Genomics has helped to initiate the era of precision medicine, with some drugs now prescribed on the basis of molecular genetic tests that indicate which patients are likely to respond or should not receive a drug because of a high risk of adverse effects. However, for precision medicine to realize its potential, the patient's history, environment, and lifestyle must also be taken into account. Improving precision medicine requires a better understanding of the underlying reasons for the variability in drug response so as to better identify which drug or combination of drugs is likely to be most effective for an individual patient, along with consideration of the optimal dose or doses for that patient. Greater individualization of dose will be an important means to achieve more precise medicine and mitigate significant variability in drug response. Achieving this will require changes in how drugs are developed, approved, prescribed, monitored, and paid for. Each of these factors is discussed in this review.