allergies
"My doctor prescribed levocetirizine for reoccurring hives about 3 months ago.  The first night I took a full dose  (5 mg) I slept like a rock; didn't get up until the late morning.  I now cut the pill in half which has eliminated any drowsiness.  It has helped the hives and I haven't experienced any unusual side effects other than when I forgot to take it for a couple of days and starting feeling nauseous.  So if/when I no longer need it I will stop using gradually instead of suddenly."