Description: Zyrtec is a prescription medicine that treats both indoor and outdoor allergy symptoms, as well as chronic hives. In fact, no other allergy medicine of its kind is approved to treat more allergies than ZYRTEC. Studies show that 1 dose of ZYRTEC works for a full 24 hours. ZYRTEC is also great for the whole family. It's the first antihistamine approved to treat indoor allergies in infants 6 months and up.