Zyrtec Side Effects. Adults and children ≥12 years of age: Somnolence, fatigue, dry mouth. Insomnia reported with cetirizine (the active ingredient contained in Zyrtec) hydrochloride-pseudoephedrine hydrochloride fixed combination.
Children 2–11 years of age: Headache, pharyngitis, abdominal pain.
Children 6 months to 2 years of age: Irritability, fussiness, insomnia, fatigue, malaise.