Siu et al have observed a rare potential adverse effect of dronedarone in patients who have had previous exposure to amiodarone. Dronedarone is an antiarrhythmic agent that can be useful in some patients with nonpermanent atrial fibrillation who have not had recent heart failure. Many clinicians, including cardiologists, are currently using this drug in specific patient populations, including young individuals, at least initially, in preference to long-term oral therapy with amiodarone, a more highly toxic agent. Nonetheless, it is clinical observations like these, and not randomized controlled studies, which can (and often do) serve the purpose of alerting us to new and important consequences of our therapies. The implied warning provided by this report in these 2 patients who have had prior exposure to amiodarone should be taken seriously, and newly described adverse effects should be actively sought and reported by physicians using newly developed approved drugs of all types.