An allergic reaction begins in the immune system, which we all have to protect us from unwanted invading bacteria, viruses, and fungi.  In people with allergies, however, the immune system mistakes otherwise harmless pollen, dust, or mold as an invader.  The immune system overreacts to the pollen, dust, or mold because it has too much immunoglobulin E (IgE).  

Allergy medicines for the most part do not get to the root cause of the allergic reaction – in immune system imbalance causing too much IgE and histamine – but instead mask the symptoms by reducing the allergic response.  

Some newer allergy medicines that lessen the allergiic response may be bought over-the-counter (OTC)

and include cetirizine (Zyrtec), levocetirizine (Xyzal), and desloratadine (Clarinex or Claritin); while others require a prescription from your doctor.  These OTC drugs may claim they are “non-drowsy” because in clinical studies they were shown at a low dose to significantly reduce allergy symptoms while not signifcantly causing drowsiness.  What you may not know, is that at higher doses they may cause drowsiness, so be careful if you take extra antihistamines before you operate machinery, drive, or sky dive!

Source: aaaai.org