Pharmacodynamics.

What a compound does to the body: its effects and mechanism.

Pharmacodynamics, usually shortened to PD, describes the biochemical and physiological effects a compound produces and the mechanism by which it produces them. Where pharmacokinetics tracks concentration over time, pharmacodynamics tracks effect against concentration: how much receptor occupancy is needed before anything measurable happens, where the response plateaus, and how the effect changes with repeated exposure. For peptides this relationship is frequently not a simple straight line. Receptor systems saturate, so beyond a certain point additional compound produces no additional response while side effects may keep increasing. Systems also adapt, so a stable concentration can produce a shrinking effect over time as receptors desensitize. And some effects are indirect, arriving through a hormonal cascade rather than at the receptor itself, which introduces a delay between peak blood level and peak effect. This is why a peptide can be almost cleared from circulation while its consequences are still unfolding. The practical reason to know the term is that PD is where the interesting claims live. Statements about how a compound works, which receptor it engages, and what downstream systems it moves are pharmacodynamic claims, and they require different evidence than statements about how long it lasts.