Retatrutide: what its phase 3 trial found, and why you can't get hold of it

Retatrutide is the obesity drug everyone is talking about right now, and also the one generating the most inflated expectations. Here's what is genuinely known, with the exact figures and the caveats that come with them.

Retatrutide is not authorised in Spain

No regulator anywhere in the world has approved it: not the FDA, not the EMA, not Spain's AEMPS. It isn't a case of it being on its way to Spain — it is still an investigational drug, and no marketing authorisation application has even been submitted in the European Union.

This isn't a technicality: it means no Spanish prescription can be written for it and no Spanish pharmacy can dispense it. If a website offers it to you, it is not selling you a medicine with any guarantees behind it — whatever turns up at your door, nobody has verified what's in it.

What it is

It is a triple agonist: it acts on the GLP-1, GIP and glucagon receptors at the same time. It is given by subcutaneous injection, once a week.

The first two receptors will already be familiar to you, even if you don't realise it: they are the ones used by the drugs that do exist today. Semaglutide acts on GLP-1; tirzepatide on GLP-1 and GIP. If you want to get to grips with the mechanism before reading on, we explain it in what GLP-1 drugs are and how they work. The third one, glucagon, is what makes retatrutide different — and also the source of a side effect of its own that you'll see further down.

What TRIUMPH-1 found

TRIUMPH-1 is a phase 3 trial with 2,339 participants, randomised to retatrutide 4, 9 or 12 mg, or placebo, over 80 weeks. Lilly announced the results in May 2026 and presented them at the ADA conference in June.

Mean weight loss at 80 weeks:

• 4 mg — −19.0%
• 9 mg — −25.9%
• 12 mg — −28.3%
• Placebo — −2.2%

The caveat almost nobody mentions: what gets counted as a result

Those figures correspond to what is known as the "efficacy estimand", which measures what happens in people who stay on treatment as planned. There is another way of counting it — the "treatment regimen estimand" — which includes everyone who was randomised, whether they dropped out or not. And it gives a different number:

• 12 mg — −25.0% (instead of −28.3%)
• Placebo — −3.9% (instead of −2.2%)

The difference versus placebo falls from 26.1 to 21.1 percentage points. Neither figure is false: they answer different questions. But when you see "28.3%" in a headline, you are looking at the more favourable of the two.

What the figure doesn't tell you

• It has not been peer reviewed. The data come from a company press release and a conference presentation, not from a publication in a scientific journal. That doesn't make them false, but it does mean no independent reviewer has yet examined the full methodology. The phase 2 data, which have been published (NEJM, 2023), showed −24.2% at 48 weeks.
• One in nine stopped because of side effects. That's 11.3% on the 12 mg dose, versus 4.9% on placebo. Nausea in up to 42.4% and vomiting in up to 25.3%.
• It has a side effect of its own. Dysaesthesia — abnormal skin sensations, such as tingling or burning — in between 5.1% and 12.5% of participants, versus 0.9% on placebo. It is attributed to the glucagon component, so it is an effect that current drugs don't have.
• It has not been compared with anything that already exists. Only with placebo. There is no trial pitting it against tirzepatide or semaglutide.

How it compares with what already exists (and why the comparison is misleading)

This is the table everyone publishes. We're publishing it too, with the warning almost nobody adds: these figures are NOT comparable with each other.

• Semaglutide 2.4 mg (Wegovy) — STEP-1: −14.9% at 68 weeks, versus −2.4% on placebo. Published in NEJM (2021).
• Tirzepatide 15 mg (Mounjaro/Zepbound) — SURMOUNT-1: −20.9% at 72 weeks, versus −3.1% on placebo. Published in NEJM (2022).
• Orforglipron 36 mg (Foundayo) — ATTAIN-1: −12.4% at 72 weeks. Published in NEJM (2025).
• Retatrutide 12 mg — TRIUMPH-1: −28.3% at 80 weeks. Lilly press release and presentation at ADA 2026. Not published in a peer-reviewed journal.
• CagriSema 2.4/2.4 mg — REDEFINE-1: −22.7% at 68 weeks, versus −2.3% on placebo. Published in NEJM (2025).

Why you can't read that list as a league table

• The trials run for different lengths of time. From 68 to 84 weeks. Weight loss curves have not plateaued by 68 weeks, so giving a drug 12 extra weeks hands it an advantage without it having done anything better.
• The populations are different. Starting weight, age, sex and comorbidities don't match. We know that the same molecule produces around half as much weight loss in people with type 2 diabetes as in those without it: mixing populations invalidates any ranking.
• The placebos don't match either. The placebo group lost −2.4% in STEP-1, −3.1% in SURMOUNT-1 and −2.2% in TRIUMPH-1. A different placebo means a different lifestyle intervention behind it — in other words, a different starting line.
• Not all the figures have been peer reviewed. Four come from the New England Journal of Medicine. The retatrutide one comes from a company press release. They are not the same class of evidence.

The only thing that does prove something: the two head-to-head trials

Only twice have two of these drugs gone up against each other in the same trial, with the same people, at the same time. They are the only data on this page that allow anyone to say one is better than another.

• SURMOUNT-5 — tirzepatide 15 mg versus semaglutide 2.4 mg, 72 weeks: −20.2% versus −13.7%. Tirzepatide won.
• REDEFINE-4 — CagriSema versus tirzepatide 15 mg, 84 weeks: −23.0% versus −25.5%. It did not meet its non-inferiority endpoint. In other words: CagriSema failed even to show it was equivalent.

Note the contrast. In the list above, CagriSema (−22.7%) appears to beat tirzepatide (−20.9%). When they were actually measured against each other, head to head, tirzepatide won. That is exactly what it means for figures from different trials not to be comparable — and why that list is not a league table.

Orforglipron and retatrutide have never been measured against anything other than placebo. When someone tells you that one of the two "is better" than what already exists, there is no trial behind that statement.

The side effects, in context

Nausea and vomiting are not unique to retatrutide: they are the class effect of this whole family of drugs, and they are the leading reason people stop treatment. How they are managed with the drugs that are available is covered in side effects of Ozempic and Mounjaro and how they are managed.

What is available in Spain today

Retatrutide is still in phase 3: it is not approved in any country, its figures have yet to go through a peer-reviewed journal, and there is no legal route to obtaining it in Spain. That's the part worth remembering.

What you don't need to wait for is addressing your weight under medical supervision. Medical weight-loss treatment is available in Spain today, with prior assessment, a prescription where appropriate from a registered doctor and ongoing follow-up from the medical team. You can check whether you're eligible in 30 seconds.

And to keep getting to grips with the subject: what GLP-1 drugs are and how they work, how Mounjaro and Ozempic differ and the only legal way to access them.