How it's designed to work

Retatrutide is designed to activate three separate metabolic receptors at once: the glucose-dependent insulinotropic polypeptide (GIP) receptor, the glucagon-like peptide-1 (GLP-1) receptor, and the glucagon receptor. Tirzepatide, by comparison, acts on only two of these (GIP and GLP-1). The added glucagon-receptor activity is thought to increase energy expenditure on top of the appetite-suppressing effects shared with other GLP-1 medicines, which is the pharmacological rationale for its larger weight-loss effect in trials so far.

Phase 2 trial results

The main published efficacy data come from a randomised, placebo-controlled Phase 2 trial in 338 adults with obesity (without type 2 diabetes), conducted from May 2021 to November 2022 and published in the New England Journal of Medicine in August 2023. At 24 weeks, mean body-weight change was −7.2% with 1mg retatrutide, −12.9% with the pooled 4mg groups, −17.3% with the pooled 8mg groups and −17.5% with 12mg, against −1.6% for placebo. At 48 weeks the effect grew further: −8.7% (1mg), −17.1% (pooled 4mg), −22.8% (pooled 8mg) and −24.2% (12mg), against −2.1% for placebo. Source: Jastreboff AM, et al. "Triple–Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial." N Engl J Med. 2023 (PubMed 37366315).

Safety and side effects

In the same Phase 2 trial, the most common adverse events were gastrointestinal — nausea, diarrhoea, vomiting and constipation — occurring more often with retatrutide than placebo and mostly during dose escalation. Reported nausea rose from roughly 14% at the 1mg dose to about 60% at 12mg, illustrating a clear dose-response pattern; a lower 2mg starting dose (versus 4mg) partly reduced these effects. Discontinuation because of adverse events occurred in 6–16% of participants on retatrutide, versus none on placebo. Serious adverse events occurred in 0–6% of retatrutide groups compared with 4% on placebo. Source: Jastreboff AM, et al., N Engl J Med. 2023 (PubMed 37366315), as above.

Phase 3 programme and other indications

Eli Lilly's Phase 3 programme for retatrutide is called TRIUMPH. By mid-2026 the company had reported positive topline results from multiple trials in this programme, including studies in adults with obesity or overweight with type 2 diabetes and in adults with severe obesity and established cardiovascular disease; across the 4mg, 9mg and 12mg doses tested, the trials reportedly met their primary and key secondary weight-loss endpoints at 80 weeks. A related Phase 3 diabetes trial (TRANSCEND-T2D-1) also reported results in mid-2026. Separately, a smaller Phase 2a trial has examined retatrutide in metabolic dysfunction-associated steatotic liver disease (MASLD, previously known as NAFLD). Sources: Eli Lilly and Company investor news releases, 2026; company statements reported via industry press (PharmExec, BioPharma Dive), 2026.

Regulatory status: not approved anywhere

As of September 2026, retatrutide has no marketing authorisation from the MHRA, the FDA or the EMA. It cannot lawfully be prescribed or sold as a medicine in the UK, and the only legal way to receive it is as a participant in an approved clinical trial. Eli Lilly has said it intends to submit a Biologics License Application to the US FDA in the first quarter of 2027; on that timeline, UK availability through the MHRA would realistically follow at the earliest in 2027–2028, with any NHS use later still.

Despite this, products marketed as "retatrutide" are sold outside the licensed supply chain, and the MHRA has repeatedly seized unlicensed stock. In a Northamptonshire operation on 28 May 2026, the MHRA's Criminal Enforcement Unit recovered around 12,000 doses of unlicensed weight-loss medicines, which it described as "unlicensed weight loss medicines, including retatrutide and tirzepatide," calling it the largest such seizure by the agency to date. The MHRA's Andy Morling said: "Medicines regulation isn't discretionary — it exists to protect people." Source: MHRA / GOV.UK, 29 May 2026. An earlier, larger raid near Northampton in October 2025 also recovered more than 2,000 unlicensed retatrutide and tirzepatide pens alongside raw chemical ingredients. Source: MHRA / GOV.UK, 24 Oct 2025

Because no licensed retatrutide product exists, none of the GPhC-registered pharmacies listed elsewhere on this site can lawfully supply it, and any UK seller offering "retatrutide" for purchase today is, by definition, operating outside the licensed medicines system — the MHRA warns such products are untested for safety, quality or effectiveness and may not even contain what they claim.