The asthmatic airway is identified and studied using inhaled agents such as histamine and methacholine. The use of antihistamines prior to the test will inhibit the test result if histamine is used to cause airway constriction. If using methacholine, this may also be true depending on whether old (e.g. benadryl) or new (e.g. desloratadine) antihistamines are used. This study will look at the effect of old and new antihistamines on inhaled methacholine challenge response in individuals with mild asthma.

Inclusion Criteria:

• Baseline forced expiratory volume in 1s (FEV1) of 70% or more of predicted
• Methacholine provocative concentration causing a 20% fall in FEV1 16mg/ml or less
• No respiratory infection or change in allergen exposure for 4 weeks prior to screening and throughout the study
• No significant medical co-morbidities
• allergy

Exclusion Criteria:

• pregnant or breastfeeding females
• unable to withhold antihistamines, anticholinergics, long acting bronchodilators and combination therapies