The significance of stereochemistry in therapeutic action is outlined and elucidated. Often only one isomer is therapeutically active, but this does not mean that the other is really inactive. It may very well contribute to the side-effects. The therapeutically non-active isomer in a racemate should be regarded as an impurity (50% or more). It is emphasized how in clinical pharmacology, and particularly in pharmacokinetics, neglect of stereoselectivity in action leads to the performance of expensive "highly sophisticated scientific nonsense". This also holds true in the development and marketing of new drugs as exemplified by various "pseudo-hybrid" drugs now reaching the clinic.