Yep....Flonase & Zyrtec is a rational combination - a nasal steroid & oral h1 antihistamine.
For most people...the second generations antihistamines lose effectiveness over time because they are competitive antagonists. This means when an antigen has a greater affinity for the receptor than the anthihistamine itself, the antigen will knock the antihistamine off.
In addition...there is speculation some of these h1 blocker may induce liver enzymes which increase rate of metabolism. The P450 enzyme system is involved in metabolism, but the specific subenzymes have not been fully identified.
So....its not a surprise your antihistamine may lose effectiveness over time. Try another one for awhile then you should be able to go back.
Also..Clarinex is desloratidine, which is the active metabolite of loratidine (Claritin). It was marketed only because claritin was approved for otc status & Schering wanted a product which would still compete in the rx market.
Probably more than any of you wanted to know...... 🙁