Dextromethorphan hydrobromide acts centrally to elevate the threshold for coughing. It has no analgesic or addictive properties. The onset of antitussive action occurs in 15 to 30 minutes after administration and is of long duration. Phenylpropanolamine hydrochloride is a sympathomimetic drug which is readily absorbed from the gastrointestinal tract and produces nasal vasoconstriction (decongestion). Phenylpropanolamine stimulates both alpha and beta adrenergic receptors, similar to ephedrine. Part of its peripheral action is indirect and is due to the displacement of norepinephrine from storage sites, but it also has a direct effect on the adrenergic receptors. Brompheniramine maleate is a histamine antagonist, specifically an Hi-receptor blocking agent belonging to the alkyl amine class of antihistamines. Antihistamines appear to compete with histamine for receptor sites on effector cells. Brompheniramine also has anticholinergic (drying) and sedative effects. Among the antihistamine effects, it antagonizes the allergic response (vasodilation, increased vascular permeability, increased mucous secretion) of nasal tissue. Brompheniramine is well absorbed from the gastrointestinal tract, with peak plasma concentration after a single oral dose of 4 mg reached in 5 hours, urinary excretion is the major route of elimination, mostly as products of biodegradation; the liver is assumed to be the main site of metabolic transformation.

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