Biotransformation, or metabolism, of drugs involves enzymatic reactions that make the drugs more polar so they can be more easily excreted from the body. These reactions are divided into two phases: phase I introduces functional groups through reactions like oxidation, and phase II involves conjugating these groups to endogenous compounds. Cytochrome P450 enzymes are a major catalyst of phase I reactions, introducing groups like hydroxyl that then undergo conjugation. Factors like other drugs, diet, age and liver function can influence a drug's biotransformation.