The short version: retatrutide activates three receptors and remains investigational. Published evidence now includes the 2023 Phase 2 obesity trial and the September 2026 Phase 3 TRIUMPH-2 report in people with type 2 diabetes. Their populations and follow-up differ. A trial result is not medicine approval; historical trial-arm figures below are not a schedule or recommendation.
Where can you buy retatrutide in Ireland?
Retatrutide is not an authorised medicine in Ireland. Advertising unauthorised human medicines is prohibited; supply and import are subject to legal restrictions. Check current HPRA and Revenue guidance.
Retatrutide remains investigational and is not available for public use. Lilly's September 2026 update, read on 2 October, says it has not been approved by any regulatory agency and is legally available only through its clinical trials. Lilly warns that consumer products sold outside those trials are illegal and their contents cannot be verified. A research-use-only label does not establish medicine authorisation. Check Lilly's current retatrutide status.
If the question is about weight-management treatment, ask a doctor about medicines actually authorised for your circumstances and obtain any prescribed medicine through a registered pharmacy. The HPRA medicine register lets you check authorisation; the HPRA also warns against buying prescription medicines online. A Phase 3 announcement is not an Irish approval or a launch date.
What Is Retatrutide?
Retatrutide (LY3437943) is a once-weekly injectable peptide compound under development by Eli Lilly. Unlike Semaglutide (which targets one receptor) or Tirzepatide (which targets two), Retatrutide is a triple agonist — it activates GLP-1, GIP, and glucagon receptors simultaneously. That additional glucagon pathway is what sets it apart in the research literature and why it has attracted significant clinical interest.
The 2023 New England Journal of Medicine Phase 2 trial reported a least-squares mean body-weight change of −24.2% at 48 weeks in its 12 mg once-weekly trial arm. The published Phase 3 TRIUMPH-2 report now adds evidence in a different population with type 2 diabetes; other work in the programme continues. Retatrutide remains investigational.
How the Triple Agonist Mechanism Works
Retatrutide activates three receptor pathways. The obesity trial measured weight outcomes; it did not establish that a particular change in resting energy expenditure explains the result or proves superiority over another medicine.
Phase 2 Clinical Trial Data
The pivotal Phase 2 trial (NCT04881760, Jastreboff et al., NEJM 2023) enrolled 338 adults with obesity (BMI ≥30 or ≥27 with weight-related comorbidity) and ran for 48 weeks. Participants without type 2 diabetes were included in the primary cohort. All arms were administered once weekly via subcutaneous injection. The figures below are what that trial measured, not a schedule to copy — retatrutide is not approved or prescribable in Ireland.
| Trial arm (once weekly) | Weight Loss at 24 Weeks | Weight Loss at 48 Weeks | Notes |
|---|---|---|---|
| Placebo | −1.6% | −2.1% | |
| 1 mg | −7.2% | −8.7% | |
| 4 mg (combined) | −12.9% | −17.1% | Two groups, starting dose 2 mg or 4 mg; the trial reports them combined |
| 8 mg (combined) | −17.3% | −22.8% | Two groups, starting dose 2 mg or 4 mg; the trial reports them combined |
| 12 mg | −17.5% | −24.2% | Highest arm in the trial; 83% of this group lost 15% or more of body weight by 48 weeks |
Source: Jastreboff AM et al., N Engl J Med 2023;389:514-526. Least-squares mean percentage change in body weight from baseline, 338 adults with obesity, 48 weeks. ClinicalTrials.gov NCT04881760.
Key finding: The 12 mg arm went from −17.5% at 24 weeks to −24.2% at 48 weeks — weight loss was still increasing across the second half of the trial rather than levelling off by week 48. The Phase 3 TRIUMPH trials run longer than 48 weeks.
Retatrutide vs Semaglutide vs Tirzepatide
The table below describes three separate placebo-controlled trials. It is historical trial context, not a head-to-head comparison or a ranking of the medicines. The participant groups, durations and analysis methods differ. For each authorised medicine's own evidence and status, read the semaglutide guide and tirzepatide guide.
| Compound | Receptors | Pivotal Trial | Mean weight change in cited trial | Duration | Status |
|---|---|---|---|---|---|
| Semaglutide 2.4mg Ozempic / Wegovy |
GLP-1 | STEP-1 | 14.9% | 68 weeks | Approved (EU/Ireland) |
| Tirzepatide 15mg Mounjaro / Zepbound |
GLP-1 + GIP | SURMOUNT-1 | 20.9% | 72 weeks | EU-authorised medicine; see current Mounjaro product information |
| Retatrutide 12mg LY3437943 |
GLP-1 + GIP + Glucagon | Phase 2 NCT04881760 | 24.2% (12 mg, 48 wk) | 48 weeks | Investigational; Phase 3 programme |
Important caveat: each trial used different populations, selection criteria, and follow-up durations. This is not a controlled comparison.
Why these trials cannot rank retatrutide and tirzepatide
These separate trials cannot establish which compound is stronger. The retatrutide Phase 2 trial studied 338 adults for 48 weeks. SURMOUNT-1 studied 2,539 adults for 72 weeks. Its abstract reports a 20.9% mean weight reduction in the highest tirzepatide trial arm under the treatment-regimen estimand. Mixing a different estimand into this table would make an already indirect comparison more misleading.
The practical difference is not the percentage. It is that tirzepatide is licensed and available in Ireland as Mounjaro, and retatrutide is not an approved medicine for public use. One is a medicine a doctor can prescribe. The other is a compound in trials.
Retatrutide targets a third receptor, glucagon, in addition to GLP-1 and GIP. The published obesity trial did not isolate the contribution of each receptor or prove that a change in energy expenditure explains the observed result.
Side Effects Observed in Phase 2
The Phase 2 abstract says the most common adverse events were gastrointestinal. They were dose-related, mainly mild to moderate and partially mitigated by a lower starting amount in the trial. Heart-rate increases were dose-dependent, peaked at 24 weeks and then declined. A 48-week Phase 2 trial does not establish long-term safety.
Published Phase 3 findings: TRIUMPH-2
Bellido and colleagues' TRIUMPH-2 report, published in The Lancet on 29 September 2026, randomised 1,152 adults with type 2 diabetes and BMI of at least 27. At 80 weeks, mean body-weight change under the treatment-regimen estimand was −18.8% in the highest retatrutide arm and −5.1% with placebo. This analysis includes outcomes regardless of treatment discontinuation and uses imputation for missing data; 84% of all randomised participants completed the study drug.
Gastrointestinal adverse events were the most common; hypotension and dysesthesia occurred more often with retatrutide. Permanent discontinuation due to adverse events or death was up to 12% across retatrutide arms versus 5% with placebo. The trial was funded by Eli Lilly. Its diabetes population differs from the 2023 obesity trial, and it did not compare retatrutide directly with another medicine. These findings do not establish long-term safety, public availability or medicine authorisation.
Source: Bellido V et al., Lancet, 29 September 2026. doi:10.1016/S0140-6736(26)01861-1. The Phase 2 and historical comparison tables above remain unchanged.
Current research and authorisation status
Lilly's September 2026 research update references the TRIUMPH-2 announcement. The TRIUMPH-2 abstract checked here is a published study report, not an authorisation decision.
Lilly continues to describe retatrutide as investigational and not approved by any regulatory agency. No Irish launch or approval date is established here. The prior HPRA check remains dated 23 September 2026; for current Irish medicine status, check the HPRA register. Supplier adverts and research-use-only labels do not establish authorisation.