Medication errors represent a significant source of preventable patient harm. Prior research has shown that medication errors occur frequently and are associated with a longer hospital stay and increased costs. This report from the Policy Research Unit in Economic Evaluation of Health and Care Interventions synthesizes the evidence regarding the burden of medication errors in the England. The authors estimate that 237 million medication errors occur annually and that 66 million of these errors may be clinically significant. The majority of potentially harmful errors likely occur in the outpatient setting where most medications in the National Health Service are prescribed. Costs associated with errors seem to vary widely. A prior WebM&M commentary described a case in which a medication error led to serious patient harm.