Flecainide and propafenone are class IC antiarrhythmic drugs. Significant coronary artery disease is a contraindication to the use of class IC antiarrhythmics as this increases the risk of proarrhythmia and sudden cardiac death.

These drugs may be proarrhythmic in the setting of left ventricular hypertrophy (wall thickness > 1.4 cm) and are contraindicated in this setting.

These agents must be used in combination with an AV blocking agent in order to prevent rapid atrial fibrillation or atrial flutter conduction (1:1 conduction) through the AV node resulting in very fast ventricular rates if a breakthrough episode occurs since class IC drugs also act to increase AV nodal conduction.