The dose that produces exactly half the maximum possible effect — a standard way of comparing how potent different compounds are.Lower EC50 = higher potency (less compound needed to achieve the same effect). Derived from in vitro functional assays. Distinct from IC50 (concentration producing 50% inhibition) and Ki (binding affinity constant). EC50 values from cell culture experiments provide comparative pharmacology data but do not directly translate to human dosing — in vivo PK, receptor distribution, and downstream signaling complexity all mediate the relationship between plasma concentration and biological effect.