Retatrutide vs. Tirzepatide: What the Trial Data Actually Show

Retatrutide and tirzepatide are two widely discussed metabolic compounds developed by Eli Lilly. Online comparisons often quote a single result from each programme, but no completed head-to-head result has been reported. TRIUMPH-5 is directly comparing them and is active, not recruiting.⁵

The mechanism difference

Tirzepatide is a dual agonist, activating GLP-1 and GIP receptors. Retatrutide goes a step further as a triple agonist, adding glucagon receptor activation on top of GLP-1 and GIP. That third pathway is the entire premise behind retatrutide's development — glucagon receptor activity is associated with increased energy expenditure, which is a mechanism the dual-agonist approach doesn't directly engage.

What each trial actually found

Tirzepatide's SURMOUNT-1 trial reported 20.9% mean weight reduction at 72 weeks with the 15 mg dose under its treatment-regimen estimand.¹ Retatrutide's peer-reviewed Phase 2 trial reported 24.2% at 48 weeks with 12 mg.² In May 2026, Lilly reported TRIUMPH-1 Phase 3 topline results: 28.3% at 80 weeks with 12 mg under the efficacy estimand; the treatment-regimen estimate was 25.0%.⁶ Those retatrutide figures currently come from a sponsor release rather than a complete peer-reviewed Phase 3 report.

Regulatory status

Tirzepatide is an approved medicine in multiple markets. Retatrutide remains investigational; Lilly's current releases continue to state that it is not approved and is legally available only to clinical-trial participants.^6,7 Sponsor-reported Phase 3 success is not regulatory approval.

In South Africa, SAHPRA publishes information for registered Mounjaro (tirzepatide) presentations, including type 2 diabetes and weight management in defined adult populations.³ Retatrutide is not a SAHPRA-registered finished medicine. Powdered or informally supplied products should not be treated as equivalent to a registered prescription product; SAHPRA advises using original packaging, registered prescribers and licensed pharmacists.⁴

Side effects

Gastrointestinal events such as nausea, diarrhoea, vomiting and constipation are common in both programmes. Lilly's TRIUMPH-1 release also reported dose-related dysesthesia and urinary-tract-infection observations for retatrutide, with adverse-event discontinuation at 11.3% for 12 mg versus 4.9% for placebo.⁶ Separate trials cannot establish whether one drug is safer; the direct comparison has not reported.

So which one should you research?

For clinical care, this is not currently a two-approved-drug choice: tirzepatide has regulatory review, registered products and post-market experience; retatrutide remains investigational despite positive sponsor-reported Phase 3 results. The head-to-head result and full Phase 3 publications will provide stronger comparative evidence.

A South African clinician can assess whether a registered tirzepatide indication fits an individual's history and contraindications. An unregistered retatrutide product sold outside a trial should not be treated as a therapeutic alternative.