Bioavailability

Bioavailability is the fraction of an administered drug dose that reaches systemic circulation in an active form. It depends on the route of administration and losses during absorption and first-pass metabolism.

Absorption (pharmacokinetics): The movement of a drug from its administration site into the bloodstream. Absorption is the entry step that sets how much of a dose can become bioavailable.
Systemic circulation: The blood flow that carries blood between the heart and the body's tissues, excluding pulmonary circulation. The endpoint in the definition is entry into this circulation.
Absolute bioavailability: The systemic availability of a nonintravenous dose relative to an intravenous dose of the same drug. It uses intravenous administration as the reference for estimating the fraction reaching circulation.
Bioavailability versus bioequivalence: The distinction between a drug's systemic availability and the regulatory comparison of exposure between drug products. Bioavailability describes an extent; bioequivalence judges whether two products have sufficiently similar exposure.
First-pass effect: The presystemic metabolism of a drug, especially in the intestinal wall and liver, before it reaches systemic circulation. It can remove a substantial fraction of an orally absorbed dose before that dose enters circulation.
Route of administration: The path by which a drug enters the body, such as oral, intravenous, or inhaled administration. The route changes the barriers and presystemic losses that determine bioavailability.
Relative bioavailability: The systemic exposure from one drug formulation or route compared with another, usually at matched doses. It is used to compare formulations when an intravenous reference is unavailable or unnecessary.
Bioavailability versus bioaccessibility: The distinction between a substance reaching systemic circulation and becoming available for absorption from its source. A drug may be released from a formulation yet still fail to reach circulation.
Drug metabolism: The chemical transformation of drugs by enzymes, often producing metabolites that are easier to eliminate. Metabolism can inactivate drug before systemic entry or convert a prodrug into its active form.
Intravenous therapy: The administration of a substance directly into a vein. Intravenous dosing bypasses absorption barriers and is conventionally assigned complete bioavailability.

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