Agonist.

A molecule that activates a receptor to produce a response.

An agonist binds a receptor and activates it, triggering a downstream biological response. A full agonist produces the maximal response the receptor system can give; a partial agonist produces a sub-maximal response even when it occupies every available receptor. This is the single most useful concept for reading peptide research, because most peptides of interest work by acting as agonists at a receptor that an endogenous hormone already uses. Semaglutide and liraglutide are agonists at the GLP-1 receptor, the same receptor the gut hormone GLP-1 activates after a meal. GHRPs such as ipamorelin are agonists at the ghrelin receptor, which is why they trigger growth-hormone release through a different route than a GHRH analog does. Agonists can also be biased, meaning they activate some of a receptor's downstream signalling pathways more strongly than others, which is one way two drugs at the same receptor can produce different effect and side-effect profiles. The misunderstanding worth correcting is that agonist means stimulant or means more is better. Receptors adapt: continuous strong activation can cause a receptor to be desensitized or internalized, so the pattern of activation over time often matters as much as the strength of binding.