Patients taking Astemizole should receive the following information and instructions. Antihistamines are prescribed to reduce allergic symptoms. Patients taking Astemizole should be advised: 1) to adhere to the recommended dose, and 2) that the use of excessive doses may lead to serious cardiovascular events. DO NOT EXCEED THE RECOMMENDED DOSE. Some patients appear to increase the dose of Astemizole in an attempt to accelerate the onset of action. PATIENTS SHOULD BE ADVISED NOT TO DO THIS, and not to use Astemizole as a p r n product for immediate relief of symptoms. Patients should be questioned about use of another prescription or over-the-counter medication, and should be cautioned regarding the potential for life- threatening arrhythmias with concurrent use of ketoconazole, itraconazole, or erythromycin. Patients should be questioned about pregnancy or lactation before starting Astemizole therapy, since the drug should be used in pregnancy or lactation only if the potential benefit justifies the potential risk to fetus or baby. In addition, patients should be instructed to take Astemizole on an empty stomach e.g., at least 2 hours after a meal. No additional food should be taken for at least 1 hour after dosing. 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.
Ketoconazole/Itraconazole, Macrolides, Including Erythromycin
Concomitant administration of Astemizole with erythromycin is contraindicated because erythromycin is known to impair the cytochrome P450 enzyme system which also influences astemizole metabolism. There have been two reports to date of syncope with Torsades De Pointes, requiring hospitalization, in patients taking combinations of astemizole 10 mg daily with erythromycin. In each case the QT intervals were prolonged beyond 650 milliseconds the time of the event; One patient also received ketoconazole and the other patient also had hypokalemia.
Concomitant administration of astemizole with ketoconazole tablets is contraindicated because available human pharmacokinetic data indicate that oral ketoconazole significantly inhibits the metabolism of astemizole, resulting in elevated plasma levels of astemizole and desmethylastemizole. Data suggest that cardiovascular events are associated with elevation of astemizole and/or astemizole metabolite levels resulting in electrocardiographic QT prolongation.
Concomitant administration with itraconazole is also contraindicated based on the chemical resemblance of itraconazole and ketoconazole. In vitro data suggest that itraconazole has a less pronounced effect on the biotransformation system responsible for the metabolism of astemizole compared to ketoconazole.
Astemizole is contraindicated in patients with known hypersensitivity to astemizole or any of the inactive ingredients.