What is meant by first-pass metabolism?

First-pass metabolism refers to the process by which the concentration of a drug is significantly reduced before it reaches systemic circulation. When a drug is administered orally, it is first absorbed from the gastrointestinal tract and transported via the portal vein to the liver, where it undergoes metabolic transformation. This metabolic process can substantially decrease the amount of active drug that enters the bloodstream, affecting the overall bioavailability of the medication.

The liver is a primary site for drug metabolism, where enzymes can modify, convert, or deactivate drugs. As a result, some medications may have limited efficacy when taken orally due to extensive first-pass metabolism, necessitating higher doses or alternative routes of administration, such as intravenous delivery, to achieve therapeutic effects. This phenomenon highlights the importance of understanding a drug’s pharmacokinetics to ensure effective dosing and utilization.