Arrhythmogenic effects appear in 5-22% of all patients. On the basis of a literary investigation on 11,547 patients with 1,114 proarrhythmic effects and of 21 own observations mechanisms, risks and diagnostics of this dangerous side effect are demonstrated. In most cases patients with severe functional disturbances of the heart and known ventricular tachyarrhythmias are affected. Not infrequently a prolonged QT-internal is found. An accumulation of the known arrhythmias or an appearance of new disturbed rhythms, frequently Torsade de pointes, may occur. Holter-ECG and programmed stimulation are suited for recognition, which are together to be applied in malignant tachyarrhythmias. The prophylaxis deals with a careful indication, aimed selection of the antiarrhythmic drugs and a subtile control of therapy in the first days after the beginning of the therapy and after every increase of dosage.