Characteristics of 7 included randomized controlled trials ^*.

*FEV1, forced expiratory volume in 1 second; PEF, peak expiratory flow; histamine PC20, provocative concentration of histamine required to cause a 20% fall in FEV1, asthma exacerbation rates; JACQ, Juniper Asthma Control Questionnaire; AQLQ, the Asthma Quality of Life Questionnaire; NR, not reported; †An asthma exacerbation was defined as an acute worsening of asthma requiring additional treatment in excess of an increase in short-acting. β2-agonist. ‡Exacerbations were defined as increase in the daily dose of albuterol to control symptoms of chest tightness or as any one of the following: nocturnal or waking respiratory symptoms on two consecutive days, a decrease of more than 15% in the FEV1, or a 2-point worsening in the Likert score. §Exacerbations were defined as periods of deterioration in asthma control in subjects who had been treated with high-dose oral prednisolone for at least 5 days. ¶Exacerbations defined as worsening of asthma requiring use of oral corticosteroids for 3 or more days, admission, or a visit to the emergency department–were corroborated by another measurement: decreased peak flow, 50% increase in rescue medication, increased frequency of nocturnal awakening due to asthma, or overall asthma symptom score of five for at least 2 of 3 successive days.