Xenobiotics ("strangers to life") are generally considered to fall into three categories: foods, drugs, and poisons. However, almost anything taken into the body has poisonous aspects. The Swiss physician Paracelsus (1493 - 1541) recognized this principle when he wrote in 1538, "What is not a poison? All things are poisons, and nothing is without toxicity. Only the dose permits anything not to be poisonous."

Considering the enormous exposure that animals have to the nonnutritive natural products found in food as well as to the myriad human-made xenobiotics, it is surprising that physiological catastrophes do not occur more often than they do. It has been postulated that the detoxication systems (remember what "detoxification" means!) of the animal body have evolved over the last billion years or so because of the selection pressure provided principally by the exposure to plant metabolites and decayed plant products. For example, in addition to nutrient molecules (e.g. starch, sugars, protein, fat, minerals, and vitamins), most vegetables and fruits contain small amounts of dozens or even hundreds of nonnutritive organic molecules. Examples of nonnutritive molecules include various aldehydes, ketones, alchohols, esters, tannins, steroids, phenols and terpenes, among others. Amazingly, enzymatic mechanisms for using or disposing of most of these molecules in animals have arisen through intense natural selection over the millenia.