Zyrtec (cetirizine) typically starts working within 20 to 60 minutes after taking it, with about 50% of people experiencing effects within 20 minutes and 95% within an hour. Peak blood concentrations are reached around 1 hour, which corresponds to the time most people feel significant relief from allergy symptoms such as sneezing, runny nose, and itchy eyes

. The effects of a single dose of Zyrtec generally last for about 24 hours, allowing for once-daily dosing. Zyrtec-D, which includes a decongestant, lasts about 12 hours due to the added pseudoephedrine component

. Onset of action can vary slightly depending on individual factors like metabolism, symptom severity, and whether it is taken with food (which may delay peak concentration by up to 1.7 hours)

. In summary:

• Starts working: 20 to 60 minutes (most within 1 hour)
• Peak effect: about 1 hour
• Duration: about 24 hours for Zyrtec; about 12 hours for Zyrtec-D

This rapid onset makes Zyrtec a fast-acting antihistamine for allergy symptom relief