01 What is Retatrutide?
In plain English.
Retatrutide is an experimental once-weekly injection being developed by Eli Lilly for obesity, type 2 diabetes and related metabolic conditions. It belongs to the same broad family as semaglutide (Wegovy / Ozempic) and tirzepatide (Mounjaro / Zepbound), but goes one step further: it acts on three gut and metabolism hormone receptors at once instead of one or two.
It is currently investigational only, meaning it is not available as a prescription medicine anywhere in the world. The only legitimate way to receive it is by enrolling in a registered clinical trial. Anything sold online labelled 'retatrutide' is an unregulated research-chemical preparation with no quality guarantee.
02 How it works
The simple version, then the science.
Retatrutide mimics three of the body's own appetite and metabolism hormones at once. GLP-1 and GIP signalling reduce appetite and improve how the body handles sugar, similar to semaglutide and tirzepatide. The glucagon component is the new piece: it nudges the body to burn more energy and may help mobilise fat from the liver, on top of the appetite suppression.
Go deeper · the proposed mechanism
Retatrutide is a single peptide engineered to bind the GIP, GLP-1 and glucagon receptors with balanced agonist activity. The GLP-1 and GIP arms drive incretin-style effects, delayed gastric emptying, increased satiety, improved insulin secretion. The glucagon arm increases resting energy expenditure and hepatic lipid oxidation, which is thought to explain the additional weight loss and the early signal on liver fat reduction seen in phase 2. Pharmacokinetics support once-weekly subcutaneous dosing.
03 What it's used for
Each use graded by how strong the evidence actually is.
- ModerateObesity / weight loss🔬 Phase 2 trial (n=338) reported ~24% mean weight loss at 48 weeks on the 12 mg dose, the largest reduction reported for any obesity drug at that point. Phase 3 (TRIUMPH programme) is ongoing; results not yet read out.
- ModerateType 2 diabetes🔬 Phase 2 trial showed substantial reductions in HbA1c and body weight in adults with T2D. Phase 3 data still pending; not approved for diabetes.
- PreclinicalMASLD (fatty liver disease)🔬 Phase 2 substudy reported large reductions in liver fat. Promising mechanistic signal; trials in MASH are ongoing.
- AnecdotalOnline "research chemical" weight-loss use💬 Widely discussed in forums and sold by unregulated vendors. There is no quality control, no medical oversight, and no way to verify the contents.
04 What the evidence says
The phase 2 obesity data, published in the New England Journal of Medicine in 2023, is genuinely striking: at the top 12 mg weekly dose, mean weight loss reached roughly 24% at 48 weeks, meaningfully above what has been seen with semaglutide or tirzepatide at the same time point. Phase 2 diabetes results in The Lancet are similarly strong on HbA1c and weight. Two caveats are important to hold simultaneously. First, phase 2 trials are smaller, shorter, and run in carefully selected populations; effect sizes routinely shrink at phase 3 and in broader real-world use. Second, there is no long-term safety record, the longest published data is around a year. Until the TRIUMPH phase 3 programme reads out and a regulator reviews it, the honest description is 'best preliminary obesity data we have ever seen, but unproven and unapproved'.
05 Dosing & administration
Reported in the literature, information not advice.
Trial protocols use once-weekly subcutaneous injection with a slow dose-titration over several months, starting at 2 mg/week and escalating to 4, 8 or 12 mg/week, to manage gastrointestinal side effects. These doses are for context only; there is no approved retatrutide product and no validated outpatient protocol. Any unsupervised use of compounded or grey-market 'retatrutide' is risky and outside the boundary of established medicine.
06 Side effects & safety
The safety profile from phase 2 looked broadly similar to other incretin drugs: mainly gastrointestinal, nausea, vomiting, diarrhoea, constipation, most often during dose escalation. The glucagon component introduces a theoretical risk of increased heart rate and changes in glucose handling that needs longer-term data to characterise properly. Because the published follow-up is short and the population studied is selective, rare or long-latency adverse events have not been ruled out. Pregnancy, breastfeeding, personal or family history of medullary thyroid cancer or MEN-2, severe gastrointestinal disease, and pancreatitis history are typical exclusions in trials of this class. Quality and content of any non-trial "retatrutide" is unknown and unverifiable.
07 Where to buy (research use only)
Vetted on quality and transparency, not an endorsement to use.
08 Legal & regulatory status
- UKNot licensed by the MHRA. Available only via clinical trial participation.
- USNot FDA-approved. Phase 3 trials in progress; no compounding pathway as no shortage of an approved retatrutide product exists.
- EUNot authorised by the EMA. Investigational only.
- SportNot named on the WADA Prohibited List, but as an unapproved investigational substance it is captured by category S0 ("Non-Approved Substances") and is prohibited at all times in tested sport.
09 Clinical studies & research
Primary sources. Read the science yourself.