A decongestant narrows the nasal passage blood vessels, prevents secretions from going back into the throat and improves airflow. The decongestant contains antihistamines that heal nasal passage swelling, resolves sinus headaches and reduces allergies. There are two types of decongestants: topical and systemic.

A systemic decongestant stimulates autonomic nervous system’s sympathetic division and thereby treats swelling of the vascular network of the respiratory tract. The decongestant also activates alpha-adrenergic receptors present in the blood vessels and decreases blood supply to the nose, reducing edema of the nasal mucosa. The decongestant contracts urinary sphincters and gastrointestinal (GI) tract, reduces insulation secretion and dilates pupils. The indirect action of the decongestants releases norepinephrine, causing peripheral vasoconstriction.

When systemic decongestants are administered orally, the GI tract absorbs them instantly. The decongestants easily enter into different fluids and tissues, such as breast milk, placenta and cerebrospinal fluid. The liver metabolizes the decongestants slowly and partly. The unchanged decongestants are excreted in urine within one day of the administration. The most commonly used systemic decongestants include pseudoephedrine and ephedrine.

The systemic decongestants may interact with other medicines. For instance, a systemic decongestant administered with a sympathomimetic, such as tyramine, epinephrine and dopamine, may stimulate central nervous system (CNS).  The combination of a systemic decongestant and monoamine oxidase (MAO) inhibitor may produce fatal hypertension.  The decongestants shall be administered carefully, especially to patients suffering from heart disorders, glaucoma, diabetes and hypertension, as it may worsen these conditions.

A topical decongestant, a vasoconstrictor, gives instant relief from mucous membrane swelling if sprayed to the nasal mucosa and nasal congestion. Popularly used topical decongestants include xylometazoline, tetrahyrozoline, phenylephrine, naphazoline and epinephrine. The decongestants activate alpha-adrenergic receptors occupying the nose’s vascular muscle and decrease blood supply to the nose. As a result, arterioles constrict. The reduction in the blood flow and capillary permeability brings down swelling. The Eustachian tubes open up, the nasal passage becomes clear and the sinuses drain easily, improving respiration. This vasoconstriction facilitates absorption of only small amount of the medicine. Thus, the topical decongestants rarely interact with other medications.

If a MAO inhibitor and topical decongestant are administered simultaneously, hypertension and severe headache may occur. Excessive use of the decongestants causes rebound nasal congestion. The rebound will resolve after a few days, if you stop using the decongestant. The topical decongestant may also cause sharp pain in the nasal mucosa and burning sensation temporarily.