The effect of phenytoin on the dispositon of disopyramide

The combining of two drugs may cause an interaction which either enhances or diminished the beneficial or adverse effects of either agent. An interaction between two antiarrhythmic drugs, disopyramide and phenytoin has been reported. In this interaction, phenytoin appears to decrease the therapeutic effect while enhancing the toxic effects of disopyramide. Disopyramide slows the conductions velocity, prolongs the effective refractory period of the action potential and decreases automaticity of normal and diseased fibers in the heart. These properties enable disopyramide to treat both atrial and ventricular arrhythmias. Phenytoin, although not approved by the FDA as an antiarrhythmic drug, is often combined with other agents such as disopyramide in the treatment of ventricular arrhythmias. The current study was a prospective controlled evaluation of the effects of phenytoin on the disposition of disopyramide in normal volunteers. The objective was to determine if an interaction existed.