Retatrutide
Retatrutide is an experimental injected medicine from Eli Lilly that acts on three hormone receptors involved in appetite and metabolism. It is in phase 3 trials for obesity and related conditions and is not approved anywhere.
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What it is
Retatrutide (development code LY3437943) is an experimental peptide medicine developed by Eli Lilly for obesity and weight-related conditions [3, 6]. As of October 2026 it is not approved by any regulator. Lilly has said it plans to apply to the FDA in early 2027 [6]. Outside authorised clinical trials, it cannot be legally sold for human use [1, 2, 6].
How it works
Retatrutide is called a "triple agonist" because it activates the receptors for three hormones: glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-1 (GLP-1) and glucagon [5, 6]. GIP and GLP-1 are gut hormones that help control blood glucose and appetite. Approved medicines such as semaglutide act on the GLP-1 receptor, and tirzepatide acts on both GIP and GLP-1 receptors [7, 8]. Adding activity at the glucagon receptor is what makes retatrutide different [5].
Approved medical uses
None in people. The FDA states it is not approved for any condition [1], and the MHRA states it has not been approved for UK use [2].
What the research shows
All of the phase 3 evidence so far comes from the TRIUMPH programme, which is sponsored by the manufacturer [3, 6]. The programme has enrolled more than 5,800 participants and is testing retatrutide in obesity, type 2 diabetes, sleep apnoea and knee osteoarthritis [6].
- TRIUMPH-1 enrolled about 2,300 adults with obesity or overweight but without diabetes [3]. At 80 weeks, people on the highest dose studied lost about 25% of body weight on average, compared with about 4% on placebo [4].
- TRIUMPH-4 enrolled 445 adults with obesity or overweight and knee osteoarthritis. Over 68 weeks, average weight loss was more than a quarter of body weight at the higher doses studied, compared with about 2% on placebo, and knee pain scores fell more than with placebo [5].
- TRIUMPH-2 enrolled 1,152 adults with type 2 diabetes and obesity or overweight. Over 80 weeks, retatrutide reduced body weight by up to about 21% and lowered HbA1c, a measure of long-term blood glucose [6]. The manufacturer reports this trial has been published in The Lancet [6].
- These results come from company-funded trials, and some were first reported in press releases before full peer review. UK pharmacists have urged careful scrutiny of the full safety data [4, 6].
Known risks and side effects
- Stomach and bowel effects: nausea, diarrhoea, constipation and vomiting were the most common side effects in phase 3 trials, and were more frequent at higher doses [4, 6].
- Stopping because of side effects: in TRIUMPH-1, up to about 11% of people on the highest dose stopped because of side effects, compared with about 5% on placebo [4]. In TRIUMPH-4 this reached about 18% at the highest dose, compared with 4% on placebo [5].
- Abnormal skin sensations (dysaesthesia): reported more often with retatrutide than placebo, in roughly 5% to 10% of people depending on dose and trial, compared with under 1% on placebo in TRIUMPH-2 [4, 6]. Most cases were mild to moderate [4, 6].
- Urinary tract infections: reported in around 10% of people at the highest dose in TRIUMPH-1 [4].
- Risks of related medicines: approved medicines in the same family carry warnings about pancreatitis, gallbladder disease, kidney injury from dehydration and thyroid C-cell tumours in rodents [7, 8]. Whether retatrutide shares these risks is still being studied.
- Unlicensed products: the FDA states retatrutide is not approved for any condition and cannot legally be used in compounding [1]. The MHRA states it has not been approved for UK use and that products sold in the UK claiming to contain it are likely to be illegal. In 2025 the MHRA seized illegally made retatrutide pens and warned that such products may be contaminated with toxic substances [2].
- Sport: as a substance with no approval for human therapeutic use, it falls under the World Anti-Doping Agency's S0 category of non-approved substances, which are prohibited at all times [9].
Blood tests commonly monitored
There are no official monitoring guidelines for retatrutide because it is not an approved medicine. For approved medicines in the same family (GLP-1 and GIP/GLP-1 receptor agonists), product labels mention:
- Blood glucose: in people with diabetes, especially alongside insulin or sulfonylureas [7, 8].
- Kidney function: in people with vomiting, diarrhoea or dehydration [7, 8].
- Calcitonin: routine testing is of uncertain value for detecting thyroid cancer [7, 8].
- Lipase and amylase: may rise; the meaning without symptoms of pancreatitis is unclear [7, 8].
If you and your doctor are monitoring treatment, Enhanced Training Studio keeps a private log on your phone: what you take and when, your blood test results over time, and how you feel. How tracking works.
Sources
- US Food and Drug Administration. FDA's concerns with unapproved GLP-1 drugs used for weight loss. FDA, updated 2026. FDA: unapproved GLP-1 drugs
- Medicines and Healthcare products Regulatory Agency. MHRA smashes major illicit weight loss medicine production facility in record seizure. GOV.UK, 2025. MHRA: illicit facility seizure
- ClinicalTrials.gov. A master protocol to investigate the efficacy and safety of LY3437943 ... in participants without type 2 diabetes who have obesity or overweight (TRIUMPH-1), NCT05929066. US National Library of Medicine. ClinicalTrials.gov NCT05929066
- Robertson J. Phase III retatrutide study demonstrates 30% weight loss. The Pharmaceutical Journal, 2026. Pharmaceutical Journal
- Willis S. Investigational weight-loss drug trial shows average reduced body weight of 30%. The Pharmaceutical Journal, 2025. Pharmaceutical Journal
- Eli Lilly and Company. Lilly's triple agonist, retatrutide, delivered substantial weight loss and A1C reduction (TRIUMPH-2). Press release, September 2026. Eli Lilly investor news
- US Food and Drug Administration / Novo Nordisk. Wegovy (semaglutide): prescribing information. DailyMed, US National Library of Medicine, revised 2026. DailyMed: Wegovy
- US Food and Drug Administration / Eli Lilly. Zepbound (tirzepatide): prescribing information. DailyMed, US National Library of Medicine, revised 2026. DailyMed: Zepbound
- World Anti-Doping Agency. The 2026 Prohibited List (S0 Non-approved substances), as published in the Dutch Treaty Series (Tractatenblad) 2026, no. 25. 2026 Prohibited List
Related
Last reviewed 7 October 2026. General information, not medical advice. Found a mistake? Email [email protected].