Which is responsible for the lower bioavailability seen in oral drugs?

A. First-Pass Effect: The first-pass effect refers to the metabolism of a drug by the liver immediately after it is absorbed from the gastrointestinal tract and before it reaches systemic circulation. Oral medications pass through the portal vein to the liver, where a portion may be metabolized into inactive compounds, reducing the amount of active drug available in the bloodstream. This phenomenon decreases oral bioavailability compared to other routes. B. Distribution: Distribution describes the movement of a drug from the bloodstream into tissues and organs. While it affects the drug’s concentration at target sites, it does not explain the reduced bioavailability of orally administered medications. Distribution occurs after the drug has already entered systemic circulation. C. Peripherally Inserted Central Catheter: A PICC line is an intravenous access device used to administer medications, fluids, or nutrition. It does not influence the pharmacokinetics of orally administered drugs or their bioavailability. D. Half-Life: The half-life of a drug is the time required for its plasma concentration to decrease by 50%. Although half-life affects dosing intervals and accumulation, it does not directly determine the proportion of drug that reaches systemic circulation after oral administration.