Retatrutide trial data, in plain English — what the Phase II results showed

The trial was published in the New England Journal of Medicine in June 2023. The lead author was Ania Jastreboff, the director of Yale's obesity medicine program. The formal title is NEJM enough that people in the field know it immediately: a Phase II, double-blind, placebo-controlled, dose-ranging study of retatrutide in adults with obesity.

The study enrolled 338 adults. The inclusion criteria required a body mass index of 30 or higher — the clinical threshold for obesity — or a BMI of 27-29.9 with at least one weight-related comorbidity such as hypertension, dyslipidemia, or obstructive sleep apnea. Adults with type 2 diabetes were excluded, which is important context: this was a study of people with obesity but without overt diabetes, which is a particular population with particular metabolic characteristics. The exclusion of type 2 diabetes means the results don't directly tell us how retatrutide performs in people with the additional metabolic challenge of insulin resistance severe enough to have progressed to diabetes.

Participants were randomly assigned to one of six dose groups: placebo, or retatrutide at 1 mg, 2 mg, 4 mg, 8 mg, or 12 mg weekly. These were subcutaneous injections, once weekly — the same route and schedule as tirzepatide and weekly semaglutide. The titration schedule varied by group, with all groups starting at lower doses and escalating over the first twelve to twenty weeks before reaching the assigned maintenance dose. This titration approach is standard in the GLP-1 class and exists specifically to manage GI tolerability: the nausea and GI symptoms that come with rapid receptor activation are substantially reduced when the dose increases gradually.

The primary efficacy endpoint was percent change from baseline body weight at 24 weeks. The full 48-week data was also reported.

At 48 weeks, the results by dose group were roughly as follows. The placebo group lost approximately 2% of body weight — a small amount that probably reflects the lifestyle counseling component of the trial rather than the drug. The 1 mg group lost approximately 8.7%. The 4 mg group lost approximately 17.3%. The 8 mg group lost approximately 22.8%. The 12 mg group lost approximately 24.2%. The dose-response relationship was clear, steep, and statistically significant across groups.

The 24% figure at 12 mg is real. It is also from a 48-week Phase II trial in 338 people at one specific dose in a population without type 2 diabetes. Each of those qualifications matters.

The comparison that gets made — to tirzepatide's approximately 20-22% in the SURMOUNT-1 Phase III trial — requires some care. SURMOUNT-1 enrolled 2,539 participants, ran for 72 weeks, and included a broader population. A 48-week Phase II trial with 338 participants is a different kind of evidence than a 72-week Phase III trial with more than 2,500. The populations, though similar in principle, differ in the details of inclusion criteria, baseline characteristics, and the mix of comorbidities. Phase II trials are designed to find the right dose range and generate a hypothesis about efficacy; Phase III trials are designed to confirm or refute that hypothesis in a population large enough to detect safety signals and to characterize response heterogeneity across diverse patients. Finding 24% in Phase II does not guarantee 24% — or anything close to it — in Phase III.

This is not a reason to dismiss the Phase II results. It is a reason to hold them correctly.

The side-effect profile in the Phase II trial was consistent with the class: nausea, vomiting, diarrhea, and constipation were the most common adverse events. The frequency and severity increased with dose. At 12 mg, approximately 42% of participants reported nausea, 20% reported vomiting, and similar proportions reported other GI symptoms. Most were mild to moderate in severity, and most occurred during the titration phase rather than at steady state. The discontinuation rate due to adverse events was roughly 16% in the highest-dose group — higher than the discontinuation rates reported for semaglutide and tirzepatide in their Phase III trials, though again this comparison has limitations due to different titration schedules and populations.

One finding worth specific attention: the higher-dose groups showed a slight increase in fasting glucose relative to baseline, consistent with the glucagon receptor activation component of retatrutide's mechanism. Glucagon receptor agonism promotes hepatic glucose production, and even with the GLP-1 component providing counterbalancing insulin stimulation, the net glycemic effect at high doses in some individuals tended slightly upward. In people without diabetes and with functioning beta cells, this was not clinically significant in the trial. In people with type 2 diabetes or pre-diabetes, this signal would require closer monitoring and more careful dose management.

Body composition was assessed in a subset of participants. Fat mass reduction drove the weight loss, as expected. Lean mass — muscle and non-adipose tissue — also declined, which is consistent with what happens during large caloric-deficit weight loss of any kind, whether pharmacological or dietary. The proportion of weight loss from lean mass was roughly comparable to what has been observed in tirzepatide and semaglutide trials, though the analyses aren't directly comparable. Whether preserving lean mass during GLP-1-class weight loss is achievable through resistance exercise, protein intake optimization, or pharmacological approaches like adjunctive growth hormone pathway support is an active area of research across the field, and not specific to retatrutide.

The Phase III program is called TRIUMPH — a set of trials that will evaluate retatrutide in larger populations over longer timeframes, in people with and without type 2 diabetes, and with outcomes beyond weight loss including cardiovascular events, fatty liver, and other metabolic endpoints. These trials were initiated in 2023 and full results are not yet available as of mid-2025. The regulatory timeline for any potential approval depends on what those results show.

The compounding question comes up in conversations about retatrutide, and the honest answer is that the timeline and regulatory context matter. The FDA's current posture on compounding of GLP-1 class drugs is in flux: compounded versions of semaglutide and tirzepatide have been widely used during shortage periods, and the FDA has engaged with this question in various ways. Retatrutide is not approved anywhere, which means its regulatory status relative to compounding is different from the approved drugs — it is not a drug that can be compounded as a version of an approved product because no approved product exists. This is a meaningful distinction for anyone attempting to access it outside of clinical trials.

What the Phase II trial tells us — honestly and without the enthusiasm that tends to outpace it — is that the triple mechanism appears to work in the direction the biology predicted. That adding glucagon receptor activation to GLP-1 and GIP agonism produces measurably greater weight loss in a Phase II trial than dual agonism achieved in its own Phase II data. That the dose-response relationship is real and steep. That the side-effect profile is manageable but more pronounced at the highest doses. And that a drug with these properties in Phase II is worth watching carefully in Phase III, with the understanding that the number that circulates — 24% — is the best-case scenario from an early-stage trial, not a guaranteed outcome from the full program.

That distinction matters, not because the data isn't promising, but because the people who need it most deserve accurate expectations. The gap between what a Phase II trial shows and what a Phase III trial confirms has ended more than a few promising drugs. It has also, sometimes, held up. We'll know more when the TRIUMPH data is public.

Until then, 24% is a hypothesis with unusually good evidence behind it.