Clinical decision support systems (CDSS) are being applied widely in patient safety, most frequently to provide alerts intended to prevent medication errors. The utility of such warnings is limited by alert fatigue—clinicians' tendency to ignore repeated alerts. This study of an alert within an electronic medical record designed to encourage participation in a clinical trial is relevant for CDSS designers, as it quantifies the degree of alert fatigue. The study found that response rates to the alert declined consistently over time in response to increased exposure to the alert. A recent commentary called for CDSS to be tailored to maximize safety outcomes while minimizing alert fatigue.