retatrutide side effects phase 2 trial Efficacy and safety of retatrutide, a GIP, GLP-1, and glucagon receptor agonist, in people with type diabetes and inadequate glycaemic control with diet and exercise (TRANSCEND-T2D-1): a double-blind, randomised, 3 Retatrutide vs Tirzepatide (2026): Differences

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The digestive system is complex, and medications like Zepbound can significantly alter gut motility and microbiota, leading to these side effects

The study authors couldnt determine whether the medication itself caused these outcomes, or whether theyre attributable to the underlying health conditions of the women taking it

Human medicines in 2025 European Medicines Agency

The brain pathway may be more significant than the endocrine route for the physiological effects of GLP-1, especially regarding insulin release, but the endocrine route becomes more pronounced with bigger meals and more L-cell activation (Deacon 2019)