It is becoming increasingly evident that metabolism of drugs and other foreign chemicals may not always be an innocuous biochemical event leading to detoxification and elimination of the compound. Indeed, several compounds have been shown to be metabolically transformed to reactive intermediates that are toxic to various organs.

Such toxic reactions may not be apparent at low levels of exposure to parent compounds when alternative detoxification mechanisms are not yet overwhelmed or compromised and the availability of endogenous detoxifying cosubstrates (glutathione, glucuronic acid, sulfate) is not limited. However, when these resources are exhausted, the toxic pathway may prevail resulting in overt organ toxicity or carcinogenesis.