Medical disclaimer
This content documents personal experience and is not medical advice. Consult a healthcare professional before starting any medication.
If you've ended up here, you're likely wondering what it is, whether it actually works, and whether you can get it in the UK. This guide answers all three.
What is Retatrutide?
Retatrutide (also known by its research code LY3437943) is a next-generation weight loss compound developed by Eli Lilly — the same pharmaceutical company behind Mounjaro.
Where Ozempic targets a single receptor (GLP-1), and Mounjaro targets two (GLP-1 and GIP), Retatrutide hits three: GLP-1, GIP, and glucagon. That triple mechanism is why researchers call it a "triple agonist" — and it's why the results are unlike anything that's come before it.
In plain terms: it's the most powerful metabolic drug ever put through clinical trials.
How does it work?
To understand why Retatrutide is such a leap forward, you need to understand what each receptor does:
GLP-1 (Glucagon-like peptide-1)
This is the hormone that tells your brain you're full and slows down digestion. It's the mechanism that makes Ozempic work. When activated, you simply stop feeling hungry the way you used to.
GIP (Glucose-dependent insulinotropic polypeptide)
GIP enhances the GLP-1 effect — amplifying appetite suppression and improving insulin sensitivity. Adding GIP is why Mounjaro outperforms Ozempic.
Glucagon
Here's what makes Retatrutide different. Glucagon directly increases energy expenditure — your body burns more calories, even at rest. It also targets visceral fat (the dangerous fat stored around your organs) specifically. No other approved weight loss drug does this.
The combination of all three creates a compounding effect: less appetite, better insulin response, and a metabolism that's actively burning more energy. The result is weight loss that dwarfs anything previously seen in clinical trials.
What do the clinical trials show?
Retatrutide is currently in Phase 3 trials under the name TRIUMPH-1 — the largest and longest obesity trial Eli Lilly has ever run.
In May 2026, Eli Lilly announced the topline results:
The headline number
28.3% average body weight reduction.
To put that in context:
- Ozempic (semaglutide): ~15% weight loss
- Mounjaro (tirzepatide): ~21–27% weight loss
- Retatrutide: 28.3% weight loss
For someone starting at 100kg, that's losing roughly 28kg — reaching 72kg — without any dramatic lifestyle changes beyond the medication.
The Phase 2 results (published 2023) showed similar figures, with participants on the highest doses losing an average of 24% of body weight in 48 weeks. Some participants lost over 30%.
These are not modest improvements. They represent a genuine step-change in what's medically achievable for weight loss.
Is Retatrutide approved in the UK?
Not yet. As of July 2026, Retatrutide has not been approved by the MHRA (the UK medicines regulator) and cannot be legally prescribed or dispensed in the UK.
Regulatory submission is expected in 2026–2027. NHS availability, if approved, is likely 2028 at the earliest — and even then, access will initially be restricted to specialist clinics and specific patient criteria, as was the case with Wegovy and Mounjaro.
That means if you want to access it now, the only route is through research peptide suppliers.
How are people in the UK accessing it now?
Retatrutide exists in what researchers call a "legal grey area." It is not a licensed medicine, not a controlled drug, and not on any UK banned import list. It's sold by specialist peptide suppliers — typically from China — as a research compound.
The "for research use only" label these suppliers use is a legal disclaimer. The reality is that thousands of people in the UK are already using it.
The typical process involves:
- Sourcing lyophilised (freeze-dried) Retatrutide powder in vials from a reputable supplier
- Reconstituting with bacteriostatic water
- Injecting weekly using either a reusable peptide pen or insulin syringes
It's more involved than loading an Ozempic pen — but the cost difference is stark. A private UK clinic prescribing Mounjaro will charge £130–200/month. A Retatrutide research kit from a peptide supplier costs roughly £50–70 and lasts several months.
That's the tradeoff: more hands-on, significantly cheaper, dramatically more powerful.
How does it compare to Mounjaro and Ozempic?
| Ozempic / Wegovy | Mounjaro | Retatrutide | |
|---|---|---|---|
| Mechanism | GLP-1 | GLP-1 + GIP | GLP-1 + GIP + Glucagon |
| Avg. weight loss | ~15% | ~21–27% | ~28%+ |
| For 100kg person | ~15kg lost | ~21–27kg lost | ~28kg lost |
| UK prescription | Available privately | Available privately | Not yet approved |
| Research peptide route | Available | Available | Available |
| Monthly cost (clinic) | £100–180 | £130–200 | N/A |
| Monthly cost (research) | ~£8–15 | ~£8–15 | ~£10–20 |
The performance gap between Retatrutide and its predecessors is real and consistent across multiple trials. For people who have tried Ozempic or Mounjaro and found the results disappointing, or who want to maximise results from the start, Retatrutide is the logical next step.
What are the side effects?
The side effect profile is broadly similar to Mounjaro and Ozempic, with some key differences:
Common (especially in the first 4–6 weeks)
- Nausea
- Reduced appetite (this is the intended effect, but it can feel uncomfortable early on)
- Constipation
- Fatigue
Less common
- Vomiting
- Mild headache
- Dizziness
Retatrutide vs Mounjaro on side effects: Retatrutide tends to cause stronger initial side effects, particularly nausea in the first month. The glucagon component increases this. This is why starting at the lowest possible dose and titrating slowly is not optional — it's essential.
The good news: most people find that side effects reduce significantly after weeks 4–6 as the body adapts. The appetite suppression remains; the nausea largely doesn't.
What's the dosing protocol?
Read this first
The schedule below is what is commonly reported by people using the research peptide route. It is a description of common practice, not a prescription, and Retatrutide has no MHRA-approved dosing in the UK. Do not treat it as instructions.
| Weeks | Dose |
|---|---|
| 1–4 | 1mg/week |
| 5–8 | 2mg/week |
| 9–12 | 4mg/week |
| 13+ | 4–8mg/week (maintenance) |
Do not rush this. The temptation to jump to a higher dose to accelerate results is understandable — resist it. The titration schedule exists specifically to minimise side effects and let your body adjust. Most people find 4mg/week is the sweet spot for long-term maintenance.
My take
I'm not a doctor, a pharmacist, or a researcher. I'm someone who spent weeks researching this, decided Retatrutide was the most rational choice given the clinical data, and started documenting the journey publicly so others don't have to piece it together alone.
The clinical evidence for Retatrutide is genuinely extraordinary. 28.3% average weight loss in Phase 3 trials is not hype — it's a number that changes what we thought was possible.
Whether you choose to access it now via the research route or wait for MHRA approval, being informed about what it is and how it works puts you ahead of most people walking into a GP's office asking for "the weight loss jab."
That's what this site is for.
Medical disclaimer
This article is based on personal research and documents the author's experience. It is not medical advice. Always consult a qualified healthcare professional before starting any medication or weight loss programme.
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