Chlorpheniramine competitively antagonizes H1 receptors, reducing capillary permeability and nasal secretions while readily crossing the blood–brain barrier to cause sedation. Its anticholinergic effects dry secretions but can impair cognition. Combination tablets often include dextromethorphan, an NMDA receptor antagonist that elevates the cough threshold centrally. The sedating and anticholinergic properties explain the guidance to limit use in young children and avoid duplicate antihistamines.