A decongestant is any pharmaceutical used to relieve congestion of the upper respiratory tract (the nose, throat, and sinuses). The two most commonly used are pseudoephedrine and phenylephrine.
Decongestants work by stimulating the effect of adrenaline and norepinephrine. This encourages blood vessels to constrict, reducing inflammation and reducing the production of mucus.
However, because their effect is usually not limited to the upper respiratory tract, the use of decongestants can lead to higher blood pressure. People with existing high blood pressure should only take decongestants on the recommendation of a physician or pharmacist.