Pharmacokinetics is the study of the movement of a drug through the compartments of the body and the transformations (activation and metabolism) that affect it. Pharmacokinetics is often referred to as ‘what the body does to the drug’. Figure PK1 shows an overview of these processes.

Drug administration

Drugs are administered by many different routes. The aim of drug administration is to achieve therapeutic levels of the drug at its site (or sites) of action. In general, this is achieved using the vascular compartment as the transport mechanism for redistribution. Drug administration is, therefore, designed to produce suitable drug levels within the blood. The choice of route for a particular drug takes into account physical properties, target site of action, consideration of possible toxic effects and the practicalities of administration. The routes are summarised in Figure PK2 in a practical classification. The enteral and topical routes are most easily accessible, but require absorption across a barrier or membrane to establish their effect.

Absorption

Absorption is the process of taking the drug from the site of administration to the blood. This is necessary for all enteral and parenteral routes except for IV administration. Systemic absorption may occur from topically administered drugs, but this is not the intended route. Absorption involves the crossing of barriers between administration site and vascular compartment with subsequent movement across the physical distance between the two. The distance within any local compartment is traversed by simple passive diffusion down the concentration gradient.