Ketoconazole should not be given to a patient being treated with astemizole because:
Ketoconazole, erythromycin and clarithromycin are potent inhibitors of microsomal enzymes. Astemizole, terfenadine and cisapride are the drugs that can block cardiac K+ channels at high concentration. If administered along with microsomal enzyme inhibitors, these drugs can result in torsades de pointes, a serious polymorphic ventricular tachycardia.