Pharmacokinetics: Principles of How the Body Processes Drugs

Core processes (ADME)

The main pharmacokinetic processes are often summarized by the acronym ADME:

• Absorption — movement of a drug from its site of administration into the bloodstream. Route (oral, intravenous, inhaled, transdermal) and formulation influence the rate and extent of absorption.
• Distribution — transfer of the drug between blood and tissues; influenced by blood flow, tissue permeability, plasma protein binding and the drug's chemical properties.
• Metabolism — biochemical modification, primarily by liver enzymes, that can activate, deactivate or prepare compounds for elimination.
• Excretion — removal of drug or metabolites from the body, mainly via urine or bile, but also by expired air, sweat or feces.

Key parameters and models

Pharmacokinetics uses measurable parameters to describe drug handling. Important terms include half-life (time for concentration to fall by half), clearance (volume of plasma cleared of drug per unit time), bioavailability (fraction of dose reaching systemic circulation) and volume of distribution (a conceptual volume relating amount of drug to concentration). Models range from simple one-compartment descriptions to multi-compartment or physiologically based models that reflect organ-specific kinetics.

History and development

The field matured in the 20th century as analytical chemistry and clinical pharmacology advanced. Early discoveries about drug metabolism and enzyme systems led to modern dosing principles, therapeutic drug monitoring and regulatory requirements for pharmacokinetic studies during drug development.

Clinical importance and distinctions

Pharmacokinetic knowledge guides dose selection, interval timing, route choice and adjustments for age, organ function or drug interactions. It explains why two drugs with similar effects may require different dosing schedules and why therapeutic monitoring is essential for narrow therapeutic index agents. Unlike pharmacodynamics, which explains the relationship between concentration and effect, pharmacokinetics explains concentration over time — together they determine clinical response.

For further reading on foundational concepts and methodology see general resources in pharmacology and drug science: drug concepts, substance disposition.