I'm convinced that prolonged use of Zyrtec makes me angrier for example. I've experimented with my allergies, and when I'm off Zyrtec, I'm sniffling and sneezing all the time... But generally happier and healthier from a mental perspective.
Every medication you take, especially with prolonged use, should be considered an experiment IMO. Even if the studies are show safety and efficacy... there are plenty of other side effects that could diminish quality of life.
Allegra doesn't have this problem for me, so I just switched to that.