allergic rhinitis
"I just took Zyrtec for the first time yesterday.  It did make me drowsy but I also didn't get much sleep the night before.  I took it because my eye was so itchy and watery it had become swollen and I was having trouble breathing.  I have had allergies for over 15 years but never took anything before this. Well I don't know if this is an unlisted side effect but I woke up in my sleep cracking up and could not stop laughing for a few minutes even after my husband woke me.  I wonder if anyone else had intense dreams while on this medication.  I don't take anything else that would cause this."