What is "bioavailability"

n. (context pharmacology English) The amount of drug which reaches the site of physiological activity after administration.

In pharmacology, bioavailability (BA) is a subcategory of absorption and is the fraction of an administered dose of unchanged drug that reaches the systemic circulation, one of the principal pharmacokinetic properties of drugs. By definition, when a medication is administered intravenously, its bioavailability is 100%. However, when a medication is administered via other routes (such as orally), its bioavailability generally decreases (due to incomplete absorption and first-pass metabolism) or may vary from patient to patient. Bioavailability is one of the essential tools in pharmacokinetics, as bioavailability must be considered when calculating dosages for non-intravenous routes of administration.

For dietary supplements, herbs and other nutrients in which the route of administration is nearly always oral, bioavailability generally designates simply the quantity or fraction of the ingested dose that is absorbed.

Bioavailability is defined slightly differently for drugs as opposed to dietary supplements primarily due to the method of administration and Food and Drug Administration regulations.

Bioaccessibility is a concept related to bioavailability in the context of biodegradation and environmental pollution. A molecule (often a persistent organic pollutant) is said to be bioaccessible when "[it] is available to cross an organism's cellular membrane from the environment, if the organism has access to the chemical."

Bioavailability, in environmental and soil sciences, represents the amount of an element or compound that is accessible to an organism for uptake or adsorption across its cellular membrane. In environmental and agricultural applications, bioavailability most often refers to availability of contaminants, such as organic pollutants or heavy metals, in soil systems and is also used frequently in determining potential risk of land application of sewage sludge or other inorganic/organic waste materials.

Almost exclusively, plant roots and soil organisms uptake contaminants that are dissolved in water. Therefore, the bioavailable fraction is often likened to the dissolved (aqueous) fraction in these systems. Depending on its chemical properties, a contaminant may or may not be found in the aqueous phase. Organic contaminants may become sorbed or sequestered in organic matter through weak Van der Waals interactions or through hydrogen- or covalent bonding. Ionic compounds, such as heavy metals, can be precipitated into the solid phase. Volatile compounds can be lost as aerosols to the soil atmosphere. In these forms, contaminants are relatively inaccessible to microbial or plant uptake and must dissociate and re-dissolve into the soil solution to become biological available.