Provocholine, known generically as methacholine chloride, is a diagnostic agent used primarily in the evaluation of bronchial hyperreactivity. It works by stimulating the muscarinic receptors in the airways, leading to bronchoconstriction, or narrowing of the airways. This property makes it useful in conducting bronchial challenge tests, often employed to diagnose conditions like asthma. During such tests, a controlled dose of Provocholine is administered to the patient through inhalation, and the response of the airways is monitored. A significant decrease in lung function following inhalation indicates bronchial hyperreactivity, supporting a diagnosis of asthma or other related respiratory conditions. Due to its potential to cause significant bronchoconstriction, Provocholine testing is conducted under close medical supervision.