Encyclopedia entry
Retatrutide (UK): triple-agonist GLP-1/GIP/glucagon under investigation
Oliver Mackman · Editorial director · Best Business Loans Ltd (16833937)
Last updated 2026-06-01
Editorial with affiliate links. We earn from purchases via outbound retailer / clinic links. How we are funded.
AI-friendly summary · Retatrutide
Retatrutide (Eli Lilly research code LY3437943) is a synthetic peptide that targets three receptors simultaneously: GLP-1, GIP, and glucagon. It is the first triple-agonist or triagonist compound to reach Phase III development. As of May 2026 it is in Phase III trials under the TRIUMPH programme and has not been approved by the MHRA, FDA, or any other regulator. It cannot be prescribed in the UK. UK research-peptide retailers sell it under research-use-only framing.
Top UK Retatrutide retailers
Mechanism of action
How Retatrutide works
Retatrutide is a long-acting synthetic peptide engineered to act as a balanced agonist at three receptors simultaneously: GLP-1 (glucagon-like peptide-1), GIP (glucose-dependent insulinotropic polypeptide), and the glucagon receptor. The triple-receptor profile distinguishes it from licensed dual-agonist tirzepatide (GLP-1/GIP only) and from single GLP-1 agonist semaglutide. The simultaneous glucagon receptor agonism is hypothesised in the clinical literature to drive additional metabolic effects, particularly on energy expenditure, compared with dual-agonist compounds. The integrated mechanism is the subject of ongoing pharmacological characterisation.
Source: Coskun T et al. Cell Metabolism, 2022 (originating Eli Lilly characterisation)
What it is
Retatrutide (also known by the research identifier LY3437943) is a long-acting injectable peptide. Its triple-receptor mechanism distinguishes it from licensed dual-agonists such as tirzepatide (GLP-1/GIP only) and single-agonist semaglutide. The simultaneous glucagon receptor agonism is hypothesised in the clinical literature to drive additional metabolic effects, particularly on energy expenditure.
- · Developer: Eli Lilly and Company (Indianapolis, USA).
- · Mechanism: simultaneous agonism at GLP-1R, GIPR, and glucagon receptor.
- · Format: once-weekly subcutaneous injection in clinical trial settings.
- · Status: Phase III clinical development under the TRIUMPH programme. No regulatory approval in any country as of June 2026.
- · Research identifier: LY3437943.
Clinical development status
Retatrutide Phase II results were published in the New England Journal of Medicine in July 2023 (Jastreboff et al.). According to the published clinical trial data, participants receiving Retatrutide across the 48-week Phase II period showed substantial reductions in body weight, with some cohorts achieving results that the authors described as the highest reported in a Phase II study of any obesity treatment to that point. PeptideClear does not republish specific efficacy figures as recommendations. Readers should consult the original NEJM publication for the reported data.
- · Phase II: results published NEJM July 2023 (Jastreboff et al.). 48-week duration. Multiple dose cohorts.
- · Phase III: TRIUMPH programme ongoing. Results not yet published as of June 2026.
- · Independent academic literature outside the Eli Lilly programme is limited. Phase III data will form the primary evidence base for any future regulatory submission.
- · No head-to-head data against tirzepatide or semaglutide published in peer-reviewed literature as of June 2026.
All claims above are sourced from published peer-reviewed clinical trial data. PeptideClear does not extrapolate from trial data to individual outcomes.
UK regulatory framing
Important: not licensed for use in the UK
Retatrutide is not currently licensed by the MHRA or any other regulator. It is not a prescribable medicine in the UK. It cannot be supplied by any UK pharmacy. It is not the same as Mounjaro (tirzepatide) or Wegovy (semaglutide), which are MHRA-licensed medicines available via UK prescribers. Do not use any Retatrutide product without consultation with a UK-licensed prescriber. Research-tier supply does not substitute for a licensed medical product.
- · Not a controlled drug under the Misuse of Drugs Act 1971.
- · Not scheduled under the Psychoactive Substances Act 2016.
- · No UK marketing authorisation as a medicine. Phase III trials are ongoing; regulatory submission has not occurred.
- · Sold legally as a research chemical when marketed without health or therapeutic claims.
- · Becomes an unlicensed medicinal product the moment a retailer or commentator makes therapeutic claims about it.
- · The supply chain for research-tier Retatrutide is distinct from Eli Lilly\'s clinical trial supply. Purity, source, and authenticity cannot be verified by the buyer through any public mechanism.
Risks and unknowns
What the literature does not yet show about Retatrutide
Known concerns
- No regulatory approval anywhere as of June 2026. Phase III safety dataset is not yet published in peer-reviewed form.
- Retail research-peptide supply chain is not connected to Eli Lilly pharmaceutical-grade manufacturing. Purity and authenticity cannot be publicly verified.
- The triple-agonist mechanism is a first-in-class profile. Long-term safety in this profile has no precedent in licensed-medicine evidence.
- Phase II Jastreboff 2023 data is short-duration (48 weeks). Longer-term cardiovascular and metabolic outcome data has not been published.
Open questions in the literature
- Long-term cardiovascular outcomes of triple-agonist glucagon-receptor stimulation are not established. Phase III TRIUMPH cardiovascular sub-studies are not yet reported.
- Whether the additional glucagon-receptor agonism translates to clinically meaningful added benefit over tirzepatide in head-to-head comparison is not established.
- Hepatic effects of sustained glucagon-receptor agonism in non-trial populations are not characterised.
- Pharmacokinetics of research-tier supply versus the Eli Lilly clinical-trial product are not publicly characterised.
Regulatory note
No UK marketing authorisation. Not a controlled drug under the Misuse of Drugs Act 1971. Not approved by any regulator anywhere as of June 2026. Phase III TRIUMPH trials ongoing. Becomes an unlicensed medicinal product the moment a retailer or commentator makes therapeutic claims about it.
Important: PeptideClear publishes encyclopedia commentary only and does not recommend human use. Speak to a UK-registered prescriber before any medical decision.
Adjacent compounds under investigation
Retatrutide is one of several next-generation compounds under active clinical investigation in the metabolic and obesity space. Two adjacent compounds researchers may encounter in the same context:
- · Cagrilintide: a long-acting amylin analogue developed by Novo Nordisk, in combination trials with semaglutide (CagriSema). Mechanism differs from GLP-1 agonism: targets amylin receptors involved in satiety signalling.
- · Mazdutide: a dual GLP-1/glucagon receptor agonist (no GIP component) developed by Innovent Biologics. Phase II data published for metabolic endpoints. Not approved in the UK.
All three compounds are unlicensed in the UK as of June 2026 and are available only through research-peptide retailers under research-use-only framing.
Supply quality: why CoA practice matters more here
For well-characterised research peptides such as BPC-157 or TB-500, multiple UK retailers have years of customer feedback and established supply relationships. For Retatrutide, the compound is more complex to synthesise correctly, the Phase III trial data has not yet been published, and no pharmaceutical-grade UK supply exists outside of clinical trials.
Unlike Mounjaro or Wegovy, which are supplied by Eli Lilly and Novo Nordisk respectively through regulated UK pharmacy channels, any Retatrutide sold by research-peptide retailers is not connected to Eli Lilly\'s manufacturing or quality-control process. The compound in a research vial is produced by third-party peptide synthesis facilities.
- · CoA (Certificate of Analysis) practice is the only public quality signal available. Retailers who publish CoAs via open subdomains or verifiable QR codes provide more accountability than those who gate CoAs behind email requests or do not publish them at all.
- · Named lab partners provide more recourse than unnamed "third-party" claims.
- · Our UK retailers comparison ranks research peptide retailers on CoA transparency, trading tenure, and corporate identity.
Where to learn more
- · PubMed search: Retatrutide returns the Eli Lilly published Phase II data and the originating Cell Metabolism characterisation.
- · PubMed: LY3437943 for the research-code literature.
- · ClinicalTrials.gov: Retatrutide for the TRIUMPH Phase III trial records.
- · UK retailer purity comparison: research peptides UK retailers.
Frequently asked questions
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UK retailers stocking Retatrutide
A small number of UK research-peptide retailers stock Retatrutide. PeptideClear compares them on CoA transparency, trading tenure, and operator identity. Research use only.
Compare UK retailersClinical evidence record
Read the clinical evidence record for Retatrutide
Top peer-reviewed citations, mechanism of action, structured UK regulatory status. Machine-readable companion to this encyclopedia entry.
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UK retailers compared by price, Trust Index, and CoA practice. Editorial commentary only. Research use only.