Bioavailability: Overview

Bioavailability is the proportion of an administered drug that enters the systemic circulation and becomes available for distribution to its site of action. It determines how much of a drug dose actually reaches the bloodstream to produce therapeutic effects. Understanding bioavailability is essential for predicting drug efficacy and establishing appropriate dosing regimens.

Intravenous administration delivers the drug directly into the systemic circulation, bypassing absorption barriers. Since the entire dose immediately enters the bloodstream without loss or metabolism before reaching circulation, its bioavailability is defined as 100% by convention, serving as the reference standard for comparing other routes.

Oral bioavailability is reduced by two primary factors: gastrointestinal absorption, which determines how much drug crosses the GI barrier, and liver biotransformation, which metabolizes the drug before it reaches systemic circulation. These processes collectively limit the amount of active drug available in the bloodstream compared to intravenous administration.

Bioavailability is determined by comparing plasma concentration-time curves for the same drug given orally and intravenously at equivalent doses. The area under each curve represents drug absorption for that route. The ratio of the oral area to the intravenous area yields the bioavailable fraction, expressed as a percentage of the intravenous dose.

The area under the plasma concentration-time curve represents the total extent of drug absorption for a given route of administration. A larger area indicates greater drug absorption and systemic exposure. Comparing areas between oral and intravenous routes allows quantification of how much drug actually becomes bioavailable through each administration method.

Intravenous administration achieves 100% bioavailability immediately, while oral administration typically yields lower bioavailability due to incomplete gastrointestinal absorption and hepatic metabolism. Plotting plasma concentration versus time for both routes reveals these differences: intravenous shows rapid peak concentration, while oral shows delayed absorption and lower peak levels.

Bioavailability comparison ensures drugs are formulated and dosed appropriately to achieve therapeutic effects. By quantifying how much drug reaches systemic circulation via different routes, developers can optimize formulations, predict clinical outcomes, and establish rational dosing strategies that account for absorption and metabolism losses.