Which organ is primarily responsible for first-pass metabolism?

• A. The kidney metabolizes many drugs before they reach systemic circulation.
• B. The Liver metabolizes many drugs before they reach systemic circulation.
• C. The Lungs metabolize many drugs before they reach systemic circulation.
• D. The Brain metabolizes many drugs before they reach systemic circulation.

Answer: (B)

First-pass metabolism is the process where an oral drug is metabolized by the liver before it ever reaches the rest of the body. The liver is the main site where this initial metabolism happens, largely due to its rich supply of drug-metabolizing enzymes such as the cytochrome P450 system. This hepatic processing can significantly reduce the amount of active drug that enters systemic circulation, which is why oral bioavailability can be low for some medications. While the kidney, lungs, and brain can participate in drug metabolism to some extent, they do not serve as the primary gatekeeper for drugs absorbed from the gut. Clinically, high first-pass metabolism means the oral dose may need to be larger or a different route of administration might be chosen to achieve therapeutic levels.