Fareston must not be used in patients with ‘ QT prolongation’ (a disruption of the electrical activity of the heart), electrolyte disturbances (altered levels of salts in the blood) especially hypokalaemia (low potassium levels), bradycardia (a very slow heart rate), heart failure (an inability of the heart to pump enough blood to the rest of the body) or a history of symptomatic arrhythmias (abnormal heart rhythms), or in patients also taking other medicines that can cause QT prolongation.