Retatrutide

Retatrutide is the compound the obesity field has been watching. It acts on three receptors rather than one or two, and in phase 3 trials it produced weight loss that no approved medicine has matched.

The evidence base is now unusually strong for something still working its way toward a licence, which is what makes it worth understanding in detail.

Why people are talking about it#

The size of the result. An average 28.3% weight reduction at 80 weeks, with nearly half of participants losing at least 30% of their body weight, is a figure that used to belong to surgery alone.

The second reason is the third receptor. Tirzepatide already covers GLP-1 and GIP. Retatrutide adds glucagon, and glucagon signalling increases energy expenditure and mobilises fat from the liver. So rather than only reducing what goes in, the drug also raises what is burned.

What people take it for#

Weight loss, principally. In the trials it is also being tested for type 2 diabetes, knee osteoarthritis pain, sleep apnoea, fatty liver disease and cardiovascular outcomes.

Weight loss#

The claim is more weight lost than tirzepatide or semaglutide can produce, and the phase 3 numbers support it. The mechanism is agonism at three receptors at once, measured in adults with obesity in the TRIUMPH programme at 80 weeks.

Type 2 diabetes#

The claim is better blood sugar control alongside the weight loss, resting on the GLP-1 and GIP arms.

Glucagon agonism raises blood sugar, which made the triple approach counter-intuitive, and the trial data indicates the incretin arms more than offset it. In TRANSCEND-T2D-1 the reported A1C reduction reached 2.0 percentage points at 40 weeks.

Osteoarthritis pain and sleep apnoea#

The claim is relief from two conditions that track closely with weight, and the mechanism is mostly indirect: less load on a knee, and less soft tissue crowding an airway.

What that looks like to a person is a knee that hurts less and a night with fewer interruptions. Lilly reported knee osteoarthritis pain reductions of up to 4.3 points and sleep apnoea severity falling by up to 36.1 events per hour in the TRIUMPH-4 and related trials.

What the evidence shows#

The phase 2 trial was published in the New England Journal of Medicine in 2023 and reported up to 24.2% mean weight reduction at 48 weeks. The phase 3 TRIUMPH programme then reported through 2026: TRIUMPH-1 found an average 28.3% weight reduction on the 12 mg dose at 80 weeks, with 45.3% of participants losing at least 30% of their body weight.

TRIUMPH-2, TRIUMPH-3 and TRIUMPH-4 reported average reductions of 20.8%, 22.6% and 28.7% respectively. In an extension, participants with a starting BMI of 35 or above reached an average 30.3% at 104 weeks.

For scale, that upper figure is roughly a third of body weight, sustained for two years, from a weekly injection.

The side effect profile is the same family as the other incretin drugs, and the rates climb with dose: nausea, diarrhoea, constipation and vomiting, mostly mild to moderate and mostly settling during treatment. One finding is specific to this drug rather than the class, an altered skin sensation reported by roughly one in eight participants on the higher doses, and Lilly describes these as generally mild and mostly resolving.

How it is taken#

A once-weekly subcutaneous injection in the trials, with doses stepped up over time in the same pattern as the approved incretin drugs. The 4 mg, 9 mg and 12 mg arms are the ones the phase 3 results describe.

Lilly has stated it intends to file for US approval in the first quarter of 2027, which would put a decision somewhere in late 2027 or 2028. Until then the dose figures above describe what was studied rather than what a prescriber would write.

How it compares#

Against tirzepatide#

Tirzepatide hits two of the same three receptors and reached about 20% average weight loss in SURMOUNT-1. Retatrutide reported up to 28.3% in TRIUMPH-1, and a head-to-head trial is running rather than finished. Tirzepatide is available on prescription today, which is the practical difference.

Against semaglutide#

Semaglutide reached about 15% in the STEP programme and has the completed cardiovascular outcomes trial that retatrutide does not yet have. On weight the gap is large; on proven hard outcomes semaglutide is still the one with the evidence in hand.

Against surgery#

Bariatric surgery has decades of durability and mortality data behind it. Retatrutide's 30.3% at 104 weeks overlaps with surgical results on weight alone, over a far shorter horizon.

Common questions#

Is it better than tirzepatide?

On average weight loss in separate trials, the figures are higher. The direct comparison has not reported yet.

When will it be available?

Lilly has said it plans to submit to the FDA in the first quarter of 2027, so a decision is unlikely before late 2027 at the earliest.

What is the skin sensation side effect?

Dysesthesia, an abnormal or unpleasant skin sensation, reported by about one in eight participants on the higher doses. It is the most distinctive finding in the programme and it is dose-related.

Will the results hold when it is reviewed?

The trial results are what they are. What changes at review is that a regulator reads the full dataset rather than a topline summary, and a label states who the drug is for.