Effective dose 50% (ED50) = the mean population dose necessary to achieve half the maximum (Emax) possible drug effect. ED50 was estimated from published analgesic dose response data (aspirin & paracetamol based on human dose-response data, others on comparative animal work).
Being prostaglandin synthetase antagonists, the mid-part of their dose-benefit curve plateaus, and in contrast a wide range of adverse events continue to increase.