Drug metabolism occurs primarily in which organ?

• A. Lungs
• B. Liver
• C. Brain
• D. Kidneys

Answer: (B)

Drug metabolism is most active in the liver because it houses the highest concentration of drug-metabolizing enzymes, especially the cytochrome P450 family and multiple Phase I and Phase II enzymes. These systems transform drugs through oxidation, reduction, or hydrolysis (Phase I) and then conjugation (Phase II) to make them more water-soluble for easier excretion. The liver also governs the first-pass effect: drugs absorbed from the gut go through the portal vein to the liver, where a portion can be metabolized before reaching systemic circulation, affecting Bioavailability. Other organs can contribute to metabolism for specific drugs, but they don’t match the liver in enzyme capacity or in overall role in preparing drugs for clearance.