Pharmacogenetics is the study of inherited genetic differences in drug metabolic pathways which can affect individual responses to drugs, both in terms of therapeutic effect as well as adverse effects.

Individual variability in drug efficacy and drug safety is a major challenge in current clinical practice, drug development, and drug regulation. For more than 5 decades, studies of pharmacogenetics have provided ample examples of causal relations between genotypes and drug response to account for phenotypic variations of clinical importance in drug therapy. The convergence of pharmacogenetics and human genomics in recent years has dramatically accelerated the discovery of new genetic variations that potentially underlie variability in drug response, giving birth to pharmacogenomics. Read ABSRACT

VLA comment:  10 % of Caucasians, for example, are unable to metabolize these modern drugs.  Hence, the epidemic of suicides and homicides.  Early clinical examples of genetic influence on drug response involved variations in single genes (i.e., monogenic inheritance) in which polymorphisms (no metabolic ability) of a single gene encoding a drug-metabolizing enzyme responsible for the metabolism and disposition of a substrate drug caused aberrant response to the drug. The phenotypic variation can be dramatic, especially when no alternate pathway exists to perform the same function.