drug metabolism
the process by which a drug is transformed in the body (in the liver and other organs), usually from a more lipid-soluble form, which makes it more readily absorbed into the body, to a more water-soluble form, which facilitates its excretion. Two phases of drug metabolism are recognized. In Phase I metabolism, the drug is oxidized, reduced, or hydrolyzed—that is, oxygen is added, oxygen is removed, or hydrogen is added, respectively (see cytochrome P450). In Phase II metabolism, functional groups (specific clusters of atoms) are added to drug molecules (e.g., by glucuronidation).