The median effective dose—the dose of a compound that produces the desired therapeutic effect in 50% of the population tested. ED50 is measured in whole organisms (in vivo), distinguishing it from EC50, which is measured in isolated cells or tissues (in vitro).

The distinction matters because a compound’s effective concentration in a petri dish can be very different from the dose needed in a living body, where absorption, distribution, metabolism, and excretion all intervene. ED50 is more clinically relevant than EC50 for predicting human dosing, but it is almost always derived from animal studies—extrapolating animal ED50 to human doses requires allometric scaling and remains imprecise.