I have been taking Anti-Histamines for years and can't say I have noticed this. In fact, I get depressed when i don't take them cos I my symptoms (allergic rashes/itching) go mad!! I have tried Clarityn, Zirtek, telfast the lot - I worry about the long term effects of taking any drug though. I find that Clarityn and Zirtek are pretty inoffensive and apart from the active ingredient shouldn't cause any reactions like depression - though who knows.
~~Stick 'em up punk it's da Fun Lovin' Criminal~~