Is it safe to double up on reactine, so taking every 12hrs instead of 24hrs?

Cetirizine: Reactine is also known as Cetirizine or zyrtec and is a new generation antihistamine. In general, antihistamines are a safe class of medications. Some patients may experience drowsiness or delayed reaction time with increased doses of antihistamines such as cetirizine. Under the supervision of a physician, higher doses of Cetirizine may be tried or other treatments may be necessary.