(Ref: KDT 6/e p645) Cisapride is a 5HT4 agonist that can block cardiac K+ channels at high concentration. When these are administered with microsomal enzyme inhibitors (like erythromycin or ketoconazole), polymorphic ventricular tachycardia can result. Mosapride and tegaserod are other 5HT4 agonists that are devoid of arrhthmogenic action.