Adverse drug events (ADEs) are common in the long-term care setting, in part because many residents are prescribed high-risk medications such as sedatives and anticoagulants. This cluster-randomized trial evaluated the effectiveness of a computerized provider order entry (CPOE) system at preventing ADEs in two long-term care facilities. The system was largely ineffective at preventing errors, with no significant difference in ADE incidence between intervention and control units. Limitations of the CPOE system likely contributed to the null result, as the system created many unnecessary alerts and had only limited decision support capabilities. An AHRQ WebM&M commentary discusses a case of a medication error associated with warfarin use at a skilled nursing facility.