Amongst the undesirable effects of medical intervention which touch on the respiratory apparatus one can distinguish schematically those disorders which affect the "passive respiratory system" (lungs, pleura, bronchi and vessels) and those which concern the "active respiratory system (SRA)" (nerve centres, respiratory muscles) which are less often described. After a brief reminder of the principles and limits of the available methods of investigation, this chapter reviews the different iatrogenic disorders of SRA according to their level and their aetiology. Peripheral disorders are touched on such as alterations of phrenic conduction (with particular mention of satellite lesions from cardiac surgery), those of neuromuscular transmission (most often induced by medication) and of intrinsic muscular properties (mainly steroid induced myopathies). Central disorders are described principally as the respiratory effects of neurotropic and non-neurotropic medications and the harmful effects of different substances on the sleep/respiration interaction. The secondary effects of therapy on SRA are all the more marked if there are underlying respiratory or neuromuscular disturbances which allow a large place for preventive measures. The diagnosis is made difficult by the complexity of the structures and functions involved and it is clear that the development of pathophysiological studies, which are still too scarce, should enable better understanding of their clinical significance.