Retatrutide cuts weight 25%; comparison awaits

People without diabetes who injected the top dose of Eli Lilly's experimental drug retatrutide once a week lost an average of 25.0% of their starting weight over 80 weeks, against 3.9% on a placebo, according to the full trial paper published on 29 September in the New England Journal of Medicine (NEJM paper). That is a large effect for a medicine still in testing. It also follows a bigger headline figure that Lilly announced from the same study earlier this year.

The trial, called TRIUMPH-1 and sponsored by Lilly, randomly assigned 2,339 adults to weekly injections of 4, 9 or 12 milligrams of the drug, or to a dummy injection (trial registration). Everyone had obesity, or was overweight with at least one weight-related condition. Because chance decided who got what, the groups started out alike, and the placebo group shows how much weight people lose simply by taking part in a trial. Lower doses also produced smaller losses, a pattern that adds confidence the drug is responsible.

Why Lilly's first number was bigger

On 21 May, Lilly led with 28.3% at the top dose, against 2.2% on placebo (Lilly's May announcement). Both are legitimate estimates from the same data, but they answer different questions.

Lilly's figure is a modelled "what if." It estimates the average loss had every participant stayed on their assigned dose and taken no other weight-loss treatment. The paper's headline instead counts everyone in the group they were originally assigned to. That includes people who stopped injecting or added another treatment along the way.

Real patients do sometimes stop, so the 25.0% describes a whole group of people handed the drug, while the 28.3% describes an idealised version of that group. The 25.0% is also neither the most any individual lost nor a figure with the placebo effect subtracted (HCPLive).

Three hormone signals in one molecule

Think of hormones as keys and receptors as the locks they fit on the surface of cells. When the right key turns, the cell acts on the message. Retatrutide is a single molecule designed to turn three of these locks at once.

Two of the locks belong to gut hormones called GLP-1 and GIP, which the body releases after eating and which help control blood sugar. GLP-1 also curbs appetite. The third lock belongs to glucagon, a hormone that normally tells the body to release stored fuel (Cell Metabolism).

The idea behind adding glucagon, drawn from earlier laboratory research, is that it might push the body to use more of its reserves while the other two signals reduce how much people eat. TRIUMPH-1 did not test that idea. It gave participants only the complete molecule, with no version lacking the glucagon signal. How much of the weight loss glucagon contributes therefore remains an inference from earlier work.

The cost, and the comparison still missing

Side effects rose with the dose. Lilly's May report listed nausea in 42.4% of top-dose participants versus 14.8% on placebo, and 11.3% stopping treatment because of adverse events of any kind versus 4.9% (Lilly's May announcement). The company described the most common stomach complaints as generally mild to moderate.

It is tempting to rank retatrutide against tirzepatide and semaglutide, the best-known drugs now prescribed for weight loss, but TRIUMPH-1 compared it only with a placebo. Those two medicines met head to head in a separate trial, SURMOUNT-5 (SURMOUNT-5 paper). Its patients, length and methods differ from TRIUMPH-1's, so setting its results beside retatrutide's would mislead.

TRIUMPH-1 also measured weight only while people kept injecting. When participants in an extension of an earlier semaglutide trial stopped treatment, they regained substantial weight (STEP 1 extension). That is a reason to study withdrawal for retatrutide, not a forecast. A trial of this size cannot reliably detect rare side effects either.

The drug is still investigational. Lilly plans to apply for US approval in the first quarter of 2027, which is a company plan rather than a date when patients can get it (Scientific American).

This placebo-controlled trial establishes that retatrutide produces substantial weight loss in people without diabetes. Whether it beats a medicine doctors can already prescribe remains untested. Anyone calling it the new benchmark should first see Lilly run that head-to-head trial, along with one showing what happens when people stop injecting.