They block the action of histamine, which triggers inflammation and the production of mucus in the upper respiratory track during an allergic reaction. These medicines attempt to prevent congestion before it occurs. They do not shrink mucus membranes to relieve nasal congestion, a problem which is usually treated with decongestants. Popular cold and allergy remedies often contain a combination of antihistimines and decongestants. Side effects for these medicines range from drowsiness to sleeplessness, and may not be advisable for patients with hypertension, or thyroid disorders.
Decongestants often have a stimulant effect.
If a particular decongestant makes a child with allergies seem hyperactive, a parent may be able to solve the problem by switching to a different decongestant medicine. However, to do this the person buying the medicine must know exactly what kind of decongestant or antihistamine they are getting. Many antihistamines have the opposite effect, causing drowsiness. There is a new generation of antihistamines, which is currently available by prescription, that does not cause this side effect. Another medicine which may be taken to help clear up mucus is called an expectorant.