My 5 year old ASD son had terrible side effects from taking Zyrtec. After roughly taking the medicine daily for 3 weeks, he begin having strong tics (head, shoulder and arm abductions) multiple times per minute, chewing on everything and severe anger issues. These were issues he had never had prior to taking this medicine. Obviously my wife and I were extreme concerned.
It took roughly 2 weeks for the medicine to totally clear his system and all issues are gone.
I personally have taken Zyrtec and it is great for allergies but something about his neurological system gave him these side effects with this drug. He seems to tolerate Allegra much better but because of this issue we have stopped giving him antihistamines.
I searched all over the Internet including wrong planet and was able to find many other families that experienced similar issues. I'm posting here to provide further information to anyone who might be experiencing issues and trying to find a root cause.