Metabolism refers to the biochemical transformation of drugs within the body primarily carried out by the liver. Drugs can be transformed into either inactive (metabolism) or active (prodrug) metabolites. Some drugs form equally active metabolites, providing long-lasting effects. First-pass metabolism describes the transformation of orally administered drugs by the liver before they reach systemic circulation, reducing their bioavailability. Metabolism occurs through two phases: phase I involves reactions like oxidation and hydrolysis that make the drug more polar, while phase II conjugates these products to make them water-soluble and excretable. Factors like age, genetics, and concurrent medications can impact an individual's drug metabolism.