Methacholine provocative tests have several advantages over other tests of airway hyperreactivity. First, a relatively low dose of methacholine is needed to exert an airway response, with no limit to the dose that can be administered. In addition, methacholine inhalation is safe, with few side effects at low concentrations, and can easily be conducted in outpatient clinics. Methacholine and histamine inhalation challenges induce bronchoconstriction at relatively equivalent concentrations (Bhagat and Grunstein 1984; Toelle et al. 1994). However, methacholine is more commonly used as an inhalation challenge agent as opposed to histamine (Scott and Braun 1991), due to histamine being associated with more systemic side effects and less reproducibility (Juniper et al. 1978; Chatham et al. 1982; Higgins et al. 1988). For example, Juniper et al. reported that following inhalation challenge with histamine or methacholine, there appeared various side effects including throat irritation, flushing, and headache. On the other hand, these symptoms were notably more frequent with histamine than methacholine and were dose related (Juniper et al. 1978).