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A neutral rundown of how Retatrutide and tirzepatide compare based on published research — useful background before diving into the rest of the Retatrutide vs tirzepatide discussion threads on this forum.

Receptor targets

Tirzepatide (Mounjaro/Zepbound, FDA-approved) is a dual agonist: it activates the GLP-1 and GIP receptors. Retatrutide is a triple agonist: it activates GLP-1, GIP, and glucagon receptors. The added glucagon receptor activity is the main mechanistic difference and is thought to contribute to increased energy expenditure and liver fat reduction.

Published research

Tirzepatide’s pivotal trial is SURMOUNT-1 (NEJM, 2022). Retatrutide’s key trials are its Phase 2 obesity trial (NEJM, 2023) and the Phase 3 TRIUMPH-1 obesity trial, which Lilly reported topline results for in May 2026. A dedicated head-to-head trial directly comparing the two compounds (ClinicalTrials.gov NCT06662383) is currently underway, with results expected around the end of 2026.

Weight-loss results reported in studies

In SURMOUNT-1, tirzepatide 15 mg produced average weight loss of about 22.5% at 72 weeks. In TRIUMPH-1, Retatrutide 12 mg produced average weight loss of about 28.3% at 80 weeks (up to 30.3% in a 104-week extension in a subgroup). Important caveat: these numbers come from two separate trials with different populations and designs — this is a cross-trial comparison, not a head-to-head result, so treat the gap between them as suggestive rather than definitive until NCT06662383 reports.

Side-effect profiles

Both compounds share a broadly similar, GI-predominant side-effect profile typical of incretin-based therapies — nausea, diarrhea, vomiting, and constipation are the most commonly reported. In TRIUMPH-1, Lilly reported discontinuation rates due to adverse events of 4.1%, 6.9%, and 11.3% for Retatrutide’s 4 mg, 9 mg, and 12 mg doses respectively, versus 4.9% for placebo.

Similarities

Both are once-weekly subcutaneous injections, both work through the incretin/glucagon hormone system, and both have been studied for obesity/overweight and type 2 diabetes.

Differences

The clearest practical difference right now: tirzepatide is FDA-approved and commercially available by prescription. Retatrutide is investigational, still in Phase 3 trials, and not available outside of clinical trial enrollment.

See also Tirzepatide Research & Experiences for tirzepatide-specific discussion, and Tirzepatide vs Semaglutide for the third leg of this comparison.

This thread is for discussing published research and trial data. Please keep comparisons neutral and sourced — this isn’t the place for treatment recommendations or dosing advice.

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