In chronic idiopathic urticaria, mast cells, for unknown reasons, release histamine.
Histamine causes the skin to break out in hives.
Hives last less than 24 hours. This medication is taken just once a day. However, drowsiness may occur at more than the recommended dose. The most common side effects occurred about as often as they did with a sugar pill, including headache, drowsiness, fatigue, and dry mouth. At the recommended dose, it won't make kids drowsy or cause hyperactivity.