Semaglutide vs retatrutide: one vs three

The shorthand people use for this comparison is wrong. Semaglutide is not a dual agonist. It acts on one receptor, GLP-1^[1]. Retatrutide acts on three: GIP, GLP-1 and glucagon^[2]. The dual agonist in this family is tirzepatide, which sits between them. So the honest framing is one receptor versus three, with a two-receptor drug already licensed in the middle.

Receptor count is also the less useful half of the comparison. Semaglutide is an approved medicine with outcome data measured in tens of thousands of patient-years. Retatrutide is an investigational compound that no regulator has authorised anywhere.

What the third receptor is supposed to add

GLP-1 and GIP receptor agonism reduce how much you eat. Glucagon receptor agonism is in retatrutide for a different reason. The discovery paper from Eli Lilly's team reported that in obese mice, weight loss on the molecule was augmented by glucagon-receptor-mediated increases in energy expenditure layered on top of the reduction in calorie intake driven by the other two receptors^[3]. The design goal is a drug that raises the body's energy output while the other two receptors hold food intake down.

The liver signal is the other reason the glucagon arm gets attention. A Phase 2a substudy of the main retatrutide obesity trial enrolled 98 participants who had metabolic dysfunction-associated steatotic liver disease and at least 10 percent liver fat at baseline. Mean relative change in liver fat at 24 weeks was minus 81.4 percent on the 8 mg dose and minus 82.4 percent on 12 mg, against plus 0.3 percent on placebo. Normal liver fat, defined as under 5 percent, was reached by 86 percent of the 12 mg group and none of the placebo group^[4].

The same paper carries a caveat. The authors report that the liver-fat reductions were significantly related to changes in body weight, abdominal fat and metabolic measures tied to insulin sensitivity^[4]. That is consistent with a direct hepatic glucagon effect, and it is equally consistent with the liver fat falling because the participants lost a lot of weight. The energy-expenditure evidence, meanwhile, is still mouse data. Neither finding has been isolated from weight loss in a human trial designed to do that.

What semaglutide has that retatrutide does not

STEP-1 randomised 1,961 adults with overweight or obesity and no diabetes to semaglutide 2.4 mg weekly or placebo for 68 weeks, both with lifestyle intervention. Mean body-weight change was minus 14.9 percent versus minus 2.4 percent^[1]. SELECT then enrolled 17,604 adults with established cardiovascular disease and a BMI of 27 or higher but no diabetes, and reported a primary cardiovascular endpoint event in 6.5 percent of the semaglutide group against 8.0 percent on placebo over a mean 39.8 months of follow-up (hazard ratio 0.80)^[2].

Retatrutide has no equivalent to SELECT. No cardiovascular outcomes trial has reported on it, and its total published exposure is a fraction of that 17,604-patient cohort. Its largest published obesity dataset is a 338-person Phase 2 trial over 48 weeks, in which the 12 mg group lost a mean 24.2 percent of body weight against 2.1 percent on placebo, with dose-dependent heart-rate increases that peaked at week 24 before declining^[5].

Regulatory status follows the evidence. Semaglutide is authorised in the European Union under the Wegovy brand for weight management in adults with a BMI of 30 or higher, or 27 and above with a weight-related condition, and the EMA describes the active substance plainly as a GLP-1 receptor agonist^[6]. Retatrutide holds no marketing authorisation from the FDA, the EMA, the MHRA or any other agency we track, and Eli Lilly's own access programme describes it as investigational and pre-approval^[10]. Our retatrutide regulation overview has the country-by-country position, and the vials circulating online under that name sit outside every framework listed there.

The head-to-head that exists is in diabetes

A direct comparison does exist. TRANSCEND-T2D-2 (NCT06260722) is a Phase 3, randomised, open-label trial of retatrutide once weekly at two dose levels against semaglutide once weekly, in an estimated 1,250 adults with type-2 diabetes and inadequate glycaemic control on metformin with or without an SGLT2 inhibitor. It started in February 2024, is listed as active and not recruiting, and has an estimated primary completion of August 2026 with study completion in January 2027. No results are posted^[7].

Its primary outcome is change from baseline in HbA1c, in people with diabetes, so it will answer which drug controls blood sugar better and nothing beyond that. A reader with obesity and no diabetes gets no weight-loss answer from it. Retatrutide's obesity head-to-head, TRIUMPH-5 (NCT06662383), runs against tirzepatide rather than semaglutide, in roughly 800 adults^[8].

So the weight-loss comparison a reader actually wants has no trial behind it. Setting the 24.2 percent from the retatrutide Phase 2 next to the 14.9 percent from STEP-1 puts a 338-person 48-week dose-ranging trial beside a 1,961-person 68-week registration trial, run years apart in different populations. Cross-trial arithmetic like that is not a fair comparison and we are not going to pretend otherwise.

What we still do not know

The open questions are specific. Nobody has shown that the glucagon arm adds anything in humans beyond what stronger appetite suppression alone would have delivered, or that retatrutide's liver-fat effect survives adjustment for weight loss. The dose-dependent heart-rate increase has been characterised over months, not years. Cardiovascular and kidney outcomes are being tested in a separate retatrutide trial that has not reported. Discontinuation is the sharpest gap. STEP-4 randomised 803 adults who had already lost a mean 10.6 percent of body weight on semaglutide, and the group switched to placebo gained 6.9 percent back over the next 48 weeks while the group that stayed on the drug lost a further 7.9 percent^[9]. No comparable withdrawal trial has reported for retatrutide.

What this means for readers

If you are comparing these two because you are choosing a treatment, there is only one of them a clinician can prescribe. Semaglutide is prescription-only everywhere we cover, and the semaglutide regulation pages set out how that works per country. Retatrutide reaches patients only through clinical trials and a single-patient pre-approval expanded-access programme, which Eli Lilly restricts to adults with a BMI of 35 or above and at least two serious or life-threatening obesity-related complications^[10]. Anything sold online as retatrutide is outside pharmaceutical quality control, and the numbers in its trials were produced under supervised titration that grey-market buyers do not get.

If you are following the pipeline instead, the date to watch is the TRANSCEND-T2D-2 readout^[7]. It will be the first controlled look at these two molecules in the same trial, even if it answers a glycaemic question rather than a weight one. We will update this page when it reports.