Antihistamines can lose their effectiveness in just three weeks of daily use; and used over the long term, many people become resistant to them and they stop working altogether.†13 For others, they hardly work at all - even in the beginning.
When your body comes into contact with whatever your allergy trigger is -- pollen, ragweed, pet dander, or dust mites - for example -- your body makes chemicals called histamines. Histamines start the process that try to get those allergens out of your body or off your skin. They cause the tissue in your nose to swell (making it stuffy), your nose and eyes to run and itch. Sometimes you may also get an itchy rash on your skin, called hives.
Antihistamines work to reduce or block histamines, thereby reducing the body’s response to the allergen.