Terfenadine undergoes extensive metabolism in the liver by a specific
cytochrome P-450 isoenzyme. This metabolic pathway may be impaired in
patients with hepatic dysfunction (alcoholic cirrhosis, hepatitis) or
who are taking drugs such as ketoconazole, itraconazole, or
clarithromycin, erythromycin, or troleandomycin (macrolide
antibiotics), or other potent inhibitors of this
isoenzyme. Interference with this metabolism can lead to elevated
terfenadine plasma levels associated with QT prolongation and
increased risk of ventricular tachyarrhythmias (such as torsades de
pointes, ventricular tachycardia, and ventricular fibrillation) at the
recommended dose. SELDANETM is contraindicated for patients with these
conditions