Cetirizine is a second-generation antihistamine used to treat allergic rhinitis, dermatitis, and urticaria. It is taken by mouth. Effects generally begin within thirty minutes and last for about a day. The degree of benefit is similar to other antihistamines such as diphenhydramine, which is a first-generation antihistamine.
Common side effects include sleepiness, dry mouth, headache, and abdominal pain. From Wikipedia
