Adverse drug reactions (ADRs) are a significant cause of morbidity and mortality. The majority of ADRs can be considered common disorders with considerable clinical variability (clinical phenotype) in which many different genes are involved together with environmental variables. Pharmacogenomics is the study of how genes affect the individual response to drugs. There is some evidence that in the future the use of pharmacogenomics could help to reduce ADRs, as it aims to predict which patients are likely to respond to a particular drug and which patients are likely to have significant ADRs. In this article some examples of genetic polymorphisms affecting drug kinetics, drug toxicity and hypersensitivity related to ADRs are illustrated.