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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. Terfenadine is contraindicated for use by patients with these conditions (See BOXED WARNING, CONTRAINDICATIONS, and DRUG INTERACTIONS).
Other patients who may be at risk for these adverse cardiovascular events include patients who may experience new or increased QT prolongation while receiving certain drugs or having conditions which lead to QT prolongation. These include patients taking certain antiarrhythmics, bepridil, certain psychotropics, probucol, or astemizole; patients with electrolyte abnormalities such as hypokalemia or hypomagnesemia, or taking diuretics with potential for inducing electrolyte abnormalities; and patients with congenital QT syndrome. Terfenadine is not recommended for use by patients with these conditions. The relationship of underlying cardiac disease to the development of ventricular tachyarrhythmias while on terfenadine therapy is unclear; nonetheless, terfenadine should also be used with caution in these patients.
Information for the Patient
Patients taking terfenadine should receive the following information and instructions. Antihistamines are prescribed to reduce allergic symptoms. Patients should be advised to take terfenadine only as needed and NOT TO EXCEED THE PRESCRIBED DOSE. Patients should be questioned about any use of any other prescription or over-the-counter medication, and should be cautioned regarding the potential for life-threatening arrhythmias with concurrent use of ketoconazole, itraconazole, clarithromycin, erythromycin, or troleandomycin. Patients should be advised to consult the physician before concurrent use of other medications with terfenadine. Patients should be questioned about pregnancy or lactation before starting terfenadine therapy, since the drug should be used in pregnancy or lactation only if the potential benefit justifies the potential risk to the fetus or baby. Patients should also be instructed to store this medication in a tightly closed container in a cool, dry place, away from heat or direct sunlight, and away from children.
Oral doses of terfenadine, corresponding to 63 times the recommended human daily dose, in mice for 18 months or in rats for 24 months, revealed no evidence of tumorigenicity. Microbial and micronucleus test assays with terfenadine have revealed no evidence of mutagenesis. Reproduction and fertility studies in rats showed no effects on male or female fertility at oral doses of up to 21 times the human daily dose. At 63 times the human daily dose there was a small but significant reduction in implants and at 125 times the human daily dose reduced implants and increased post-implantation losses were observed, which were judged to be secondary to maternal toxicity.
Pregnancy Category C
There was no evidence of animal teratogenicity. Reproduction studies have been performed in rats at doses 63 times and 125 times the human daily dose and have revealed decreased pup weight gain and survival when terfenadine was administered throughout pregnancy and lactation. There are no adequate and well-controlled studies in pregnant women. Terfenadine should be used during pregnancy only if the potential benefit justifies the potential risk to the fetus.
Nonteratogenic Effects: Terfenadine is not recommended for nursing women. The drug has caused decreased pup weight gain and survival in rats given doses 63 times and 125 times the human daily dose throughout pregnancy and lactation. Effects on pigs exposed to terfenadine only during lactation are not known, and there are no adequate and well- controlled studies in women during lactation.
Safety and effectiveness of terfenadine in children below the age of 12 years have not been established.
Last reviewed on RxList: 12/8/2004
This monograph has been modified to include the generic and brand name in many instances.
Additional Seldane Information
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