How does methacholine give you asthma symptom ?

Bronchospasm : Methacholine induces bronchospasm in susceptible patients. Once it is inhaled, you undergo pulmonary function testing looking for airway obstruction. Once present, it is them reversed with a bronchodilator inhaler. This fulfills the definition of asthma: reversible obstruction. Most patients will not experience any symptoms during this test, even if obstruction occurs. It is a very safe test.