Pharmacodynamics

Pharmacodynamics describes what a drug does to the body — the biochemical and physiological effects mediated through drug-receptor interactions. Most drugs exert effects by binding to specific receptors (proteins on cell surfaces or intracellularly) to either activate them (agonists) or block them (antagonists). Key pharmacodynamic concepts include: potency (concentration of drug needed to produce 50% maximal effect — EC50), efficacy (maximum effect a drug can produce regardless of dose), and therapeutic index (ratio of TD50/ED50, indicating the margin between therapeutic and toxic doses). The dose-response relationship follows a sigmoidal curve on a log-dose plot, with the steep portion representing the dosing range where small changes produce large effect changes. Receptor interactions are further classified as competitive (reversible, surmountable by increasing agonist dose) or non-competitive (irreversible or allosteric, reducing maximum response regardless of agonist concentration).