My body acts weird. Everytime I use Zyrtec (cetirizine) drops I get very little drowsy only on the first dose. My tolerance goes away if I stop it for long. Why?

Expected. That's true with most sedating antihistamines. One does get used to the drowsiness after a couple of weeks. Whether this is due to a faster breakdown of the drug by the liver or reconditioning of the brain- no one knows However it is believed that some impairment in performance does happen when measured by performance tools even though you may not be feeling it.