A 10-milligram doze of Zyrtec should take effect within 10 to 20 minutes of consuming the medication, according to InhousePharmacy.biz. This single dosage of Zyrtec should last for up to 24 hours.
According to InhousePharmacy.biz, Zyrtec is used to reduce or eliminate signs of seasonal allergies from pollen as well as reactions to domestic year-round allergens, such as dust mites, pet dander and mold. Symptoms associated with such allergies are sneezing, a runny nose and itchy or watery eyes. The allergy symptoms may also lead to difficulty sleeping, says InhousePharmacy.biz. Zyrtec treats allergy symptoms, but it does not cure allergies.