What is 'first-pass metabolism'?

First-pass metabolism refers to the initial processing of a drug in the liver before it reaches systemic circulation. When a drug is administered orally, it is absorbed from the gastrointestinal tract and enters the portal circulation, which leads to the liver. Here, enzymes metabolize the drug, often reducing its concentration before it enters the bloodstream. This process can significantly affect the bioavailability of the drug, which is the portion that actually reaches systemic circulation in an active form.

Understanding this concept is essential, especially in pharmacology, as it influences dosing and efficacy of oral medications. Some drugs might be heavily metabolized by the liver, leading to a reduced therapeutic effect, while others might be minimally affected and maintain their desired activity levels. First-pass metabolism is a crucial factor that helps in the design of drug regimens and informs healthcare providers about potential drug interactions and optimal routes of administration.