Methacholine Challenge: Comparison of Doubling and Quadrupling Methacholine Dose Regimes Using the Tidal Volume Method

Inclusion Criteria: * You must be over 18 years of age; and * You must have a diagnosis of asthma; and * Excluding asthma, you are not aware of any other lung conditions or diseases. Exclusion Criteria: * You have a known hypersensitivity (i.e. an overreaction of your immune system) to methacholine or other parasympathomimetic agents or cholinesterase inhibitors (i.e. agents that act in a similar way via your nervous system - to be discussed with study personnel); or * You are a nursing mother or if you are a woman of child bearing potential who is, may be, or intends to become pregnant during testing as the effects of methacholine inhalation in these situations are not known; or * Your baseline lung function is poor (ie. your FEV or forced expiratory volume in one second, is less than 65% of your predicted values). This will be performed and explained by study personnel prior to commencing methacholine inhalation testing; or * You have had any respiratory infections for the last four weeks; or * If you have allergies, and have been exposed to agents that trigger your asthma within the last four weeks; or * If you have any significant chronic medical condition.