Adverse drug events – many of which are preventable – are a major source of patient harm. This cross-sectional study explored the causes of preventable adverse drug events (ADEs) and how patients, families, and providers can prevent them. The study finds that the most common causes of preventable ADEs involved providers not ensuring that patients and/or family understood the medication plan or could identify symptoms of side effects. The researchers used the study findings to develop learning messages for providers, patients, and families and to implement a preventable ADE surveillance system.