Absorption and Bioavailability

Absorption is the process by which a drug moves from its site of administration into the systemic circulation, and bioavailability is the fraction of an administered dose that reaches the systemic circulation in unchanged form. For a drug given by any route other than intravenous, bioavailability is less than complete, and understanding why is central to interpreting how a dose becomes an exposure.

Definition

Bioavailability (F) is the rate and extent to which the active drug is absorbed and becomes available at the site of action, conventionally measured as the fraction of an extravascular dose reaching the systemic circulation relative to an intravenous dose, estimated from the area under the concentration-time curve.

Scope

This topic covers the mechanisms and barriers of drug absorption, the definition and estimation of bioavailability, the first-pass effect, and the physicochemical determinants captured by classification frameworks such as the Biopharmaceutics Classification System. It is a reference and educational entry, not a guide to product selection or dosing.

Core questions

• By what mechanisms does a drug cross absorptive membranes into the circulation?
• What determines the fraction of a dose that reaches the systemic circulation intact?
• How is bioavailability quantified and compared between formulations?
• How do solubility and permeability shape oral absorption?

Key concepts

• Passive diffusion and carrier-mediated transport
• Rate and extent of absorption
• First-pass (presystemic) metabolism
• Area under the concentration-time curve (AUC)
• Absolute and relative bioavailability
• Solubility and permeability
• Bioequivalence

Key theories

Biopharmaceutics Classification System (BCS)

Amidon and colleagues proposed classifying drugs by aqueous solubility and intestinal permeability, predicting when in vitro dissolution can stand in for in vivo bioavailability and clarifying the rate-limiting steps in oral absorption.

Mechanisms

Most drugs are absorbed across the gastrointestinal epithelium by passive diffusion of the un-ionized, lipid-soluble form, supplemented by carrier-mediated transport for some compounds. The extent reaching the systemic circulation depends on the fraction absorbed across the gut wall and the fraction surviving presystemic (first-pass) metabolism in the gut wall and liver. Solubility and permeability set the rate-limiting step for oral drugs, as formalized in the Biopharmaceutics Classification System; bioavailability is then estimated from the area under the concentration-time curve relative to an intravenous reference.

Clinical relevance

Bioavailability explains why the same dose can produce different exposures by different routes or formulations and underlies the concept of bioequivalence used when comparing products. This entry explains these principles for educational reference and does not provide recommendations on product choice or dosing.

Evidence & guidelines

Regulatory agencies issue technical guidance on bioavailability and bioequivalence studies and on biopharmaceutics-based biowaivers; the underlying science is summarized in standard pharmacokinetics texts.

History

The quantitative concept of bioavailability developed alongside the recognition that formulation could affect drug exposure. The Biopharmaceutics Classification System, introduced by Amidon and colleagues in 1995, gave the field a mechanistic basis for relating in vitro dissolution to in vivo absorption and shaped subsequent regulatory thinking on bioequivalence.

Key figures

• Gordon Amidon
• Hans Lennernäs
• Malcolm Rowland
• Thomas Tozer

Seminal works

• amidon-1995

Frequently asked questions

Why is intravenous bioavailability considered 100%?

An intravenous dose enters the systemic circulation directly without crossing an absorptive barrier or undergoing first-pass metabolism, so by definition the entire dose is available; other routes are compared against it.

What is the first-pass effect?

It is the metabolism of an orally absorbed drug in the gut wall and liver before it reaches the systemic circulation, which can substantially reduce bioavailability for some drugs.