The active ingredient of Zyrtec is cetirizine. It works by blocking the receptors of chemical signals that cause upper respiratory symptoms. During an allergic reaction, your body's immune system releases histamines that causes sneezing, itching, stuffy or runny nose, cough or sinus headaches. Cetirizine blocks the action of histamines on your body.
Your body doesnt become addicted to antihistamines nor does it develop tolerance to antihistamines. Once you take them for months to years, you don’t need increasing doses just to get the same protection that you got originally. If you have nasal allergies, and you inhale some of the allergens to which you’ve become sensitized, white cells burst open, releasing histamine, which quickly begins inflammation in your nose and you get symptoms. If there was some antihistamine in your body at the time, this inflammation is blocked. Sort of like the windshield of your car keeping dust from hitting your face when you drive.
If you remove the windshield of your car, dusts will again hit your face. If you then replace the windshield, it won’t remove the dusts already in your eye. Does this mean that you or your car are “addicted” to the windshield?