Adverse drug reactions (ADRs) are common, are associated with morbidity and mortality, and are costly to healthcare systems. Genomic factors predispose to ADRs, but these vary depending on the drug, patient, and disease. Genomic testing can not only help to predict and prevent ADRs but can also be used in other ways (diagnosis, closer monitoring of those at risk, pre-emptive genotyping, and understanding of mechanism), all of which will be important in the future to improve the benefit-risk ratio of drugs. In the era of precision medicine, such genomic data will need to be integrated with other forms of data to develop a comprehensive and integrated approach to improve responses to medicines used in patients.