Adverse drug reaction (ADR) A noxious and unintended reaction that occurs alongside the intended principal effect of a drug, for which a causal relationship between the use of the drug and the adverse reaction is suspected

Adverse drug reaction (ADR) ADRs can be both type A (pharmacological/toxic) and type B (hypersensitivity)

Type A („augmented“: pharmacological/toxic drug reaction) Disease manifestations due to predictable, dose-dependent pharmacological/toxic effects typical for a drug at the recommended dose (e.g., sedative effect of older antihistamines, hair loss caused by cytostatics) or at higher doses (intoxication)

Type B („bizarre“: hypersensitivity reactions) Individual, unpredictable clinical reaction to a drug, i.e., disease manifestations occur in specifically predisposed patients; a distinction is made between two forms:

Drug allergy: Hypersensitivity is based on an immunological reaction (types I–IV according to Coombs and Gell)

Non-immunological drug hypersensitivity: An immunological (allergic) reaction mechanism cannot be detected. This form of reaction was formerly further subdivided into: _ Drug intolerance: typical symptoms of the pharmacological effect (toxicity) develop already at low doses, which are usually tolerated _ Drug idiosyncrasy: Symptoms differ from the pharmacological effect of the substance. Reactions involving symptoms that correspond to allergic disease were also referred to as pseudo-allergies