Brompheniramine is a first-generation H1 receptor antagonist that decreases histamine-mediated vasodilation and itching while crossing the blood–brain barrier to induce sedation. Pseudoephedrine stimulates alpha-adrenergic receptors, shrinking nasal mucosal vasculature to reduce congestion and improving eustachian tube drainage. Dextromethorphan acts centrally on the medullary cough center as an NMDA receptor antagonist/sigma-1 agonist, raising cough threshold. The combined anticholinergic and sympathomimetic actions require careful dosing in young children and those with cardiovascular or urinary tract disease.