My AME told me not to take Clarityn on flying days but yet so many people seem to be taking it without any problems. I can't see why it shouldn't be allowed personally anyway as it is certainly in the non-drowsy class.
I've heard plenty of pilots refer to fexofenadine being fine though. This is usually referred to by it's trade name Telfast (or if you're in the US, Allegra-D).
I agree with one of the former respondents though, altitude and a/c can work wonders.
Happy flying.
Nigel