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Signs and symptoms of overdosage may be absent or mild (e.g., headache, nausea, confusion); but adverse cardiac events including cardiac arrest, ventricular arrhythmias including torsades de pointes and QT prolongation have been reported at overdoses of 360 mg or more and occur more frequently at doses in excess of 600 mg, and QTc prolongations of up to 30% have been observed at a dose of 300 mg b.i.d. Seizures and syncope have also been reported. USE OF DOSES IN EXCESS OF 60 MG B.I.D. IS NOT RECOMMENDED. (See BOXED WARNING, CLINICAL PHARMACOLOGY, and ADVERSE REACTIONS).
In overdose cases where ventricular arrhythmias are associated with significant QTc prolongation, treatment with antiarrhythmics known to prolong QTc intervals is not recommended.
Therefore, in cases of overdosage, cardiac monitoring for at least 24 hours is recommended and for as long as QTc is prolonged, along with standard measures to remove any unabsorbed drug. Limited experience with the use of hemoperfusion (n=1) or hemodialysis (n=3) was not successful in completely removing the acid metabolite of terfenadine from the blood.
Oral LD50 values for terfenadine were greater than 5000 mg/kg in mature mice and rats. The oral LD50was 438 mg/kg in newborn rats.
CONCOMITANT ADMINISTRATION OF TERFENADINE WITH KETOCONAZOLE (NIZORAL) OR ITRACONAZOLE (SPORONOX) IS CONTRAINDICATED. TERFENADINE IS ALSO CONTRAINDICATED IN PATIENTS WITH DISEASE STATES OR OTHER CONCOMITANT MEDICATIONS KNOWN TO IMPAIR ITS METABOLISM, INCLUDING SIGNIFICANT HEPATIC DYSFUNCTION, AND CONCURRENT USE OF CLARITHROMYCIN, ERYTHROMYCIN, OR TROLEANDOMYCIN. QT PROLONGATION HAS BEEN DEMONSTRATED IN SOME PATIENTS TAKING TERFENADINE IN THESE SETTINGS, AND RARE CASES OF SERIOUS CARDIOVASCULAR EVENTS, INCLUDING DEATH, CARDIAC ARREST, AND TORSADES DE POINTES, HAVE BEEN REPORTED IN THESE PATIENT POPULATIONS. (See WARNINGS and DRUG INTERACTIONS).
Terfenadine is contraindicated in patients with a known hypersensitivity to terfenadine or any of its ingredients.
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
- Seldane Drug Interactions Center: terfenadine oral
- Seldane Side Effects Center
- Seldane FDA Approved Prescribing Information including Dosage
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