Dextromethorphan is reported to act within 30 minutes of oral administration and to exert an effect for up to 6 hours. Pseudoephedrine is a decongestant and acts on alpha-adrenergic receptors in the mucosa of the respiratory tract, producing vasoconstriction. Also, drainage of sinus secretions may be increased and obstructed eustachian ostia may be opened. Oral decongestants have a prolonged duration of action but may cause more systemic effects. However, pseudoephedrine has the least CNS stimulatory effect.
Pseudoephedrine is absorbed from the gastrointestinal tract.
The rate of excretion is accelerated in acidic urine. The onset of action is 15 to 30 minutes and the duration of action is about 4 hours. Guaifenesin has an expectorant action which increases the output of respiratory tract fluid by reducing adhesiveness and surface tension. The increased flow of less viscous secretions promotes ciliary action and facilitates the removal of mucus. Guaifenesin is absorbed from the gastrointestinal tract.