What does the first-pass effect refer to?

The first-pass effect specifically pertains to the initial metabolism of a drug before it enters systemic circulation. When a drug is administered orally, it is absorbed from the gastrointestinal tract and transported to the liver via the portal vein. During this process, the liver metabolizes a portion of the drug, which can significantly reduce the amount of active compound that reaches the systemic circulation. This phenomenon is crucial for understanding how orally administered medications can have different bioavailability, which can impact their efficacy.

Drugs that undergo extensive first-pass metabolism may require higher doses to achieve the desired therapeutic effect compared to those that are less affected by this process, leading to important considerations in drug dosing and route of administration.