Produced naturally inside the body — as opposed to something you’d administer from outside.Endogenous peptides (GLP-1, insulin, oxytocin, alpha-MSH) have defined physiological roles produced by specific cells in response to specific signals. The fact that a peptide is endogenous does not make exogenous administration of it safe, effective, or physiologically equivalent. Exogenous doses, timing, receptor context, and feedback regulation all differ substantially from endogenous physiology. "It’s natural" is not a safety or efficacy argument.