GLP-1 Agonists Compared: Semaglutide, Tirzepatide and Retatrutide

The Three Generations of Incretin Mimetics

In less than a decade, the field of GLP-1 agonists has fundamentally changed — from single receptor activation (GLP-1 monoagonism) to dual activation (GIP + GLP-1) to triple agonism (GIP + GLP-1 + glucagon). This development reflects one of the most exciting advances in modern metabolic research.

Mechanistic Comparison

1. Semaglutide — GLP-1 monoagonist. Selectively binds GLP-1R. Half-life ~7 days through albumin binding.
2. Tirzepatide — GIP/GLP-1 dual agonist. Binds GIPR and GLP-1R. Half-life ~5 days. Approved as Mounjaro/Zepbound.
3. Retatrutide — GIP/GLP-1/Glucagon triple agonist. Additionally binds glucagon receptor. Not yet approved (Phase 3).

Clinical Efficacy Overview

Mean weight reduction after 68–72 weeks: Semaglutide 2.4 mg: -14.9 %, Tirzepatide 15 mg: -22.5 %, Retatrutide 12 mg: -24.2 % (Phase 2).

- Comparison from clinical Phase 3 programmes

Why Does Efficacy Increase with Each Generation?

The increasing efficacy is explained by the additive activation of multiple metabolic signalling pathways. GIP receptor agonism improves the insulin response to meals and directly modulates adipose tissue. Glucagon receptor agonism increases hepatic fat oxidation and thermogenesis — despite nominally opposing individual effects, a synergistic pattern emerges.

Side Effect Profile and Research Implications

With increasing efficacy, all three compounds show similar gastrointestinal side effects (nausea, vomiting, diarrhoea) that are often dose-limiting. Gastrointestinal effects are receptor-class specific rather than compound-specific, as they are triggered by GLP-1 receptor activation in the gut.

Retatrutide is not yet approved. All three compounds are available from Studien Peptide as high-purity research compounds for authorised laboratories.

- ⚗️ Research note