TRIUMPH-5: retatrutide versus tirzepatide, result pending

TRIUMPH-5 is the trial that will finally compare retatrutide with tirzepatide, the drug sold as Mounjaro, in the same people at the same time. It has not reported. Until it does, every claim that retatrutide "beats" Mounjaro is a guess dressed up as a fact.

Why the comparison has not been made yet

Retatrutide's TRIUMPH-1 result, 25.0% average weight loss at 80 weeks on the treatment-regimen estimand,^[2] is a bigger number than tirzepatide produced in its own main obesity trial. That is where "retatrutide beats Mounjaro" comes from. It is not a fair comparison, for reasons the how to read a trial page goes through:

• Different trials, different people. Each drug was tested in its own trial with its own entry rules, starting weights and countries. Small differences in who joins can move an average by several points.
• Different lengths. The tirzepatide trial and TRIUMPH-1 did not run for the same number of weeks, and weight loss on these drugs is still changing at 72 weeks.
• Different ways of counting. Which estimand is quoted, and how missing data were handled, differ between trials and between the press release and the paper.
• Different eras. TRIUMPH-1 is a 2026 topline result; tirzepatide's data have been through peer review and regulatory scrutiny, which is why it has a UK licence and a NICE recommendation.^[3]

The only way round all of that is to randomise the same people to one drug or the other in the same trial. That is what TRIUMPH-5 does.

What the trial will answer

In about 800 adults with obesity, assigned at random and with neither participants nor investigators knowing who is on which drug, it will measure which group has lost more weight after 80 weeks.^[1] Because the two groups are alike in everything but the drug, a difference between them can be attributed to the drug. It will also, in the same people, compare the side effects: stomach effects, drop-out rates, heart rate, and whether dysesthesia, which has not been reported with tirzepatide, appears in one group and not the other.

What it will not answer

• Whether either drug prevents heart attacks or strokes. That takes an outcomes trial. See TRIUMPH-Outcomes.
• What happens after stopping. The maintenance question belongs to TRIUMPH-6, covered on the other indications page.
• Whether retatrutide is "worth it" for a given person. More average weight loss is only better if the extra side effects, and the extra unknowns of a newer drug, are acceptable. That is a judgement for regulators, NICE and, eventually, a person and their doctor.
• Anything about a product sold online. The trial compares two pharmaceutical-grade drugs made by the same company. It has nothing to say about an unlicensed vial.

What to look for when it reports

The result will arrive as a company press release first. When it does, check:

1. The estimand. Is the headline the treatment-regimen or the efficacy figure, and are both given for both drugs?^[4]
2. The doses compared. The registry lists one retatrutide arm and one tirzepatide arm without stating the target doses.^[1] A comparison of retatrutide's top dose with a lower tirzepatide dose would mean something different from a comparison of both drugs at their highest.
3. The population. The registry says adults with obesity; look for the starting BMI and whether people with diabetes were included.
4. No placebo. There is no placebo group, so there is no placebo-adjusted figure. The result is a difference between two active drugs, and how big that difference is matters as much as which way it points.
5. Side effects and drop-outs side by side. Two drugs in the same people is the first fair test of whether retatrutide's stomach effects, heart-rate rise and dysesthesia are worse than tirzepatide's.
6. Statistical significance and size. A difference of one or two percentage points could be real and still not matter much.

Where to check

Search NCT06662383 on ClinicalTrials.gov. The "primary completion date" is the earliest point at which a result could exist, and the "study status" will change from "active, not recruiting" to "completed" when the last visit has happened.^[1] This page and the news section will be updated when Lilly announces. The misinformation tracker records the versions of the comparison that circulate in the meantime.