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First-Pass Metabolism

Meaning

First-Pass Metabolism, also known as pre-systemic metabolism, is the phenomenon where the concentration of a drug or orally administered substance is significantly reduced before it reaches the systemic circulation. This reduction occurs primarily in the liver, but also involves the gut wall and gut flora, immediately following absorption from the gastrointestinal tract. It is a critical pharmacokinetic factor that dictates the bioavailability and effective dosing of oral hormonal therapies and supplements.