Despite the efficacy of implantable cardioverter-defibrillators in preventing sudden death associated with ventricular arrhythmias, 40% to 70% of patients with an implantable cardioverter-defibrillator receive adjunctive antiarrhythmic drug therapy. The most common aims of drug therapy are to reduce the frequency of ventricular tachycardia and fibrillation triggering shock, to alter the tachycardia characteristics in order to enhance the efficacy of antitachycardia pacing, and to suppress supraventricular tachyarrhythmias. Antiarrhythmic drugs may, however, interfere with implantable cardioverter-defibrillator function by raising defibrillation and pacing thresholds. Furthermore, drugs may adversely affect the sensing of ventricular tachycardia and fibrillation by the implantable cardioverter defribillator. Careful prescription and follow-up electrophysiologic testing is critical in ensuring a cooperative effect of drug and implantable cardiovascular-defibrillator after initiation of new therapy.