Levocetirizine 5 mg: Reduction of Symptoms, Airway Resistance and Sleep Impairment in Persistent Allergic Rhinitis

Inclusion Criteria: * Subjects who demonstrate their willingness to participate in the study and to comply with its procedures by signing a written informed consent. * Subjects aged between 18 and 50 years (inclusively), of either sex and any race. * Women of childbearing potential have to use an acceptable method of birth control. * Subjects have to be able to understand and to adhere to the dosing and visit schedules, and to agree to record symptom scores, NPIF measurements, adverse events, concomitant medications and intake of rescue medication accurately and consistently in a daily diary. * Subjects have to suffer from symptoms of allergic rhinitis on more than 4 days a week and for more than 4 weeks per year (ARIA criteria). * History of at least two years of persistent allergic rhinitis (as defined by ARIA criteria). * T5SS (Total Five Symptom Score, sum of scores evaluating the severity of five symptoms of rhinoconjunctivitis) assessed on visit 1 is \> 8; in particular, nasal obstruction is rated ≥ 2. * A CAP or prick test obtained within 12 months before visit 1 demonstrating sensitization to one or more allergens which anticipate symptoms of persistent allergic rhinitis on at least 4 days per week during the treatment period. * On visit 2: Average T5SS calculated from diary reports is \> 8 during the screening period; average rating of nasal obstruction is ≥ 2. Exclusion Criteria: * Women who are pregnant or nursing. * Subjects who have not observed the designated washout periods for any of the prohibited medications. * Subjects unable to understand the nature, scope, and possible consequences of the study or being suspected of non-compliance. * Subjects who have used any investigational product within 30 days prior to enrollment or any investigational antibodies for asthma or allergic rhinitis in the past 90 days. * History of alcohol or drug abuse. * Any disease of the upper and lower respiratory tract except for allergic rhinitis during the last 2 weeks before visit 1, 2 and 3; any significant impairment of nasal patency. * Subjects with current evidence of hematopoietic, cardiovascular, hepatic, renal, neurologic, psychiatric or autoimmune disease; or a tumor or conditions which may interfere with the absorption, distribution, metabolism or excretion of the study medication. * Any deviation from normal in physical examination and any disease (except for allergic rhinoconjunctivitis), which might deteriorate significantly due to the subject's participation, or require impermissible medication or interfere with study evaluation. * History of hypersensitivity to the study drug or its excipients.