Background: Flecainide, a class IC antiarrhythmic drug, is used frequently in patients with supraventricular arrhythmias, mostly atrial fibrillation and less commonly for ventricular arrhythmias. With its strong affinity to sodium channels in the cardiac cells it affects phase 0 of the action potential leading to conduction slowing which is reflected in widening of the QRS and it can also prolong the PR interval. (1) These unspecific electrocardiogram (ECG) changes may mimic myocardial ischemia in patients undergoing myocardial stress testing. The current case report reflects the diagnostic difficulties arising from this phenomenon. Case summary: A 66-year-old patient who had been treated with flecainide for a decade for atrial fibrillation showed significant QRS widening and ST segment depression during exercise testing. The patient did not exhibit symptoms of myocardial ischemia. A subsequent single photon emission computed tomography (SPECT) revealed a 10-15 % cardiac ischemic area. Invasive coronary angiogram showed a chronic, complete obstruction of the left circumflex artery, which was percutaneously revascularized. Discussion: This case demonstrates how pharmacological effects of flecainide and other class IC antiarrhythmic drugs may lead to suspicion of cardiac ischemia due to QRS widening and unspecific ST-segment changes. In these patients, accurate ECG interpretation is important to avoid unnecessary invasive procedures like coronary angiography. On the other hand, significant underlying coronary artery disease may nonetheless be present, warranting invasive diagnostic testing. Non-invasive cardiac imaging may facilitate the process of distinguishing real ischemia from flecainide induced ECG changes.