Question: how long do you have to take zyrtec before it works?
Answer: Most people experience at least 24 hours of relief after a single dose. Zyrtec, on the other hand, is fast acting. People who take it may feel relief in as little as one hour. Antihistamines like Zyrtec and Claritin are designed to calm the histamine reaction your body has when it's exposed to an allergen.