This article is for research and educational purposes only. Not medical advice.
The last five years have seen extraordinary progress in incretin-based pharmacology. What began with GLP-1 receptor agonists treating type 2 diabetes has evolved into a class of compounds producing weight loss outcomes that rival bariatric surgery — and the science is still advancing.
Three compounds now define the research landscape: semaglutide (single GLP-1 agonist), tirzepatide (GLP-1/GIP dual agonist), and retatrutide (GLP-1/GIP/glucagon triple agonist). Understanding their differences requires looking at both mechanism and clinical evidence.
Semaglutide (Ozempic/Wegovy, Novo Nordisk) was the compound that redefined the category. It binds exclusively to the GLP-1 receptor, producing:
The STEP programme established its efficacy. STEP-1 (2021, n=1,961) showed mean weight loss of 14.9% over 68 weeks at 2.4 mg weekly. The SELECT cardiovascular outcomes trial (2023) added a 20% reduction in major adverse cardiovascular events — transforming semaglutide from a weight loss drug to a metabolic medicine.
Semaglutide is currently approved by both the FDA and TGA. It has the longest safety record of the three compounds and the most human outcome data.
Tirzepatide (Mounjaro/Zepbound, Eli Lilly) added GIP receptor co-agonism to GLP-1 activity. GIP — glucose-dependent insulinotropic polypeptide — is an incretin hormone with effects on insulin secretion and adipose tissue metabolism. The combination appeared to produce synergistic rather than simply additive effects.
The SURMOUNT-1 trial (2022, n=2,539) showed mean weight loss of 20.9% at the 15 mg dose — the highest recorded in a randomised controlled trial for any pharmacological agent at that time. Approximately one-third of participants lost ≥25% of body weight.
The direct head-to-head SURMOUNT-5 trial confirmed tirzepatide's superiority over semaglutide: approximately 47% greater relative weight loss.
Tirzepatide has FDA approval for both diabetes and obesity; TGA approval in Australia is expanding. For a detailed Australian-focused comparison of tirzepatide and semaglutide — including PBS listing status, private costs in AUD, and clinical decision considerations — see tirzepatide vs semaglutide in Australia.
Retatrutide (LY3437943, Eli Lilly) adds a third mechanism: glucagon receptor agonism alongside GLP-1 and GIP activity. The hypothesis is that glucagon receptor activation — which in isolation raises blood glucose — produces net metabolic benefits when co-administered with GLP-1: increased thermogenesis, greater hepatic fat clearance, and elevated basal metabolic rate, without the hyperglycaemic consequence because GLP-1-mediated insulin secretion counteracts it.
The Phase II trial (Jastreboff et al., NEJM, 2023) enrolled 338 adults with obesity and ran 24 weeks with dose escalation. At the 12 mg dose:
For Australian researchers tracking this compound, RetaLABS has published a detailed the Australian research peptide range comparing the Phase II data in more depth. Those looking to source research-grade material can also access research-grade retatrutide through their product range.
The full Phase II trial is on PubMed: Jastreboff AM et al. Triple Hormone Receptor Agonist Retatrutide for Obesity. NEJM 2023.
| Compound | Mechanism | Peak trial weight loss | Approval status | |---|---|---|---| | Semaglutide 2.4 mg | GLP-1 | ~15% (68 wks) | FDA + TGA approved | | Tirzepatide 15 mg | GLP-1 + GIP | ~21% (72 wks) | FDA approved, TGA expanding | | Retatrutide 12 mg | GLP-1 + GIP + Glucagon | ~18% (24 wks) | Phase III (TRIUMPH programme) |
Note: retatrutide's 24-week Phase II figure is not directly comparable to the longer trials above — Phase III results (expected 2025–2026) will provide the definitive comparison.
All three share GI adverse effects — nausea, vomiting, constipation, diarrhoea — primarily driven by gastric emptying slowdown and central satiety effects. These are dose-escalation dependent and typically diminish after the titration phase.
The glucagon component in retatrutide adds complexity: the long-term tolerability of triple agonism, particularly in populations with varying degrees of insulin resistance, will require Phase III characterisation. Initial Phase II safety data was acceptable.
As covered in our article on semaglutide and muscle loss, lean mass outcomes during significant weight reduction are an important secondary endpoint across all three compounds. Tirzepatide's GIP component and retatrutide's glucagon component may each have distinct effects on muscle metabolism — this is an area of active research interest.
For Australian researchers and clinicians, these three compounds represent different stages of the regulatory and research lifecycle. Semaglutide and tirzepatide are in clinical use; retatrutide remains investigational. The research community continues to follow the TRIUMPH Phase III programme closely as it will establish retatrutide's long-term safety and efficacy profile. For those not yet on pharmaceutical therapy, the evidence for natural compounds that support GLP-1 pathways — including berberine and dietary fibre — provides context on non-pharmacological options at the other end of the spectrum. As the PBS landscape for these agents evolves, the Wegovy PBS listing Australia 2026 guide tracks current subsidy status and eligibility for the most accessible semaglutide pathway. For the full Australian picture on semaglutide access — including PBS cost, private prescription pricing, supply status, and compounded semaglutide warnings — see the Ozempic Australia 2026 cost and access guide.
The progression from semaglutide to tirzepatide to retatrutide reflects a coherent mechanistic logic: each generation adds a receptor target that amplifies the metabolic effect of the previous. The efficacy data tracks this logic. What remains to be established for retatrutide is the long-term safety, cardiovascular outcomes, and lean mass data that will ultimately determine its clinical positioning relative to the two approved agents.
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