 Xenobiotics such as synthetic drugs, natural poisons and antibiotics are detoxified by a set of xenobiotic-metabolizing enzymes.In humans, these include cytochrome P450 oxidases, UDP-glucuronosyltransferases, and glutathione "S"-transferases.This system of enzymes acts in three stages to firstly oxidize the xenobiotic (phase I) and then conjugate water-soluble groups onto the molecule (phase II).