No interaction studies have been performed with levocetirizine (including no studies with CYP3A4 inducers); studies with the racemate compound cetirizine demonstrated that there were no clinically relevant adverse interactions antipyrine (with antipyrine pseudoephedrine, cimetidine, ketoconazole, erythromycin, azithromycin, glipizide and diazepam).
Theophylline: A small decrease in the clearance of cetirizine (16%) was observed in a multiple dose study with theophylline (400 mg once a day); while the disposition of theophylline was not altered by concomitant cetirizine administration.
Ritonavir: In a multiple dose study of ritonavir (600 mg twice daily) and cetirizine (10 mg) daily, the extent of exposure to cetirizine was increased by about 40% while the disposition of ritonavir was slightly altered (-11%) further to concomitant cetirizine administration.
Food: The extent of absorption of levocetirizine is not reduced with food, although the rate of absorption is decreased.
Alcohol: In sensitive patients, the concurrent administration of cetirizine or levocetirizine and alcohol or other CNS depressants may cause additional reductions in alertness and impairment of performances.