Basic pharmacokinetic principles and their application to psychotropic drugs.

The application of pharmacokinetic principles to the administration of psychotropic medications provides a rational approach to understanding factors influencing the time course and intensity of drug action. When plasma concentrations of a given medication can be linked to levels at receptors in the brain, the pharmacodynamic effects of that drug can then be predicted. The pharmacokinetic parameter of most interest to clinicians is clearance, which describes the rate of drug removal per unit of plasma concentration. For any given drug, this parameter varies widely from person to person and may be markedly altered both in elderly patients and in patients with either kidney or liver disease. The clearance of a given medication can also be either increased or reduced by coadministration of other drugs. Such drug:drug interactions can potentially result in either reduced therapeutic effect or toxicity. Elimination half-life is determined by clearance and by another pharmacokinetic parameter, volume of distribution. Elimination half-life will be related to both the time necessary for plasma concentrations to reach steady state with repeated dosing and for the drug to be washed out after it is discontinued.