Enzymes that metabolise foreign compounds exhibit a large degree of interindividual variability in their levels of expression. In a number of instances this variability can be accounted for by null or variant alleles resulting from mutations in genes encoding these enzymes. Human variability in drug metabolism can be determined by biochemical and pharmacological assays. In cases where a genetic change has been characterised, polymerase chain reaction techniques have been developed to diagnose metabolism deficiencies. Genetic differences in certain foreign compound metabolising enzymes such as glutathione S-transferase M1, N-acetyltransferase 2 and CYP2D6 have been shown to be associated with risk for developing environmentally and occupationally based diseases such as cancer. Drug therapy can also be compromised by the existence of genetic deficiencies in a number of enzymes, including CYP2D6. It is anticipated that determination of an individual's drug metabolism capabilities by use of phenotyping and genotyping tests will allow for more rational and safe drug administration protocols.