Pharmacokinetics of psychotropic drugs in special populations.

Patients treated for psychiatric illness may have a variety of characteristics that alter the pharmacokinetic profiles of psychotropic drugs. Genetic background, age, health status, and personal habits can change the body's ability to absorb, distribute, and metabolize medications. Most psychotropic drugs, with the exception of lithium, are eliminated via biotransformation in the liver rather than renal excretion, and a number of studies have demonstrated distinct phenotypes for hepatic metabolism involving the cytochrome P450 system. "Slow" metabolizers are likely to develop higher plasma concentrations of several different classes of psychotropic drugs, including tricyclic antidepressants. Advancing age reduces renal function but has little effect on hepatic metabolism. Volume of distribution may also be increased in elderly patients because of their greater percentage of adipose tissue. Ethnic background can significantly influence both drug metabolism and the pharmacodynamics of a variety of drugs. Thus, the physician should carefully consider patient characteristics when prescribing psychotropic medications and should engage in therapeutic drug monitoring when there is any doubt about the plasma drug levels that a given dosing regimen will achieve.