Allegra is a fexofenadine antihistamine used to treat hives and hay fever in adults and children as young as six months of age, while Zyrtec uses cetirizine for the same ailments and is approved for adults and children as young as two years of age, says About.com. Both medications were formerly available by prescription but are now available over-the-counter, notes WebMD.
Patients ages 12 and over take Allegra in 60-milligram doses twice a day, while those ages 2 to 11 take 30 milligrams twice a day, and patients six months to two years old can have15 milligrams twice a day, says About.com. The dosage for Zyrtec is 10 milligrams a day for patients six years of age and older, and five milligrams a day for patients between two and six years of age. Higher dosages of both drugs are sometimes helpful in treating hives. Both drugs are effective within an hour of ingestion for immediate relief and provide excellent symptom control with daily usage. Studies do suggest that Zyrtec works faster than Allegra, lasts longer and is more effective overall, especially in the treatment of hives.
The most common side effect of Allegra is headache, compared to fatigue with Zyrtec, notes About.com. Allegra shows little-to-no incidence of fatigue and sedation.