Is it safe to take another 24-hour zyrtec (cetirizine) antihistamine if there's still about 8 hours left before the previous one wears off?

Zyrtec (cetirizine): You can take another xyrtec but you may become very sleepy.

Follow dosing: It's best to adhere to the dosing recommended. It would be hard to overdo it with the zyrtec (cetirizine) , the way you are suggesting, but it begs the question- how well does the rx work , if you want more before the proper dosing interval ? I would think you need to talk with your doctor about other options to achieve safe allergy control. Sometimes more than one rx is necessary.