Retatrutide produced a mean weight reduction of up to 24.2% over 48 weeks in Eli Lilly’s Phase 2 trial — and, unusually, the weight loss had not plateaued when the study ended. That single fact is why retatrutide (LY3437943) is currently the most talked-about compound in obesity research. This guide lays out exactly what the retatrutide weight loss results show, where the numbers come from, how it stacks up against semaglutide and tirzepatide, and the all-important UK research context. No hype — just the published data, accurately.
On this page
- What is retatrutide?
- The headline weight-loss results
- How retatrutide drives weight loss
- Beyond weight: liver fat & metabolic health
- Retatrutide vs semaglutide vs tirzepatide
- Phase 3: the TRIUMPH programme
- Side effects in the trial
- The UK research context
- FAQ
Key takeaways
- Up to 24.2% mean body-weight reduction at the 12 mg dose over 48 weeks (Phase 2), versus roughly 2.1% on placebo.
- That’s around 26 kg on average — and the curve was still falling at week 48, with no clear plateau.
- It’s a triple agonist (GLP-1 + GIP + glucagon) — the glucagon arm is what sets it apart from semaglutide and tirzepatide.
- It remains investigational: no MHRA or FDA marketing authorisation, and in the UK it’s supplied only as a research compound.
- Phase 3 (TRIUMPH) is ongoing and will decide its clinical future.
What is retatrutide?
Retatrutide (development code LY3437943) is a once-weekly injectable peptide developed by Eli Lilly. It’s described as a “triple agonist” because it activates three metabolic receptors at once:
- GLP-1 (glucagon-like peptide-1) — curbs appetite, slows gastric emptying
- GIP (glucose-dependent insulinotropic polypeptide) — supports insulin response and appetite control
- Glucagon receptor — the differentiator, linked to increased energy expenditure and fat metabolism
Semaglutide (Wegovy/Ozempic) hits one of those receptors; tirzepatide (Mounjaro/Zepbound) hits two. Retatrutide hits all three — the mechanistic reason its trial numbers run higher.
The headline weight-loss results
The pivotal data come from the Phase 2 trial published in the New England Journal of Medicine (Jastreboff et al., 2023): 338 adults with obesity, randomised to retatrutide (1, 4, 8 or 12 mg) or placebo, for 48 weeks.
- 12 mg dose: −24.2% mean body weight at 48 weeks — roughly 26 kg.
- 8 mg dose: around −22%.
- Placebo: approximately −2.1%.
- No plateau: at the highest doses the weight curve was still declining at week 48 — meaning the 24.2% may understate the full effect over a longer period.
That last point is what excited researchers most: most weight-loss agents flatten out, but retatrutide hadn’t. You can read the primary data in the NEJM Phase 2 publication. (For comparison, the “28.7%” figure floating around some sites isn’t the headline published mean — the robust, citable number is 24.2% at 48 weeks.)
How retatrutide drives weight loss
The triple mechanism works on three fronts at once:
- Appetite regulation — the GLP-1 and GIP activity reduces hunger signalling and increases satiety, so less food is eaten without constant willpower.
- Energy expenditure — the glucagon component is associated with increased fat metabolism and higher metabolic activity, in theory burning more rather than only eating less.
- Blood-sugar control — improved glucose handling and reductions in HbA1c were seen in trial participants.
It’s the glucagon arm — absent from semaglutide and tirzepatide — that researchers credit for the extra edge.
Beyond weight: liver fat and metabolic health
The results that arguably impressed clinicians most weren’t on the scales. In the trial, retatrutide was associated with dramatic reductions in liver fat, with a large proportion of participants who had fatty liver (steatosis) reaching normal liver-fat levels. Improvements in blood pressure, lipids and insulin sensitivity were also reported.
This is the angle urologist Dr Alex Tatem highlighted on The Diary of a CEO when he described an unreleased compound that “tortures belly fat at a disproportionate rate” while delivering “the best improvements we’ve ever seen in… liver health” — predicting a “trillion-dollar drug when it comes out.” The visceral-fat-and-liver profile of retatrutide fits that description closely. (Our full breakdown of his interview is here.)
Retatrutide vs semaglutide vs tirzepatide
Approximate mean weight-loss figures from each compound’s pivotal trials:
| Compound | Receptor targets | Approx. mean weight loss |
|---|---|---|
| Semaglutide (Wegovy) | GLP-1 | ~15% (STEP trials, 68 wks) |
| Tirzepatide (Mounjaro/Zepbound) | GLP-1 + GIP | ~20–22.5% (SURMOUNT) |
| Retatrutide | GLP-1 + GIP + glucagon | ~24.2% (Phase 2, 48 wks, still falling) |
A direct caveat for honesty: these come from different trials of different lengths and populations, so they’re indicative, not a head-to-head. But the trend — more receptors, more effect — is consistent.
Phase 3: the TRIUMPH programme
Retatrutide is now in Phase 3 trials (the TRIUMPH programme), the stage that evaluates long-term safety and effectiveness across much larger populations and decides whether it reaches the market. Until those complete and a regulator grants authorisation, retatrutide remains an investigational compound — promising data, not an approved medicine.
Side effects in the trial
Like other metabolic peptides, the side-effect profile was mostly gastrointestinal and dose-dependent:
- Nausea, vomiting, diarrhoea and general GI discomfort were the most common.
- Effects were more frequent at higher doses and during dose escalation.
- Some participants discontinued over tolerability.
Gradual dose titration was used to limit these — a standard approach across the GLP-1 class.
The UK research context
This is the part the hype tends to skip. Retatrutide has no MHRA marketing authorisation and no FDA approval — it is still in clinical trials. In the UK it cannot legally be sold or prescribed as a weight-loss medicine. What UK suppliers offer is retatrutide as a research compound, for laboratory use only — not for human consumption.
So when you read “retatrutide weight loss results,” you’re reading clinical trial outcomes from Eli Lilly’s controlled studies — not a claim about a product you can buy and use. If you’re sourcing it for research, the only thing that separates a credible supplier from a risky one is verification: a batch-specific, third-party Certificate of Analysis confirming identity and ≥99% purity (see our quality testing page). Researchers handling lyophilised vials can work out exact concentrations with our free retatrutide reconstitution calculator.
Frequently asked questions
How much weight did people lose on retatrutide?
In Eli Lilly’s Phase 2 trial, the 12 mg dose produced a mean reduction of about 24.2% of body weight over 48 weeks (roughly 26 kg), versus around 2.1% on placebo — and the loss had not plateaued by the study’s end.
Is retatrutide better than tirzepatide or semaglutide?
On trial averages, retatrutide’s ~24% exceeds tirzepatide’s ~20–22% and semaglutide’s ~15%, largely thanks to its added glucagon activity. But these are separate trials, and Phase 3 head-to-head data will tell the fuller story.
Is retatrutide approved or available in the UK?
No. It’s investigational, with no MHRA authorisation, and is supplied in the UK only as a research compound — not for human consumption.
Does retatrutide help with fatty liver?
Trial data showed substantial reductions in liver fat, with many participants who had steatosis reaching normal liver-fat levels — one of the most notable findings beyond weight loss.
What are the main side effects?
Predominantly gastrointestinal — nausea, vomiting and diarrhoea — and dose-dependent, which is why trials used gradual dose escalation.
Final thoughts
Retatrutide’s Phase 2 numbers — ~24.2% mean weight loss in 48 weeks, still falling, with striking liver-fat improvements — are why it’s described as potentially the most powerful obesity compound in development. The honest framing matters too: it’s investigational, the Phase 3 results aren’t in, and in the UK it’s a research compound only. If you’re researching it, start with verification — see current lab-tested stock on our retatrutide UK page, or the per-vial detail for Retaklik (Retatrutide) 60mg and Retatrutide 2.0 (45mg).
Source: Jastreboff AM, et al. Triple–Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial. New England Journal of Medicine, 2023.
This article is for informational and research purposes only and reports published clinical-trial data. Retatrutide is an investigational compound supplied for laboratory research use only and is not for human consumption. It has no MHRA or FDA marketing authorisation. Nothing here is medical advice.