There is much evidence of a variation in the response to psychotropic medications across different ethnic groups, and there are many explanations for this phenomenon. In general, these explanations span three intersecting domains.

The first domain is that related to genetics. Certain patterns of genetic polymorphisms are more or less prevalent in particular ethnic groups. These polymorphisms are known to cause variations in the biological processes underlying the pharmacological actions of psychotropic drugs. The processes of drug metabolism are the primary research area. The second domain relates to the interaction between human biology and the wider environment. For example, enzymes that operate within metabolic pathways can be modulated by factors such as diet, alcohol usage, nicotine, illicit drug use and the use of alternative/complementary therapies. The third domain relates to variations in the expression of mental illness across cultures and ethnicities. This can lead to misdiagnosis, mistreatment and a variation in perceived treatment response. These three domains do not provide a complete explanatory model, but they go some way towards an understanding of the biopsychosociocultural context of psychopharmacotherapy.

This chapter will concentrate on the first domain, that of genetics. I shall start by introducing three areas of scientific endeavour: pharmacogenetics, pharmacokinetics and pharmacodynamics. These three disciplines, taken together in the context of culture and ethnicity, describe the developing field of ethnopsychopharmacology. Pharmacogenetics is the investigation of the genetic factors that influence an individual's reaction to a drug (Arranz et al, 2007). Pharmacokinetics is the determination of what happens to the drug in the body; this includes its absorption, distribution, metabolism and excretion. Pharmacodynamics is the study of the biochemical and physiological processes that form the basis for the pharmacological action of a drug, which includes the desired therapeutic effects and the undesired side-effects. The processes involved are complex and often incompletely understood.

Most pharmacogenetic and pharmacokinetic research has focused on the genes encoding the drug-metabolising enzymes. Nearly all psychotropic drugs are metabolised by the cytochrome P450 (CYP) enzymes in the liver (notable exceptions include lithium and lorazepam). The enzymes primarily responsible for psychotropic drug metabolism include CYP1A2, CYP2C19, CYP2D6 and CYP3A4.