Semaglutide, tirzepatide, and retatrutide get compared constantly. They act on overlapping metabolic pathways, but each one hits a different set of receptors. Those differences are what set realistic expectations for what the research actually shows.
Receptor targets and mechanism
Semaglutide is a GLP-1 receptor agonist. It mimics glucagon-like peptide-1, a gut hormone that stimulates insulin secretion in a glucose-dependent way, slows gastric emptying, and acts on the brain to reduce appetite. It is a modified analogue of human GLP-1 with a fatty acid chain that stretches its half-life to roughly seven days. That long half-life is what supports once-weekly dosing. The published pharmacology literature describes the mechanism in detail.
Tirzepatide adds a second target. It is a dual GLP-1 / GIP receptor agonist. GIP (glucose-dependent insulinotropic polypeptide) is a separate incretin hormone, and the literature describes co-agonism at both receptors as producing additive or synergistic effects on insulin secretion and weight-relevant pathways. The molecule itself is a single synthetic compound engineered to switch on both receptors at once.
Retatrutide goes one step further: a triple GLP-1 / GIP / glucagon receptor agonist. Glucagon receptor activation raises energy expenditure and drives hepatic fat metabolism. The rationale described in clinical-stage research is straightforward. Add glucagon agonism and you may amplify weight reduction beyond what dual agonism delivers. Retatrutide is still investigational. It has not received regulatory approval as of the date of this article.
What the trials report
| Compound | Receptor targets | Regulatory status (approx.) | Reported weight-loss findings |
|---|---|---|---|
| Semaglutide (Wegovy / Ozempic) | GLP-1 | FDA-approved (obesity 2021, T2D earlier) | STEP trials reported approximately 15% mean body weight reduction over ~68 weeks at the 2.4 mg weekly dose in adults with obesity |
| Tirzepatide (Zepbound / Mounjaro) | GLP-1 + GIP | FDA-approved (obesity Nov 2023, T2D 2022) | SURMOUNT-1 reported approximately 21% mean body weight reduction at the 15 mg dose over 72 weeks; the head-to-head SURMOUNT-5 trial reported greater mean reduction than semaglutide |
| Retatrutide | GLP-1 + GIP + glucagon | Investigational (Phase 3 program ongoing as of mid-2026; not approved) | Phase 2 (NEJM, 2023) reported approximately 24% mean body weight reduction at the 12 mg dose over 48 weeks; first Phase 3 readout (TRIUMPH-4) reported in Dec 2025 |
The figures above come from reported trial results as cited in the literature and regulatory communications. Individual outcomes vary, and the trials differ in design, population, and duration. Read them as population-level research findings, not outcome guarantees.
Retatrutide moves into Phase 3
When retatrutide's Phase 2 data first appeared, mid-stage results were all anyone had. That changed. Eli Lilly's first Phase 3 readout, TRIUMPH-4, was announced on December 11, 2025. In adults with obesity or overweight and knee osteoarthritis, the company reported an average body weight reduction of roughly 28.7% at the 12 mg dose over 68 weeks, versus about 2.1% on placebo. Lilly went on to report additional Phase 3 readouts (the TRIUMPH program) during 2026 and has said it intends to file for regulatory approval. As of this article, retatrutide is still investigational and holds no marketing approval in any indication.
Regulatory status
Semaglutide and tirzepatide hold FDA approval for specific indications, type 2 diabetes and chronic weight management, under their brand names. Full prescribing information is available through the FDA's drug database. Both have also cleared regulatory review elsewhere, including EMA approval in the European Union.
Retatrutide is being developed by Eli Lilly and remains in clinical development. It does not hold regulatory approval for any indication. Material sold under the retatrutide name outside the clinical trial context is sold for research use only and is not approved for human consumption.
This class moves fast. Regulatory decisions and published trial data that post-date this article may change the picture described here.