Which statement is true about first-pass metabolism?

• A. It occurs when drugs absorbed from the GI tract pass through the liver before reaching systemic circulation.
• B. It occurs only with intravenous administration.
• C. It increases the amount of drug reaching systemic circulation.
• D. It occurs in the lungs.

Answer: (A)

First-pass metabolism describes how drugs absorbed from the gut are metabolized by the liver before reaching systemic circulation, which can substantially reduce their bioavailability. After absorption, the drug travels via the portal vein to the liver, where hepatic enzymes metabolize part of it before it enters the bloodstream. This is why oral doses can require higher amounts or different formulations, and why intravenous administration bypasses this effect by delivering the drug directly into systemic circulation. The other statements aren’t correct because first-pass metabolism is not limited to intravenous administration, it generally decreases (not increases) the amount reaching circulation, and it is not primarily located in the lungs.