It is estimated that over three-quarters of a million people are injured or die in hospitals each year from adverse drug events. The majority of medical errors result from poorly designed health care systems rather than from negligence on the part of health care providers. While there is considerable evidence that information technology can be used to significantly reduce medication errors and adverse events, information technology, to be effective, must be implemented using a systems approach. This paper reviews three studies that have used a systems approach to investigate the causes of medication errors and the effectiveness of information technology in preventing adverse drug events. Significant reduction of medication errors and adverse drug events requires systemic implementation of information technology, improvements in the reporting of errors, and integration with other systems designed to detect and prevent errors.