Drug metabolism is that the chemical alteration of a drug by the body. Most drugs must undergo the liver, which is that the primary site for drug metabolism. Once within the liver, enzymes convert prodrugs to active metabolites or convert active drugs to inactive forms. Drug metabolism is that the term wont to describe the biotransformation of pharmaceutical substances within the body in order that they will be eliminated more easily. the bulk of metabolic processes that involve drugs occur within the liver, because the enzymes that facilitate the reactions are concentrated there. The metabolism of pharmaceutical drugs is a crucial aspect of pharmacology and medicine. The enzymes of xenobiotic metabolism, particularly the glutathione S-transferases also are important in agriculture, since they'll produce resistance to pesticides and herbicides. Drug metabolism is split into three phases.