Histamines are chemicals the body releases during an allergic reaction. When an allergen is present, the body creates the histamine that binds itself to receptor cells in nasal tissues, nerve endings, and nearby blood vessels. These blood vessels begin to enlarge and leak fluid and increase mucus production, which leads to sneezing, itching, redness, and swelling. The end result is nasal congestion, wheezing, or irritated skin. Antihistamines prevent the histamine from binding to the histamine site by getting to these same places first: The antihistamine chemicals bind themselves to the receptor cells.
If you take antihistamines before exposure to an allergen, you may not develop symptoms. After the exposure has already taken place and some histamine has bound to the receptor cells, the antihistamine is not effective and will not reverse the allergic process. Instead, it stops the continuation of the allergic response going forward. That’s why you will frequently see antihistamine medications combined with a decongestant medication. The antihistamine keeps further allergic reactions
from occurring, while the decongestant deals with symptoms already in the works. Antihistamines are found in both OTC and prescription formulations. They also appear in pill, liquid, nasal spray, and eyedrop forms.
Antihistamines will relieve only the symptoms related to allergies: If you are taking the medication correctly, the antihistamine will stop or quell the allergic symptom response. This will include nasal and sinus swelling, sneezing, itching, hives, and asthmatic flare.
Unfortunately, antihistamines are probably the most abused medicines available. Antihistamines will not work if you do not suffer from
allergies, yet many physicians incorrectly prescribe antihistamines for their patients who have a cold even when the patient does not have allergies. Furthermore, patients very often self-prescribe OTC antihistamines or combination medicines that include antihistamines whether or not they have allergies.
For example, you might decide to take an antihistamine if you have a cold or for acute, recurrent acute or chronic sinusitis, believing that the antihistamine will dry up your congestion. But this is not the case. If allergies are not the cause of your sinus infection (and certainly they are never the cause of a cold) an antihistamine will do nothing. They will not disrupt the mucus production, clear the infection, or decrease nasal congestion. If you have congestion and mucus production with a sinus problem, a decongestant would be a better choice.