Flecainide is a class 1C antiarrhythmic agent that blocks cardiac sodium channels, slowing conduction velocity and stabilising abnormal heart rhythms. It is primarily used to treat supraventricular arrhythmias such as atrial fibrillation and flutter, where restoring and maintaining normal rhythm is critical. However, flecainide can exacerbate ventricular arrhythmias in patients with structural heart disease, making careful patient selection essential.