– Methacholine bromide (Acetyl-β-methylcholine bromide) is a potent muscarinic-3 (M3) agonist that specifically acts on acetylcholine receptors located on smooth muscle, inducing bronchoconstriction and narrowing of the airways. Its high sensitivity in identifying bronchial hyperresponsiveness (BHR), a hallmark of asthma, makes it useful in the assessment of airway hyperresponsiveness (AHR) as a diagnostic tool for individuals exhibiting asthma-like symptoms with normal resting expiratory flow rates [1] [2] [3] [4].