First-pass metabolism refers to what process?

• A. When some drugs are metabolized by the liver into an inactive form of excretion
• B. The process by which drugs are excreted unchanged by the kidneys
• C. The conversion of drugs into active metabolites in the lungs
• D. The binding of drugs to plasma proteins to prevent excretion

Answer: (A)

First-pass metabolism is the liver’s processing of a drug after it’s absorbed from the gut and before it reaches the rest of the body. As the drug travels through the hepatic portal vein to the liver, liver enzymes often metabolize it into a form that is less active or inactive, which reduces the amount that reaches systemic circulation (lower bioavailability). That’s why oral doses can be less effective than other routes that bypass this initial liver step. The other options describe different processes—excretion by the kidneys, metabolism in the lungs, or drug binding to plasma proteins—none of which capture this pre-systemic hepatic metabolism.