes major illicit weight loss medicine production facility in record seizure. GOV.UK / MHRA press release, 24 October 2025. Primary source
  4. Lilly's triple agonist, retatrutide, delivered powerful weight loss in pivotal Phase 3 obesity trial (TRIUMPH-1 topline results). Eli Lilly and Company, press release, 21 May 2026. Primary source
  5. Semaglutide for managing overweight and obesity (TA875). NICE, 8 March 2023. NHS / NICE guidance
  6. Tirzepatide for managing overweight and obesity (TA1026). NICE, 23 December 2024. NHS / NICE guidance
  7. UK first in Europe to authorise orforglipron for weight management and type 2 diabetes. GOV.UK / MHRA press release, 10 August 2026. Primary source
  8. What to know about retatrutide. Eli Lilly and Company, 23 July 2026. Primary source
  9. Lilly's triple agonist, retatrutide, delivered weight loss of up to an average of 71.2 lbs along with substantial relief from osteoarthritis pain in first successful Phase 3 trial (TRIUMPH-4 topline results). Eli Lilly and Company, press release, 11 December 2025. Primary source
  10. Lilly's triple agonist, retatrutide, successful in two additional Phase 3 obesity trials, delivering significant improvements in weight and A1C (TRIUMPH-2 and TRIUMPH-3 topline results). Eli Lilly and Company, press release, 23 July 2026. Primary source
  11. Eli Lilly will file for approval of retatrutide obesity drug in 2027. CNBC, 23 July 2026. Secondary source

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