Behavior disorder symptoms and conditions

My son has been on Zyrtec for the past year - he is now 4. He the sweetest disposition one minute and is a monster the next hating us and wishing we were dead. We assumed that it was his age and possibly a behavior disorder until I read the other entries. We took him off immediately and he is back to the kid we lost last year. This needs to be taken seriously. When I researched the side effects on the drug website they do not mention behavior issues as a possible side effect. My son will live with a runny nose and a smile on his face.