[Introduction to adverse drug reactions and drug-drug interaction]

Adverse drug reactions are very common during a systemic drug therapy. Most of these are fortunately mild or moderate and transient. Serious adverse drug reactions often lead to hospitalisation. Drug-drug interactions can also contribute substantially to differences in drug response. Drug-induced inhibition of drug metabolising enzymes is usually competitive and causes an increased exposure of the drug whose metabolism is inhibited. This usually results in a higher incidence of adverse drug reactions and an increased severity. Conversely enzyme induction increases the metabolic capacity thereby reducing the exposure of a drug with sometimes loss of drug effects (nonresponse). Detailed knowledge of the causative mechanisms will help detecting and avoiding severe adverse drug reactions.