Medication errors represent a significant source of preventable harm. This large meta-analysis, including 81 studies, found that approximately 1 in 30 patients is exposed to preventable medication harm, and more than one-quarter of this harm is considered severe or life-threatening. Preventable medication harm occurred most frequently during medication prescribing and monitoring. The highest rates of preventable medication harm were seen in elderly patient care settings, intensive care, highly specialized or surgical care, and emergency medicine.