I wrote to the CAA about acceptable antihistamines. A CAA Dr replied saying that Zirtek (Cetirizine) is NOT acceptable for initial issue of class 1. he recommended i change to Clarityn (loratadine) or some other drug (fexofenadine i think it was called). so i'd maybe advise you to try to go without the zirtek and see how you are on just the clarityn.
hope this helps
cheers
rod