The proarrhythmia fenomenon – an induction of new arrhythmia or worsening of preexisting arrhythmia – is well known in antiarrhythmic drugs for many years. In majority of cases the proarrhythmia presentation consists of QT interval prolongation and ventricular tachycardia torsade des pointes, which can be even cause of death. Recently it has become apparent that proarrhythmia can occur during treatment with a long list of noncardiovascular drugs. Incidence of the phenomenon has not been quantified, the mechanism remains unclear. A factor of particular importance is a predisposition of individual patient based on variations in function of cardiac ion channels and drug - metabolizing enzymes. Risk factors of proarrhythmia are rewieved and possibilities for prevention are proposed.