My friend that I am staying with this weekend takes Zyrtec. About an hour or so after she takes one in the morning, she feels like she needs a nap. She has no medical problems other than allergies.

I took it for a while and had the same problem. I switched to Singulair. The docs don't know how it works but it works great on some people and does nothing for others. I also tried claritin which also did nothing for my allergies.