Important regulatory notice. Retatrutide is an investigational compound currently in Phase 3 clinical trials and is not licensed for human or veterinary use in the United Kingdom by the MHRA, EMA or FDA. This page is a research-context summary of the published Phase II clinical-trial protocol design only. It is not dosing advice, not a usage protocol, and not intended for human or animal administration. Peptides Lab UK supplies retatrutide as a research-use-only laboratory reference compound with batch-specific HPLC certificates of analysis. The only legal access route to retatrutide for human use in the United Kingdom is participation in an MHRA-authorised clinical trial via the NIHR Be Part of Research portal.
Quick Summary. Retatrutide is an investigational triple-receptor agonist (GLP-1, GIP, glucagon) developed by Eli Lilly and currently in late-stage clinical development. The published Phase II trial protocol used a slow titration design starting at 1 mg per week. The information below is a literature reference only. It is not dosing guidance and Peptides Lab UK does not endorse any human use of this compound.
Where the Phase II protocol comes from
The dose schedule discussed in the public retatrutide literature comes from the Phase II trial published in the New England Journal of Medicine in June 2023 (Jastreboff et al.). That trial was conducted under regulatory authorisation in clinical settings. It was not a laboratory-use protocol, and it is not transferable outside an authorised clinical trial environment. Anyone interested in the trial design itself should read the primary source on the NEJM website, doi 10.1056/NEJMoa2301972.
Phase II trial protocol design (literature reference)
The published Phase II protocol, as reported in the peer-reviewed literature, used a 4-week step titration design with the following pattern.
| Trial week | Reported step | Trial purpose of the step |
|---|---|---|
| Weeks 1 to 4 | 1.0 mg per week | Receptor sensitisation phase |
| Weeks 5 to 8 | 4.0 mg per week | First escalation |
| Weeks 9 to 12 | 8.0 mg per week | Second escalation |
| Week 13 onward | 12.0 mg per week | Maximum dose tested in the trial |
This is a description of how the clinical trial was designed, taken from the published paper. It is not a recommendation for personal use and it cannot legally be replicated outside an authorised clinical-trial setting in the United Kingdom.
Why the trial used a slow step design
Investigators in the Phase II trial described the slow titration as a tolerability strategy. Faster escalation in early dose-finding work was associated with higher gastrointestinal adverse-event rates, which is consistent with the wider GLP-1, GIP and glucagon-receptor agonist class. The published rationale is included in the supplementary material of the NEJM paper.
Reconstitution information for laboratory reference samples
For laboratory researchers handling retatrutide as a lyophilised reference compound, standard peptide-handling practice applies: bring the vial to room temperature before opening, add reconstitution solvent slowly down the side of the vial rather than directly onto the lyophilised cake, swirl gently rather than shake, and refrigerate the reconstituted material at 2 to 8 degrees Celsius. These are general laboratory handling notes for a research-grade reference compound. They are not instructions for human or animal use.
UK regulatory position
Retatrutide has not received a marketing authorisation in the United Kingdom. The MHRA position, repeated in coverage by The Guardian and the BBC during 2025 and 2026, is that retatrutide marketed for human use in the UK outside an authorised clinical trial is unlicensed. UK retailers presenting the compound with dosing tables framed as personal-use guidance are operating outside the regulatory framework. Licensed alternatives in the UK weight-management space currently include tirzepatide (Mounjaro), semaglutide (Wegovy) and liraglutide (Saxenda), all of which require a registered prescriber.
If you are a researcher
If you are a laboratory researcher and need a reference sample of retatrutide for in-vitro or analytical work, what you should look for in any supplier is straightforward: batch-specific certificate of analysis, third-party HPLC purity data, mass-spectrometry identity confirmation, and clear research-use-only labelling. Peptides Lab UK supplies retatrutide on that basis. We do not provide dosing or therapeutic guidance and our products are not for human or veterinary use. View retatrutide research compound →
If you are a member of the public
If you are reading this page because you have seen retatrutide marketed online for weight loss in the United Kingdom, please understand that the compound is not licensed for that use. The Guardian (4 April 2026) reported that the MHRA is investigating clinics making therapeutic claims about unregulated peptide products. The only legal route to retatrutide for human use in the UK is an MHRA-authorised clinical trial. The NIHR Be Part of Research portal at bepartofresearch.nihr.ac.uk lists current trials.
Frequently Asked Questions
Is retatrutide licensed in the UK?
No. Retatrutide is investigational and has not received a marketing authorisation from the MHRA. It is currently in Phase 3 clinical development (the TRIUMPH programme). A licensing decision would follow Phase 3 readout — public estimates suggest no UK availability before late 2026 or 2027 at the earliest.
What dose of retatrutide was used in the Phase II trial?
The published Phase II trial (Jastreboff et al., NEJM, June 2023) used a slow titration design: 1.0 mg/week for weeks 1–4, 4.0 mg/week for weeks 5–8, 8.0 mg/week for weeks 9–12, and 12.0 mg/week from week 13. This is a description of the clinical trial design only — it is not personal dosing guidance and cannot be used outside an authorised clinical-trial setting.
Can the Phase II protocol be used as personal dosing guidance?
No. The Phase II trial was a regulated clinical-trial setting with full medical oversight, monitoring and safety governance. It is not transferable to personal use, and any attempt to replicate it outside an authorised trial sits outside UK law.
Why does Peptides Lab UK supply retatrutide if it is not licensed?
We supply retatrutide as a research-use-only laboratory reference compound for in-vitro work, identity standards and analytical chemistry, with batch-specific HPLC verification. That market sits outside the human-medicines framework. We do not market retatrutide for human or veterinary use and we do not provide dosing or therapeutic guidance.
Where is the original Phase II trial paper?
Jastreboff AM et al, New England Journal of Medicine, June 2023. DOI: 10.1056/NEJMoa2301972. PubMed PMID: 37369579.
When might retatrutide become licensed in the UK?
The Phase 3 TRIUMPH programme is scheduled to read out across 2026. A regulatory review and licensing decision in the UK would follow that data readout. Most public-pharmacy estimates suggest no UK availability before late 2026 or 2027, dependent on MHRA authorisation.
Research use only. Information on this page is a literature-reference summary of published Phase II protocol design and current UK regulatory status. It is not medical, clinical or therapeutic advice. Retatrutide is not licensed in the United Kingdom. Peptides Lab UK supplies research-use-only laboratory reference compounds with batch-specific certificates of analysis. Products are not for human or veterinary use. If you are considering retatrutide for a clinical purpose, the only legal UK route is an MHRA-authorised clinical trial via bepartofresearch.nihr.ac.uk.
