Common side effects of Zyrtec include drowsiness, fatigue and dry mouth, according to Drugs.com. Other side effects that may occur in children under 11 years old include pharyngitis, abdominal pain and headaches. Serious side effects, such as weakness or difficulty urinating, may require medical attention, states WebMD.
Zyrtec is an oral medication used to treat allergy symptoms such as runny nose, watery eyes, hives, itching and sneezing, explains WebMD. It can either be an over-the-counter medication or one prescribed by a doctor. It comes in either white tablets, purple chewable tablets or as a liquid medicine, according to RxList. The active ingredient in Zyrtec is cetirizine hydrochloride, an antihistamine.