The short version: retatrutide is an experimental once-weekly injectable from Eli Lilly that activates three receptors instead of one or two. It is not approved in Ireland, the EU, or anywhere else — Phase 3 is still running, so no doctor here can prescribe it. Its Phase 2 weight-loss numbers were larger than anything published before, which is why you keep hearing the name. A big Phase 2 result is not the same thing as a licensed medicine. The trial doses below are reported because that is what the researchers measured — they are not a schedule, and nobody here is telling you to use them.
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.
Phase 2 results published in the New England Journal of Medicine in June 2023 showed body weight reductions that, at the highest doses, exceeded any previously reported figures for a weight loss drug in clinical trials. Phase 3 trials (the TRIUMPH series) are now underway.
How the Triple Agonist Mechanism Works
The glucagon component matters because it adds a separate dimension to fat loss: instead of only reducing calories in (via appetite suppression), it also increases the rate at which the body burns energy at rest. Research suggests this dual action — less in, more out — accounts for the superior weight loss figures seen in Retatrutide trials compared to GLP-1 and dual GLP-1/GIP agents.
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
Direct head-to-head trials between these compounds do not exist yet. The comparison below draws on each compound's own pivotal trial data. Methodology, population, and duration differ, so comparisons should be treated as approximate.
| Compound | Receptors | Pivotal Trial | Max Weight Loss | 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 | 22.5% | 72 weeks | Approved (EU/Ireland for T2D) |
| Retatrutide 12mg LY3437943 |
GLP-1 + GIP + Glucagon | Phase 2 NCT04881760 | 24.2% (12 mg, 48 wk) | 48 weeks | Phase 3 (TRIUMPH trials) |
Important caveat: each trial used different populations, selection criteria, and follow-up durations. This is not a controlled comparison.
Retatrutide vs Tirzepatide: which is stronger?
On the published numbers, retatrutide edges it — 24.2% mean weight loss at 48 weeks in the 12 mg Phase 2 arm, against 22.5% at 72 weeks for tirzepatide in SURMOUNT-1 — and it got there in less time, with weight loss still increasing between weeks 24 and 48. That comparison is weaker than it looks. Different trials, different populations, different lengths, and one is a Phase 2 in 338 people while the other is a completed Phase 3 in over 2,500. No head-to-head trial exists.
The practical difference is not the percentage. It is that tirzepatide is licensed and available in Ireland as Mounjaro, and retatrutide is not available at all. One is a medicine a doctor can prescribe. The other is a compound in trials.
The mechanism difference is the third receptor. Tirzepatide works on GLP-1 and GIP, which mainly reduce how much you eat. Retatrutide adds glucagon, which is thought to raise energy expenditure as well. Whether that holds up across a full Phase 3 programme is exactly what the TRIUMPH trials are for.
What is it actually like? One person's experience
Keith has used retatrutide himself, during a deliberate six-week calorie deficit alongside bloodwork-monitored TRT. The honest summary: the deficit felt far more manageable than previous cuts, with much less of the constant hunger that usually wrecks adherence around week three. Lipid markers improved — though a calorie deficit improves those on its own, so that cannot be pinned on the compound.
In his own words, and it is the part almost nobody writes: “Appetite suppression is the whole point in a deficit, and it is also the trap. Coming back up to maintenance afterwards, eating enough was genuinely hard work. I had to plan meals I did not want. And that matters, because getting leaner is not the same as looking better. Fuller muscles, better sessions and stable energy did more for how I actually looked than a lower number on the scale ever did. If the goal is maintenance or a lean bulk rather than a cut, something that flattens your appetite is working against you.”
“I came out of that deficit on purpose, back up to maintenance, and I am starting my second lean bulk now. Maintenance is not the failure state after a diet. It is where the training gets better and the muscle actually gets built. It is just the phase nobody posts about, because a smaller number is easier to photograph.”
That is one data point from one person, not evidence. It is written up in full, with the limitations stated plainly and no protocol or dose, in my TRT journey and what I actually learned.
Side Effects Observed in Phase 2
The adverse event profile was consistent with the GLP-1 class — the same broad category as Ozempic and Mounjaro:
- Nausea — most common, particularly during dose escalation phases
- Vomiting and diarrhoea — dose-dependent; more frequent in the 8 mg and 12 mg trial arms
- Decreased appetite — expected mechanism; reported as a side effect by some participants
- Injection site reactions — mild, consistent with other subcutaneous injectables
- Discontinuation rate — higher in the 12 mg arm than the lower arms; Phase 3 escalates more slowly for that reason
The Phase 3 TRIUMPH trials are using slower dose escalation schedules to reduce GI adverse events, which was a lesson taken directly from the Phase 2 data.
Where Research Stands in 2026
Phase 2 results published in NEJM. Immediately became one of the most-cited obesity trial publications of the year.
TRIUMPH-1, TRIUMPH-2, and TRIUMPH-3 Phase 3 trials initiated. Broader populations including people with type 2 diabetes and cardiovascular risk.
Phase 3 data readout expected. Eli Lilly targeting regulatory submission to FDA and EMA based on trial timelines.
Potential EU/Irish regulatory approval. Subject to Phase 3 results meeting endpoints and regulatory review timelines.
Research Updates
New data will be added to this section as Phase 3 trial results are published. Last review: July 2026.
- July 2026: No Phase 3 data published yet. TRIUMPH trials ongoing. No change to Phase 2 conclusions.