What best defines drug metabolism?

• A. Process of distributing drug to tissues
• B. Process of absorbing drug in the stomach
• C. Process of excreting drug by kidneys
• D. Process of body chemically changing drug into a form to be excreted

Answer: (D)

Drug metabolism is the body's chemical transformation of a drug into a more water-soluble form to facilitate its excretion. Most metabolism occurs in the liver through enzymes (notably the cytochrome P450 system) and often happens in two phases: Phase I reactions that introduce or reveal polar groups, and Phase II reactions (conjugation) that attach polar molecules like glucuronic acid or sulfate. These changes convert lipophilic drugs into metabolites that are easier to eliminate by the kidneys or via bile. This process is different from absorption, distribution, or excretion, which are related to how the drug enters the body, where it goes, and how it leaves the body.