While circulating through the body, a drug undergoes chemical changes as it is broken down in a process called metabolism, or biotransformation. Most of these changes occur in the liver, but they can take place in other tissues as well. Various enzymes oxidize (add oxygen to), reduce (remove oxygen from), or hydrolyze (add water to) the drug. These changes produce new chemicals or metabolites that may continue to be medically active in the body or may have no activity at all. A drug may be broken down into many different metabolites. Eventually, most drugs or their metabolites circulate through the kidney, where they are discharged, or eliminated, into the urine. Drugs can also be excreted in the body's solid waste products, or evaporated through perspiration or the breath.