Increased incidence of side effects after encainide: a newly developed antiarrhythmic drug.

In a clinical trial the efficacy of encainide, a newly developed class I antiarrhythmic agent, was compared with the well-known mexiletine. Nine patients with different underlying cardiac disease and chronic complex ventricular ectopies (documented by 24-h Holter monitoring, confirmed during the initial placebo period) entered the study. The dosage of encainide was increased from 25 to 75 mg three times daily and the antiarrhythmic effect monitored by repeated 24-h Holter registration and in some patients by treadmill exercise testing. During the clinical followup we noted a high incidence of so-called "minor side effects" (headache, dizziness, blurred vision, tremor, and nausea), which caused us to terminate the study. In all instances adverse effects emerged before ectopic activity was suppressed satisfactorily prohibiting further increment of dosage. These results indicate that encainide cannot be regarded as an antiarrhythmic drug of first choice in routine clinical application.