<p>Zafirlukast may be administered with other therapies routinely used in the management of asthma and allergy. Inhaled steroids, inhaled and oral bronchodilator therapy, antibiotics and antihistamines are examples of agents which have been co-administered with Zafirlukast without adverse interaction.<br />
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Zafirlukast may be administered with oral contraceptives without adverse interaction. Co-administration with Warfarin results in an increase in maximum prothrombin time by approximately 35%. It is therefore recommended that if Zafirlukast is co-administered with Warfarin, prothrombin time should be closely monitored. The interaction is probably due to an inhibition by Zafirlukast of the cytochrome P450 2C9 enzyme system. In clinical trials co-administration with Theophylline resulted in decreased plasma levels of Zafirlukast, by approximately 30%, but with no effect on plasma Theophylline levels. However, during postmarketing surveillance, there have been rare cases of patients experiencing increased Theophylline levels when co-administered with Zafirlukast.<br />
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Co-administration with Terfenadine resulted in a 54% decrease in AUC for Zafirlukast, but with no effect on plasma Terfenadine levels. Coadministration with Acetylsalicylic acid (650 mg four times a day) may result in increased plasma levels of Zafirlukast, by approximately 45%.<br />
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Co-administration with Erythromycin will result in decreased plasma levels of Zafirlukast, by approximately 40%. The clearance of Zafirlukast in smokers may be increased by approximately 20%</p>