Bioavailability and Half-Life
Start with the big picture
Absolute bioavailability compares exposure after an extravascular dose with exposure after intravenous administration; relative bioavailability compares non-IV formulations. Oral bioavailability may be reduced by incomplete absorption, first-pass metabolism, efflux transport, drug instability, or interactions. Extraction ratio helps explain how strongly first-pass effects influence some drugs. Bioequivalence assessment uses confidence intervals for AUC and Cmax against a reference product. For first-order elimination, half-life is constant and depends on volume of distribution and clearance; it is not constant when elimination becomes zero-order. Half-life informs dosing intervals, time to steady state, and washout, while physiologic or disease-related changes in distribution or clearance can alter it. In multicompartment kinetics, the clinically relevant half-life refers to the terminal elimination phase.
What you'll learn
- Distinguish absolute from relative bioavailability using their exposure comparisons.
- Identify processes that can reduce oral bioavailability.
- Explain how extraction ratio relates to first-pass loss and bioavailability.
- Relate first-order half-life to volume of distribution and clearance.
- Describe how half-life informs steady state, washout, and dosing considerations.
Continue your study
Work through the complete notes and reinforce the topic with the study tools available in the full lesson.
