As the growth of the elderly population continues, the burden on the health care system and society will also increase. Since chronic diseases such as hypertension, coronary artery disease, arthritis, stroke, cancer and diabetes mellitus are more prevalent with age, the number of people with multiple chronic diseases will also increase. These patients are likely to be treated for some or all of their conditions with drug therapies. When used appropriately, drugs may be the single most important intervention in the care of an older patient, but when used inappropriately they no longer provide therapeutic benefit, and they may even endanger the health of an older patient by causing an adverse drug reaction (ADR). Factors believed to be responsible for increased adverse reactions in elderly patients are polypharmacy (including prescription and over-the-counter medications), increased drug-drug interaction, pharmacokinetic changes, pharmacodynamic changes, the pathology of aging and compliance. The exact role that age plays in ADRs is not clear. This is in part because few older patients are included in the large randomised trials, and so much of the information used to ascertain the age-associated risks of drugs comes from observational studies. Although the interactions of aging, concurrent comorbidities and polypharmacy are known, older patients do appear to be at increased risk. Improvements in the management of drug therapies of older patients can lead to improvements in their overall health, functioning and safety, as well as providing potential benefits to society by ameliorating some of the burden of their health care.