Ask five Australians comparing weight-loss drugs which one wins, and you'll usually get the same arithmetic: retatrutide did 28.3%, Mounjaro did about 20.9%, therefore retatrutide is roughly eight points better. It's a tidy sum. It's also not a comparison — those two numbers come from two different trials, run years apart, in different groups of people, for different lengths of time.

The actual retatrutide vs Mounjaro comparison — one protocol, one randomisation, both drugs side by side — exists, it's called TRIUMPH-5, and it has been running since late 2024. Here's what it is testing, what it can't answer, and why the Australian context makes it matter more here than the raw percentages suggest.

Retatrutide vs Mounjaro: why the numbers you've seen don't line up

The retatrutide figure comes from TRIUMPH-1, the phase 3 obesity trial Eli Lilly reported in topline form. At 80 weeks, adults with obesity or overweight plus a weight-related condition, but without diabetes, lost an average of 19.0% on 4mg, 25.9% on 9mg and 28.3% on 12mg, against 2.2% on placebo. Some 45.3% of the 12mg group lost 30% or more of their body weight. A subgroup with a baseline BMI of 35 or above who continued in an extension reached an average of about 30.3% at 104 weeks.

The tirzepatide figure comes from SURMOUNT-1, published in the New England Journal of Medicine in 2022. In 2,539 adults with obesity or overweight without diabetes, the 15mg dose produced a mean weight change of −20.9% at 72 weeks, with 63% losing at least 20%.

Both are large, randomised, placebo-controlled trials. Neither tells you what happens when the two drugs are given to the same population under the same protocol. The trials differ in duration (80 weeks vs 72), in enrolment criteria, in the era of clinical practice they were run in, and even in their placebo arms (−2.2% vs −3.1%) — which is a quiet signal that the two participant groups weren't behaving identically. Cross-trial subtraction is a rough guide at best. More on how the programme is structured in our clinical trials overview.

TRIUMPH-5: the first direct retatrutide vs tirzepatide trial

TRIUMPH-5 (registered as NCT06662383) is a phase 3, randomised, double-blind study evaluating retatrutide against tirzepatide in adults with obesity. Per the trial registry: roughly 800 participants, both participants and investigators masked, an estimated duration of about 89 weeks, and an active comparator rather than a placebo. Entry requires obesity plus at least one self-reported unsuccessful attempt to lose weight through diet; people with a self-reported weight change of more than 5kg in the 90 days before screening, or prior/planned bariatric surgery, are excluded. The registry lists a start date of 1 November 2024 and an estimated primary completion in November 2026, with the study recorded as active and no longer recruiting.

That design is the whole point. Randomising people to one drug or the other, under one protocol, removes the population and protocol differences that make TRIUMPH-1 vs SURMOUNT-1 an apples-to-oranges exercise.

What a head-to-head can settle — and what it can't

A direct comparison can answer: how much more (or less) weight does one produce than the other in the same people; how the tolerability profiles compare when both are titrated under identical rules; and how many participants stop treatment in each arm. That last one matters, because a drug that produces bigger average loss but is harder to stay on doesn't necessarily win in the real world. In TRIUMPH-1, Lilly reported that the most common adverse events were gastrointestinal, generally mild to moderate, and concentrated during dose escalation — see our safety page for the fuller picture.

What TRIUMPH-5 will not settle: cardiovascular outcomes, long-term event rates, or what happens after people stop. Those are separate questions being studied in other arms of the retatrutide programme, and they take years longer to read out. It also won't be a peer-reviewed paper on day one — like most pharma readouts, the first version of the result will almost certainly be a company announcement, with the full publication following later. Treat topline press releases accordingly.

What this means in Australia

The Australian framing is different from the American one, because here the two drugs sit in completely different regulatory categories.

Tirzepatide (Mounjaro) is TGA-registered and dispensed through Australian pharmacies. Retatrutide is not. It has no TGA registration and no marketing authorisation anywhere in the world; Lilly has signalled regulatory filings from 2027. In Australia, lawful access today is through enrolment in a Lilly clinical trial. Our Australia page tracks the status.

There's a funding layer too. In its review of equitable access to GLP-1 obesity treatments, the PBAC noted that the sponsors of tirzepatide and liraglutide had not made submissions to list those medicines on the PBS for overweight or obesity — so weight-management use of tirzepatide sits outside PBS subsidy and is paid privately. That precedent is worth holding onto, because a strong TRIUMPH-5 result would say nothing about whether a future retatrutide would be subsidised here. Registration and reimbursement are separate hurdles, and with roughly 66.8% of Australian adults living with overweight or obesity, the second one is where population-scale access is decided.

If you're new to the mechanism — three receptors versus tirzepatide's two — start with what is retatrutide.

Want the TRIUMPH-5 readout when it lands? Join the free updates list and we'll send the numbers, plus the caveats, as they're published.

FAQ

Is retatrutide better than Mounjaro?

No one can say yet with confidence. The commonly quoted 28.3% (retatrutide, TRIUMPH-1) and 20.9% (tirzepatide, SURMOUNT-1) figures come from two separate trials with different durations and populations, so subtracting them isn't a valid comparison. The first direct head-to-head study, TRIUMPH-5, is still running.

When will the retatrutide vs tirzepatide head-to-head results come out?

The trial registry lists an estimated primary completion date of November 2026 for TRIUMPH-5. Results are typically announced by the sponsor first and peer-reviewed publication follows later, so an exact reporting date isn't fixed.

Can I get retatrutide in Australia instead of Mounjaro?

Not through normal channels. Retatrutide is investigational and has no TGA registration, whereas tirzepatide (Mounjaro) is TGA-registered and available on private prescription. Lawful retatrutide access in Australia is via participation in a Lilly-run clinical trial.

Does retatrutide cause more side effects than tirzepatide?

That question is exactly what a head-to-head trial is designed to answer, and it hasn't reported yet. In TRIUMPH-1, Lilly described the most common adverse events as gastrointestinal, generally mild to moderate, and mostly occurring during dose escalation — but that's from a placebo-controlled trial, not a direct comparison against tirzepatide.

Sources

This article is for general information only and is not medical advice. Retatrutide is an investigational drug that is not approved by the TGA or any other regulator. Speak to your GP or a qualified health professional about any weight-management treatment. retatrutide.net.au is an independent Australian information site and is not affiliated with, endorsed by, or sponsored by Eli Lilly and Company.