I had been somewhat depressed for a while. After 2 days of being on Zyrtec, I was nearly suicidal. I continued to take it for a few more days because it did help with my allergies. I wasn't sure that it had anything to do with my depression until I stopped using it (after a week). Being off of it for 2 days changed my complete outlook on life. I am much happier and positive. I only tried Zyrtec because my insurance company refused to pay for Allegra, which works very well with no side effects.