Most of the retatrutide conversation in Australia has been about the number on the scales — and fair enough, given the headline 28.3% average body weight reduction at 12mg over 80 weeks in TRIUMPH-1. But there's a quieter storyline in Eli Lilly's phase 3 program that may matter just as much to Australian patients: what happens to painful, worn-out knees.

That storyline comes from TRIUMPH-4, the first phase 3 retatrutide trial to report results (topline data released by Lilly in December 2025). It didn't study weight loss alone. It studied adults living with overweight or obesity and knee osteoarthritis — and it measured pain and physical function alongside kilograms.

Why knees are an Australian problem, not a footnote

Osteoarthritis is one of the most common reasons Australians end up in an orthopaedic waiting room. According to Australian Orthopaedic Association National Joint Replacement Registry data summarised by clinicians, roughly 98% of knee replacements in Australia are performed because of knee osteoarthritis, with an average patient age of around 69 and a slight majority of women.

Research published in the Medical Journal of Australia has noted that Australia's osteoarthritis burden is growing — driven by population growth, ageing, high rates of obesity and sports-related knee injuries — and that joint replacement numbers are projected to keep rising.

The wait can be long. Australian Institute of Health and Welfare reporting has repeatedly shown total knee replacement among the longest-waiting elective procedures in the public system, with median waits historically measured in the hundreds of days. Months of waiting means months of pain, reduced mobility, poorer sleep and less capacity to exercise — which, cruelly, makes weight management harder at exactly the moment it would help most.

With about 66.8% of Australian adults living above a healthy weight, the overlap between excess weight and aching joints is enormous.

What TRIUMPH-4 actually tested

TRIUMPH-4 was a global, randomised, double-blind, placebo-controlled study of retatrutide's two highest doses — 9mg and 12mg — over 68 weeks, in 445 participants with obesity or overweight plus knee osteoarthritis.

It used co-primary endpoints: body weight and knee pain, the latter measured with the WOMAC index (the Western Ontario and McMaster Universities Osteoarthritis Index), a standard patient-reported tool covering pain and physical function.

The results, in plain English

  • On the efficacy estimand, participants on 12mg lost an average of 28.7% of body weight (about 32.3kg) at 68 weeks; those on 9mg lost 26.4%.
  • Placebo participants lost about 2.1%.
  • More than half of those on 12mg lost at least 25% of their body weight, per reporting of the topline data.
  • Both doses met all primary and key secondary endpoints, including reductions in knee pain and improvements in physical function.

Lilly's own commentary framed it around mobility rather than aesthetics, noting that people with obesity and knee osteoarthritis often live with pain and restricted movement and "may eventually require total joint replacement".

That's the interesting bit. A drug being assessed on whether people can walk, climb stairs and sleep better is a different proposition from a drug assessed only on dress sizes.

Why a triple agonist might matter here

Retatrutide targets three receptors — GLP-1, GIP and glucagon — versus semaglutide (Ozempic, Wegovy) acting on one, and tirzepatide (Mounjaro, Zepbound) acting on two. More on the mechanism in our explainer: what is retatrutide.

For joints, the plausible driver is simple mechanical load: less body mass through the knee generally means less pain. But there's an open scientific question about whether GLP-1-class medicines also influence inflammation in the joint itself. TRIUMPH-4 wasn't designed to settle that, and nobody should claim it did.

It's also worth being clear-eyed about body composition. Glucagon receptor activation has raised theoretical questions about lean mass, and a phase 2 body-composition substudy published in The Lancet Diabetes & Endocrinology reported that retatrutide produced greater fat mass reduction than lean mass loss, with the proportion of lean mass loss to total weight loss described as similar to other obesity treatments. For anyone with dodgy knees, protecting muscle around the joint is not optional — it's the whole point. Resistance training and adequate protein remain standard advice from clinicians, regardless of medication.

The rest of 2026: seven more readouts

TRIUMPH-4 and TRIUMPH-1 are not the end of it. Lilly has said it expects seven additional phase 3 readouts across 2026, spanning type 2 diabetes, moderate-to-severe obstructive sleep apnoea, chronic low back pain, MASLD (fatty liver disease), maintenance dosing strategies, and cardiovascular and renal outcomes.

Earlier readouts have already landed: TRANSCEND-T2D-1, published in The Lancet in 2026, reported an average 16.8% body weight reduction at 40 weeks on 12mg in people with type 2 diabetes.

Read more about the trial program on our clinical trials page, and see the tolerability picture — gastrointestinal side effects and dose escalation — on our safety page.

What this means for Australians right now

Be clear on the status: retatrutide is investigational and is not TGA-approved. It isn't available on prescription in Australia, and Eli Lilly has indicated regulatory filings from 2027, which means Australian availability, pricing and any PBS consideration are still some distance away. Our Australia page tracks the local regulatory picture as it develops.

In the meantime, if knee osteoarthritis is affecting you, the evidence-based path hasn't changed: talk to your GP, get a referral to a physiotherapist or exercise physiologist, discuss existing approved medications if appropriate, and ask about the GP Management Plan and allied health pathways available under Medicare.

What TRIUMPH-4 adds is a reason for cautious optimism — and a hint that the next generation of metabolic medicines may be judged on function and quality of life, not just kilograms.

Want the next readouts explained in plain English? Join our free Australian updates list — we email when there's genuine news, including TGA milestones.

FAQ

Can retatrutide be prescribed in Australia for knee osteoarthritis?

No. Retatrutide is investigational and has not been approved by the TGA for any indication, including knee osteoarthritis. TRIUMPH-4 is a clinical trial result, not an approval.

Did TRIUMPH-4 show retatrutide prevents knee replacement surgery?

No. TRIUMPH-4 measured body weight, WOMAC pain and physical function over 68 weeks. It was not designed to test whether the medicine prevents or delays joint replacement surgery, and no such conclusion should be drawn.

When might Australians be able to access retatrutide?

Eli Lilly has signalled regulatory filings from 2027. Australian access would depend on TGA registration and subsequent supply and reimbursement decisions, so no timeline is confirmed.

Disclaimer: This article is general information only and is not medical advice. It does not replace consultation with a qualified Australian health professional — please speak with your GP or specialist about your individual circumstances. Retatrutide is an investigational medicine and is not approved by the TGA. retatrutide.net.au is an independent Australian information resource and is not affiliated with, endorsed by, or sponsored by Eli Lilly and Company or any pharmaceutical manufacturer.