first-pass metabolism
Initial metabolism of an orally administered drug by the liver before reaching systemic circulation.
Full Definition
First-pass metabolism, also called first-pass effect, refers to the metabolic process where orally administered drugs are absorbed from the gastrointestinal tract and transported directly to the liver via the portal circulation before entering the systemic bloodstream. During this hepatic transit, enzymes may significantly reduce the amount of active drug that reaches systemic circulation. This phenomenon is particularly important in psychopharmacology for drugs with high hepatic extraction ratios, such as some tricyclic antidepressants and antipsychotics. First-pass metabolism can be bypassed using alternative routes like sublingual, transdermal, or injectable administration.
Usage
Usage note: Always hyphenate; ensure distinction from general hepatic metabolism in pharmacokinetic discussions.
In Context
- "The drug's poor oral bioavailability is due to extensive first-pass metabolism, necessitating higher oral doses." — Pharmacokinetic study report
- "Sublingual administration bypasses first-pass metabolism, providing more predictable drug levels." — Dosing guideline