Drugs that are described as antiarrhythmic drugs may actually aggravate arrhythmia in several ways and these are termed proarrhythmic effects. The most common type of proarrhythmia is a paradoxical increase in the frequency of episodes of the target arrhythmia. This type of effect had not been suspected until recently and has not been widely publicized. It is a phenomenon common to all antiarrhythmic drugs when they are used to treat arrhythmias based on a re-entrant mechanism (the most common mechanism of clinical arrhythmias). Different drugs vary in their tendency to produce this type of proarrhythmic response. These differences are explicable in terms of the relative effects of the drugs on refractoriness and conduction times in the re-entrant circuit. Proarrhythmic effects are most important in the treatment of ventricular tachycardias because recurrences are often fatal. Proarrhythmic effects on ventricular tachycardia can now be predicted at electrophysiological study before commencement of long-term therapy, and potentially dangerous treatment can be avoided. The key to proper treatment to proarrhythmia is to recognize that it is a drug-induced problem and to withdraw the offending drug.