The CancelRx tool is designed to improve communication between electronic health record (EHR) systems and pharmacy dispensing software. However, interoperability issues can limit the tool’s usefulness and result in inadvertent dispensing of discontinued medications. This evaluation of discontinued medications at one health systems over a one-month period found that only one-third to one-half of discontinued medications were e-prescribed using the same EHR system and would result in a CancelRx message to the pharmacy; the remainder of discontinued medications were patient-reported or reconciled from outside sources.