Retatrutide (LY3437943) is an investigational peptide developed by Eli Lilly that acts at three receptors at once: the glucose-dependent insulinotropic polypeptide (GIP) receptor, the glucagon-like peptide-1 (GLP-1) receptor and the glucagon receptor. In 2026 it is not a licensed medicine in the UK. This guide sets out what the published trials measured, the phase 3 programme on clinicaltrials.gov, which weight-management medicines hold UK licences, and where retatrutide sits in UK law. It is general information, not medical or legal advice.
Receptor targets
Semaglutide, the active substance in Wegovy, is a GLP-1 receptor agonist (MHRA, 2024). Retatrutide acts at the GIP, GLP-1 and glucagon receptors, which is why the trial literature calls it a "triple-hormone-receptor agonist" (Jastreboff et al., NEJM 2023). The phase 3 papers describe it as under clinical development for type 2 diabetes, obesity and related complications (Bajaj et al., 2026).
The phase 2 trials: published endpoints
The phase 2 obesity trial (NCT04881760) was double-blind and placebo-controlled and enrolled 338 adults with a BMI of 30 or higher, or 27 to below 30 with a weight-related condition, who received retatrutide at one of several dose levels, or placebo, by once-weekly injection for 48 weeks. The primary endpoint was the percentage change in body weight at 24 weeks, with the change at 48 weeks as a secondary endpoint; the figures are in the published paper. The most common adverse events were gastrointestinal and mostly mild to moderate, and increases in heart rate peaked at 24 weeks before declining (PMID 37366315). These are trial results for a weekly injection, not a description of any product.
A parallel phase 2 trial (NCT04867785) randomised 281 adults with type 2 diabetes to placebo, dulaglutide or one of several retatrutide regimens. The primary endpoint was the change in HbA1c at 24 weeks, with body weight at 36 weeks among the secondary endpoints, and gastrointestinal events were reported in 35% of retatrutide participants against 13% on placebo (Rosenstock et al., Lancet 2023).
The phase 3 programme on clinicaltrials.gov
A search of the clinicaltrials.gov register on 9 October 2026 returned 14 phase 3 interventional studies of retatrutide, all sponsored by Eli Lilly and Company. Seven are shown here.
| Register entry | Population | Enrolment | Status | Primary completion |
|---|---|---|---|---|
| NCT05929066 (TRIUMPH-1) | Obesity or overweight | 2,335 | Completed | 6 April 2026 |
| NCT05929079 (TRIUMPH-2) | Type 2 diabetes with obesity or overweight | 1,152 | Completed | 16 June 2026 |
| NCT05882045 (TRIUMPH-3) | Obesity with cardiovascular disease | 1,946 | Completed | 16 April 2026 |
| NCT05931367 (TRIUMPH-4) | Obesity or overweight with knee osteoarthritis | 445 | Completed | 14 November 2025 |
| NCT06354660 (TRANSCEND-T2D-1) | Type 2 diabetes managed by diet and exercise alone | 537 | Completed | 22 January 2026 |
| NCT06383390 (TRIUMPH-Outcomes) | Cardiovascular and kidney outcomes in obesity | 10,000 (estimated) | Active, not recruiting | February 2029 |
| NCT06662383 (TRIUMPH-5) | Retatrutide compared with tirzepatide in obesity | 800 (estimated) | Active, not recruiting | November 2026 |
Three phase 3 results had appeared in PubMed by 9 October 2026, all in trials of once-weekly injection against placebo. TRIUMPH-1 randomised 2,339 adults with obesity without diabetes for 80 weeks; the primary endpoint was the change in body weight, and the most common adverse events were gastrointestinal (Jastreboff et al., NEJM, online 29 September 2026). TRIUMPH-2 randomised 1,152 adults with obesity and type 2 diabetes for 80 weeks; the primary endpoint was the change in body weight at week 80, with gastrointestinal events more frequent with retatrutide (Bellido et al., Lancet, online 29 September 2026). TRANSCEND-T2D-1 randomised 537 adults with type 2 diabetes for 40 weeks; the primary endpoint was the change in HbA1c, with body weight among the secondary endpoints (Bajaj et al., Lancet, online 6 June 2026).
What is licensed in the UK for weight management
Retatrutide is not. The MHRA's release of 24 July 2026, No summer shortcut for safe weight loss, names the GLP-1 medicines that are prescribed: "Semaglutide (Wegovy), tirzepatide (Mounjaro) and liraglutide (Saxenda) should only be taken as prescribed by a healthcare professional." It also states that retatrutide "has not been authorised for use in the UK".
The MHRA authorised tirzepatide (Mounjaro) for weight management on 8 November 2023 for adults with a BMI of 30 or more, or 27 to 30 with a weight-related health problem. Semaglutide (Wegovy) was already approved for weight management when the MHRA added a cardiovascular indication on 23 July 2024, and on 11 June 2026 it approved a semaglutide tablet, noting: "As with all GLP-1 receptor agonists, this is a prescription-only medication." For NHS use, NICE recommends semaglutide (TA875, March 2023) and tirzepatide (TA1026, December 2024) only for adults who meet BMI and weight-related comorbidity criteria.
Capsule versus injection
Every trial paper cited here used once-weekly subcutaneous injection. Of the 33 retatrutide studies on clinicaltrials.gov on 9 October 2026, 32 describe it as given by subcutaneous injection and none describes an oral form. Results from injection trials cannot be assumed to apply to a capsule. Free Muscle's retatrutide listing is a capsule; its product page says so and makes no claim that an oral capsule reproduces trial results.
UK legal status in 2026
Retatrutide is not a licensed medicine in the UK. Under regulation 2 of the Human Medicines Regulations 2012, a substance presented as treating disease, or that may be administered to people to modify a physiological function by a pharmacological or metabolic action, is a medicinal product, and regulation 46 prohibits selling, supplying or offering an unauthorised medicinal product. The MHRA has applied these rules to retatrutide in practice: on 28 May 2026 its Criminal Enforcement Unit seized around 12,000 doses of unlicensed weight loss medicines, "including retatrutide and tirzepatide, as well as peptide products", and two men were arrested on suspicion of offences under the Regulations.
Free Muscle lists retatrutide as a research compound, supplied in capsule form to adults aged 18 or over for laboratory research only. It is not presented as a medicine or for weight management, and no information on use is given.
In sport, retatrutide is not named on the 2026 WADA Prohibited List. Section S0 prohibits at all times any pharmacological substance not addressed elsewhere on the List with no current approval by any governmental regulatory health authority for human therapeutic use.
Research compounds sold by Free Muscle are supplied for laboratory research use only and are not for human consumption. They are not authorised as medicines by the MHRA, EMA or FDA, are not food supplements, and are sold only to adults aged 18 or over.
Frequently asked questions
Is retatrutide licensed in the UK?
No. The MHRA stated in July 2026 that retatrutide "has not been authorised for use in the UK". Its phase 3 trials are registered on clinicaltrials.gov, and several are still running.
Which weight-management medicines are licensed in the UK?
The MHRA names semaglutide (Wegovy), tirzepatide (Mounjaro) and liraglutide (Saxenda), all prescription-only. A semaglutide tablet was approved in June 2026. NICE recommends semaglutide (TA875) and tirzepatide (TA1026) for NHS use within BMI and comorbidity criteria.
What did the retatrutide phase 2 obesity trial measure?
Percentage change in body weight at 24 weeks in 338 adults given weekly injections for 48 weeks, with gastrointestinal events the most common adverse events. The figures are in the published paper.
Has any phase 3 trial been published?
Yes. TRIUMPH-1 (obesity without diabetes) appeared in the NEJM and TRIUMPH-2 (obesity with type 2 diabetes) in the Lancet, both online on 29 September 2026, and TRANSCEND-T2D-1 (type 2 diabetes) in the Lancet in June 2026.
Were retatrutide capsules tested in the trials?
No. The published trials used once-weekly subcutaneous injection, and none of the retatrutide studies on clinicaltrials.gov describes an oral form.
Is retatrutide prohibited in sport?
It is not named on the 2026 WADA Prohibited List. Section S0 covers pharmacological substances with no approval from any governmental health authority for human therapeutic use, so athletes should check with their anti-doping organisation.
Sources
- Jastreboff AM et al. Retatrutide for obesity, phase 2. N Engl J Med 2023;389:514. PMID 37366315
- Rosenstock J et al. Retatrutide in type 2 diabetes, phase 2. Lancet 2023;402:529. PMID 37385280
- Jastreboff AM et al. TRIUMPH-1, phase 3. N Engl J Med 2026, online ahead of print. PMID 42814954
- Bellido V et al. TRIUMPH-2, phase 3. Lancet 2026;408:1455. PMID 42810372
- Bajaj HS et al. TRANSCEND-T2D-1, phase 3. Lancet 2026;407:2402. PMID 42250575
- ClinicalTrials.gov records NCT05929066, NCT05929079, NCT05882045, NCT05931367, NCT06354660, NCT06383390, NCT06662383.
- MHRA press releases: 24 July 2026; 8 November 2023; 23 July 2024; 11 June 2026; 29 May 2026.
- NICE technology appraisals TA875 (semaglutide) and TA1026 (tirzepatide).
- Human Medicines Regulations 2012, regulations 2 and 46.
- World Anti-Doping Agency: The Prohibited List (2026).
All sources were read on 9 October 2026.
Related reading: Do oral peptides actually work? · Listings: Retatrutide and the research peptides collection.