Which pharmacokinetic factor affects the percentage of an orally administered drug dose that reaches systemic circulation?

• A. Protein binding
• B. Volume of distribution
• C. First pass effect
• D. Half-life

Answer: (C)

First-pass effect determines how much of an orally taken drug actually reaches the systemic circulation. After absorption in the gut, the drug travels via the portal vein to the liver (and also the gut wall), where metabolic processes can substantially reduce the amount that enters the general circulation. The portion that escapes this metabolism becomes the bioavailable fraction, setting the percentage of the oral dose that reaches systemic circulation. Protein binding, volume of distribution, and half-life describe what happens after the drug is in the bloodstream—how much is free, how it distributes into tissues, and how long it stays—but they don’t determine the initial fraction entering circulation from an oral dose. Drugs with extensive first-pass metabolism have lower oral bioavailability and may require higher oral doses or different routes to achieve therapeutic levels.