Idiosyncratic drug reactions (IDRs) are unpredictable adverse drug reactions that do not occur in most patients but when they do occur they can be life-threatening. They are often called dose-independent but this is not true - nothing is dose independent - but what is true is that the incidence may not vary within the narrow range of doses that are used clinically and IDRs do not occur in most patients at any dose. Examples of IDRs are troglitazone-induced liver failure often leading to death or liver transplant, clozapine-induced agranulocytosis (a essential type of white blood cell suddenly disappears making the patient susceptible to infection) and sulfonamide-induced toxic epidermal necrolysis (a very severe skin rash much like a thermal burn and treated in a burn unit). This type of adverse drug reaction is responsible for a significant number of deaths and their unpredictable nature makes them virtually impossible to prevent. IDRs also introduce a significant amount of uncertainty into the process of drug development because they are not predicted by animal testing and it is only after many patients have been treated that the extent of the problem can be determined, often leading to the withdrawal of an otherwise good drug.

Idiosyncratic Drug Reactions