Semaglutide or tirzepatide — which one does the research actually support? It is the most common comparison question in this field, and the honest answer has become more interesting than a straight two-way contest, because a third compound now has phase 3 data that beats both.

This page covers the mechanisms, the trial evidence as it stands, and where each compound sits in the Australian context. Nothing here is medical advice, and none of these compounds is supplied by us for human use.

The mechanisms, and why they are not the same drug twice

Both semaglutide and tirzepatide are built on incretin biology — mimicking gut hormones that signal satiety, slow gastric emptying and stimulate insulin release in response to meals. The difference is how many receptors each molecule engages.

• Semaglutide is a single-target agonist. It acts on the GLP-1 receptor alone, with a half-life of roughly seven days achieved through chemical modification that slows clearance.
• Tirzepatide is a dual agonist. It targets GLP-1 and GIP (glucose-dependent insulinotropic polypeptide) receptors in a single molecule. GIP is a second incretin involved in insulin secretion and in fat storage and mobilisation, and adding it appears to produce an effect greater than either mechanism alone.
• Retatrutide is a triple agonist — GLP-1, GIP and glucagon. The glucagon arm is the addition that separates it from the other two, contributing to energy expenditure rather than appetite signalling alone.

That progression — one receptor, then two, then three — is the single most useful thing to understand here, because the trial results track it fairly closely.

What the trials report

Semaglutide

In STEP 1, semaglutide at 2.4mg weekly produced average weight loss of approximately 14.9% of body weight over 68 weeks. Its landmark cardiovascular result came from SELECT, published in late 2023, which reported a 20% reduction in major adverse cardiovascular events in people with obesity and established cardiovascular disease who did not have diabetes. That finding is what shifted how the wider medical community regards this drug class.

Tirzepatide

In SURMOUNT-1, tirzepatide at its highest dose of 15mg weekly produced average weight loss of approximately 20.9% over 72 weeks, with a meaningful proportion of participants exceeding 25%. A large observational analysis of real-world outcomes in over 40,000 patients found those prescribed tirzepatide lost significantly more weight than those on semaglutide at twelve months, a finding that has held up in subsequent analyses.

On cardiovascular outcomes, SURPASS-CVOT was published in the New England Journal of Medicine in December 2025: 13,299 patients with type 2 diabetes and atherosclerotic cardiovascular disease, compared against dulaglutide over a median four years. Cardiovascular death, myocardial infarction or stroke occurred in 12.2% on tirzepatide versus 13.1% on dulaglutide — meeting non-inferiority, but not superiority. Tirzepatide did produce substantially greater weight loss in that trial (11.6% versus 4.5%), which did not translate into a superior cardiovascular endpoint. SURMOUNT-MMO, the separate outcomes trial in obesity without diabetes, is still running.

Retatrutide

In TRIUMPH-1, reported in May 2026, 2,339 adults with obesity or overweight plus at least one weight-related condition and no diabetes were followed for 80 weeks:

• 4mg — average weight loss of 19.0%
• 9mg — average weight loss of 25.9%
• 12mg — average weight loss of 28.3%

Participants with a baseline BMI of 35 or above who continued on 12mg reached an average of 30.3% at 104 weeks, and at the top dose 45.3% of participants lost 30% or more of their body weight — a threshold that has historically been associated with bariatric surgery rather than pharmacology.

We cover the full TRIUMPH-1 data in detail, along with TRIUMPH-2 and TRIUMPH-3 in type 2 diabetes and cardiovascular disease, and the cardiometabolic picture more broadly.

Reading the numbers side by side

Cross-trial comparison is imperfect — different populations, durations and endpoints — and no single trial has randomised participants to all three compounds at once. With that caveat, the pattern is consistent enough to be worth stating plainly:

• Semaglutide — ~14.9% at 68 weeks (STEP 1). Single agonist. The strongest cardiovascular outcome evidence of the three.
• Tirzepatide — ~20.9% at 72 weeks (SURMOUNT-1). Dual agonist. Cardiovascular non-inferiority against dulaglutide; the obesity outcomes trial is still open.
• Retatrutide — 28.3% at 80 weeks, up to 30.3% at 104 weeks (TRIUMPH-1). Triple agonist. The newest data, and the largest weight effect reported so far.

So the answer to the question in the title has changed. On weight endpoints the research now supports tirzepatide over semaglutide, and retatrutide over both. On cardiovascular outcomes semaglutide still has the deepest evidence base, because it has been studied longest.

The Australian context

Semaglutide and tirzepatide are prescription-only medicines in Australia and are accessed through a registered GP or specialist, not through online suppliers. If you are looking at them in that capacity, that is the route. We also cover the semaglutide PBS listing and what it means, and what the law says about peptides in Australia for the wider regulatory picture.

Retatrutide is not an approved medicine anywhere. It is an investigational compound in active phase 3 development, which is precisely why it exists as a research reference material rather than a prescription.

Research-grade considerations

For laboratory work with any of these compounds, the structural complexity is the practical problem. All three are large, heavily modified peptides that require sophisticated synthesis and degrade readily if handled or stored badly. A purity figure means very little without the method and the batch behind it.

We publish certificates by product in our certificate of analysis library, with identity confirmed by mass spectrometry and purity measured by HPLC against a specification of 99% or greater. To be clear about what that covers: we do not supply semaglutide or tirzepatide. Of the compounds on this page, the one we hold is retatrutide, as R3TA 10mg and R3TA 20mg.

Common questions

Is tirzepatide better than semaglutide?

On weight endpoints, the published data consistently favours tirzepatide. On cardiovascular outcomes the picture is less clear: semaglutide has SELECT, tirzepatide has non-inferiority against dulaglutide in SURPASS-CVOT, and the obesity outcomes trial has not reported. Better depends on which endpoint you are asking about.

Where does retatrutide fit?

It is the next step in the same mechanistic progression — a third receptor added to the two tirzepatide engages — and TRIUMPH-1 reported the largest weight effect of the three. It is also the least mature: phase 3, not approved, with a shorter evidence base than either of the others.

Do you sell semaglutide or tirzepatide?

No. Neither is in our catalogue. R3TA is the only compound on this page we supply, and it is supplied strictly as reference material for laboratory research. See what we hold and how to choose for the full range.

Why compare a prescription medicine with an investigational compound?

Because the mechanistic line runs straight through all three, and understanding where the incretin field has got to is difficult without seeing them together. GLP-1 peptides in Australia covers the class as a whole.