Retatrutide: what the trials show so far, and what is still a press release

Retatrutide is the strongest weight-loss drug yet tested in people, and the peer-reviewed trials already show it. In the published phase 2 obesity trial, the top dose produced 24.2% weight loss at 48 weeks against 2.1% on placebo, and 83% of people on it lost at least 15% of their body weight [2]. The first published phase 3 trial, in type 2 diabetes, found up to 15.3% at 40 weeks [5]. The larger figures you will see quoted, up to 28%, come from Eli Lilly press releases for phase 3 trials that are not yet in a journal [7,8]. It is not approved anywhere we checked, and outside trials and a narrow US access programme it isn't legally available.

What it is

Retatrutide (development code LY3437943) is a 39-amino-acid peptide from Eli Lilly that activates three hormone receptors at once: GIP, GLP-1 and glucagon [1]. The first two are the same targets as tirzepatide. Glucagon is the odd one out, because it normally raises blood sugar. The reasoning is that glucagon also increases energy expenditure and pushes the liver to burn fat, while the GLP-1 and GIP activity holds blood sugar down and cuts appetite [1].

That is mechanism theory, drawn mostly from animal work and early human pharmacology in the discovery paper [1]. Nobody has yet shown in a controlled human trial how much of retatrutide's extra effect comes from the glucagon arm. The molecule carries a fatty side chain for albumin binding, which gives it a half-life long enough for weekly injection [1].

The human evidence

Two tiers of evidence exist, and they should be kept apart: peer-reviewed papers you can read and check, and company announcements of trials that have not yet been published.

There is no large real-world dataset for retatrutide to sit alongside them, the way there is for older peptides. It is too new, and outside a trial almost nobody can get it legally, so the trial record is the whole of the human record for now.

Peer-reviewed: the phase 2 obesity trial

Side effects were mostly gastrointestinal and dose-related [2], and starting at 2 mg rather than 4 mg reduced them. Heart rate rose with dose, peaked at 24 weeks and then eased [2].

Peer-reviewed: type 2 diabetes and liver fat

A phase 2 trial in 281 people with type 2 diabetes compared retatrutide with placebo and with dulaglutide, an established GLP-1 drug [3]. At 24 weeks, HbA1c fell by up to 2.02 percentage points on retatrutide against 1.41 on dulaglutide. By 36 weeks, weight had fallen 16.94% on the 12 mg dose against 2.02% on dulaglutide [3].

A substudy of the obesity trial looked at 98 participants who had fatty liver (at least 10% liver fat) [4]. At 24 weeks, liver fat fell by 81.4% on 8 mg and 82.4% on 12 mg, against a 0.3% rise on placebo. On 12 mg, 86% got their liver fat back into the normal range [4]. That is imaging data, not biopsy evidence of reduced liver inflammation or scarring, which is what regulators want for a liver-disease licence.

A post-hoc analysis of both phase 2 trials, written by Lilly staff, reported falls in apolipoprotein B of about 21–24% and, in the non-diabetic trial, a 54.8% fall in C-reactive protein, a marker of inflammation [6]. These are risk markers, not heart attacks prevented.

Peer-reviewed: the first phase 3 paper

TRANSCEND-T2D-1 is the only phase 3 trial published in a journal as of this writing [5].

Weight loss is smaller here than in the obesity trials, partly because it was shorter and partly because people with diabetes consistently lose less on these drugs [5]. Two deaths occurred in the 4 mg group; investigators judged both unrelated to the drug [5].

Company-reported: the TRIUMPH phase 3 trials

The results below are company numbers, not journal ones. They come from Lilly press releases, so the full data are not public. They are the largest trials run and the source of most coverage, which is why they are worth knowing about.

Those headline figures use what Lilly calls the efficacy estimand, an estimate of what would have happened if everyone had stayed on the drug. Placebo groups lost 2.1% in TRIUMPH-4 and 2.2% in TRIUMPH-1 [7,8]. For TRIUMPH-1, Lilly also gave the treatment-regimen figure, which counts people who stopped: 25.0% on 12 mg against 3.9% on placebo [8]. That's still a very large effect, but it is the number closer to real-world use.

Two other details from TRIUMPH-1 get quoted a lot. Lilly says 45.3% of people on 12 mg lost at least 30% of their body weight by 80 weeks, against 0.5% on placebo, and that in a subgroup of 532 people who started with a BMI of 35 or more and stayed on for 104 weeks, the 12 mg group averaged 30.3% [8]. Both are efficacy-estimand figures from a press release, and the second is from a subgroup.

In TRIUMPH-4, the knee trial, pain scores on the WOMAC scale fell by about 75% on retatrutide against 40% on placebo [7]. A 40% improvement on placebo is large, which is common in pain trials and a reminder of why a control group matters.

The diabetes trials tell a more modest story. In TRIUMPH-2, weight fell 12.7%, 19.1% and 20.8% on 4, 9 and 12 mg, with HbA1c falling by up to 1.6 points [9]. That's in line with the pattern seen for semaglutide and tirzepatide, where diabetes blunts the weight response.

What the animal and lab work suggests

The discovery work in mice and early human dosing established that the molecule hit all three receptors and lowered weight and glucose [1]. Beyond that, animal studies matter less than usual here, because the human trial programme is so large. We are not aware of a published human study isolating what the glucagon component does over the long term, for instance to muscle or bone.

Safety and side effects

Gut. As with other drugs in this class, nausea, diarrhoea, constipation and vomiting dominate. In TRIUMPH-1, nausea affected 42.4% on 12 mg against 14.8% on placebo [8].

Stopping for side effects. This rises with dose and weight loss. In TRIUMPH-1, 11.3% on 12 mg stopped because of adverse events, against 4.9% on placebo; in TRIUMPH-4 it was 18.2% on 12 mg against 4.0% [7,8]. In TRIUMPH-2 and TRIUMPH-3 the figures were 7.7% and 13.5% on 12 mg, against about 5% on placebo [9]. The peer-reviewed phase 3 diabetes trial, at lower weight loss, saw 2–5% [5].

Dysaesthesia. This is the new signal. Dysaesthesia means abnormal skin sensations such as tingling, burning or sensitivity to touch. Lilly reported it in 5.1–12.5% of retatrutide groups in TRIUMPH-1 against 0.9% on placebo, and in 20.9% on 12 mg in TRIUMPH-4 [7,8]. The company describes it as mostly mild to moderate and says most cases resolved during treatment [8]. Nobody has published an explanation for it yet.

Heart rate. Dose-dependent rises in heart rate appeared in phase 2, peaking at 24 weeks [2]. The phase 3 cardiovascular data will matter here.

Legal and regulatory status

Retatrutide is not approved as a medicine in the UK or US. In August 2026 the MHRA said it has not been authorised for medicinal use in the UK and cannot be legally supplied outside clinical trials [12]. In the US, Lilly says it plans to submit an application to the FDA in the first quarter of 2027 [9].

Since mid-2026 Lilly has run a pre-approval expanded access programme in the US, registered on ClinicalTrials.gov, for a small number of patients whose doctors apply on their behalf [10,11]. That isn't a licence and doesn't make the drug available to the public.

For athletes, the 2026 WADA Prohibited List bans at all times any pharmacological substance not approved for human therapeutic use by a regulator, a category (S0) that applies to retatrutide for as long as it stays unapproved [13].

Anything sold online as retatrutide is, by definition, not the trial product. If you are looking at it anyway, purity is one of four things that matter on a certificate of analysis. Sterility, endotoxin and heavy metals are the others, and a report showing purity alone has answered one question out of four. We walk through what to look for in how to read a certificate of analysis.

What is still open

• The full peer-reviewed TRIUMPH results, including all side-effect tables and the treatment-regimen numbers for every trial [7–9].
• What causes dysaesthesia and whether it fully resolves after stopping [8].
• Whether glucagon activity affects lean mass, bone or heart rhythm over years rather than months.
• Whether it reduces heart attacks and deaths, not just risk markers [6].
• What happens to weight when people stop; no published retatrutide trial has yet tested withdrawal.

What this means

Retatrutide works, and it works harder than anything else in its class has been shown to. That is not a press-release claim: the peer-reviewed phase 2 obesity trial produced 24.2% weight loss at 48 weeks, 83% of people on the top dose lost at least 15% of their body weight, and the liver substudy cut liver fat by more than 80% [2,4]. For a drug this young, the direction and the size of the effect in people are unusually clear.

You will keep seeing the 28% figure quoted as settled fact. We will keep separating it from the published number, but the gap between 24% and 28% is a difference of estimand and duration rather than evidence that the drug is weaker. The press-release figures use Lilly's efficacy estimand over 68 to 80 weeks; the peer-reviewed figure is a shorter 48-week trial. The reason the full TRIUMPH data are not in a journal yet is who owns them. Retatrutide is Lilly's own asset, in a market the company is trying to lead, and it releases the numbers it chooses ahead of an FDA filing it says will come in the first quarter of 2027 [9]. Nobody is short of money to test this molecule. The delay is about control of the data, not about funding.

The open question is the long-run effect of the glucagon arm. Glucagon raises heart rate and burns tissue, and no trial has yet separated what it does to muscle and bone from the weight loss itself. Dysaesthesia, the tingling and burning skin sensations Lilly reports in up to 20.9% of people on the top dose in TRIUMPH-4, is the signal to watch, and no one has published an explanation for it [7,8]. Those are reasons to want the published data sooner, not reasons to doubt the drug.

Our read, stated plainly: the effect is real and it is the largest in the class, so the honest position is confidence in that effect alongside patience on the long-term safety picture. If you can wait for the TRIUMPH papers, wait. If you are set on it sooner, know that it is licensed nowhere we checked, that anything sold online is not the trial product, and that the biggest numbers in every article you read, including ours, stay company figures until a journal says otherwise.

What readers report

First-hand accounts, shared with permission. They are self-reported, not evidence, and sit alongside the research above rather than in place of it.

Common questions

How much weight did people lose on retatrutide in trials?

In the peer-reviewed phase 2 obesity trial, the top dose produced 24.2% weight loss at 48 weeks against 2.1% on placebo. Larger figures of up to 28% come from Eli Lilly press releases for phase 3 trials not yet in a journal.

What are the side effects of retatrutide?

Side effects were mostly gastrointestinal and dose-related, including nausea, diarrhoea, constipation and vomiting. Heart rate rose with dose in phase 2. Lilly reported dysaesthesia, abnormal skin sensations such as tingling or burning, in up to 20.9% on 12 mg in TRIUMPH-4.

Is retatrutide legal in the UK?

No. In August 2026 the MHRA said retatrutide has not been authorised for medicinal use in the UK and cannot be legally supplied outside clinical trials. Anything sold online as retatrutide is, by definition, not the trial product, so the trial results do not describe it.

Is retatrutide approved by the FDA?

Not yet. Lilly says it plans to submit an FDA application in the first quarter of 2027. A pre-approval expanded access programme exists in the US for a small number of patients whose doctors apply, but it is not a licence.

Educational content only — not medical advice. Many peptides discussed on HPR are not approved for human use. Talk to a qualified clinician before making any decision about your health.