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.