My allergist wants me to take 2 zyrtec (cetirizine) morning/night for my chronic hives. how many hours in between each dose should i wait? thanks

Every 6 hrs: You can take 2 Zyrtec (cetirizine) every 12 hrs or 1 every 6 hrs. Some may even go up to 1 tab every 4 hrs . Do note that Zyrtec (cetirizine) in higher doses can cause drowsiness and dryness in the mouth, How often you need to take the medication depends on how well you can control your hives. If one twice daily does it, that's all you need. If you need one every 6 hrs, so be it.

10-12 hours: The important point it to take your medication regularly If the itching starts then it's time to take the next dose. These medications are very safe except for a little drowsiness. Follow the directions you were given but please don't accept breakthrough itching. There are other medications that can be added to give you comfort.