LD 50/ ED 50 is ?

Ans. is 'a' i.e., Therapeutic index Therapeutic index Drugs usually have more than one action. The maximal response for different action of a drug may be different ie the DRCs (Dose response curves) for different effects of a drug may be different. The extent of separation of DRCs of a drug for different effects is a measure of its selectivity, e.g. the DRCs for bronchodilatation and cardiac stimulation are quite similar in case of isoprenaline but far apa in case of salbutamol - the latter is more selective. The gap between the therapeutic effect DRC and the adverse effect DRC defines the safety margin or the therapeutic index.