bioavailability

Bioavailability is a pharmacokinetic concept defined as the proportion of an administered dose that reaches the systemic circulation in an unchanged form. It is a measure of the rate and extent to which the active compound becomes available at the site of action. For intravenous administration, bioavailability is defined as 1 (100%). For other routes, it is less than 1 due to incomplete absorption and presystemic (first-pass) metabolism in the liver or gut wall.

Key factors affecting bioavailability include formulation and physicochemical properties such as solubility, permeability, and chemical stability; the route of administration and dosage; and physiological conditions like gastric emptying, intestinal transit time, regional blood flow, and hepatic first-pass metabolism. Food can alter absorption, either increasing or decreasing it, and various drug interactions, disease states (for example, gastroparesis or inflammatory bowel disease), age, and genetic differences in metabolism can also influence the amount of drug that reaches the systemic circulation.

Measurement is commonly quantified by comparing the area under the plasma concentration-time curve (AUC) after non-intravenous administration to the AUC after intravenous administration: F = (AUC_nonIV / AUC_IV) × (Dose_IV / Dose_nonIV). This ratio reflects the fraction of the dose that becomes systemically available.

Applications include assessing different routes and formulations (bioequivalence), informing dosing regimens, and supporting generic drug approval and pharmacokinetic modeling. Some drugs have inherently low oral bioavailability due to poor solubility, poor permeability, or extensive first-pass metabolism. In nutrition, bioavailability can also refer to the fraction of a nutrient absorbed and utilized by the body.