“…The lower limit of the recommended therapeutic range for serum flecainide concentration, which remains somewhat controversial, varies from 200 to 400 ng/mL for suppression of ventricular arrhythmia [16,18,20]. We previously reported that the serum flecainide concentration was recommended to maintain ≥300 ng/mL in patients with supraventricular tachyarrhythmias [21][22][23]. However, according to a previous 5 report, some patients showed sufficient effectiveness of flecainide at concentrations <300 ng/mL [21].…”