Zyrtec (cetirizine) is an over-the-counter antihistamine that treats allergy symptoms like sneezing, runny nose, itchy or watery eyes, and itching of the nose or throat. It also treats hives, including chronic cases that last six weeks or longer. Relief starts within 20 to 60 minutes of taking a single dose and lasts a full 24 hours.
How Zyrtec Works
When your body encounters an allergen like pollen, pet dander, or dust mites, immune cells release a chemical called histamine. Histamine latches onto receptors in your nose, eyes, throat, and skin, triggering the familiar cascade of sneezing, itching, swelling, and watery eyes. Zyrtec works by competitively blocking those receptors, essentially sitting in histamine’s parking spot so it can’t bind and cause symptoms.
Zyrtec is classified as a second-generation antihistamine, meaning it was designed to cause less drowsiness than older options like Benadryl. It does cross into the brain less than first-generation antihistamines, but more than some of its newer competitors, which is why drowsiness remains its most notable side effect.
Seasonal and Year-Round Allergies
Zyrtec is approved for hay fever and other upper respiratory allergies. That covers both seasonal triggers (tree pollen in spring, grass in summer, ragweed in fall) and year-round triggers like dust mites, mold, and pet dander. The specific symptoms it targets are sneezing, runny nose, itchy or watery eyes, and itching of the nose or throat.
It won’t relieve nasal congestion on its own. If stuffiness is your main complaint, you’d need a separate decongestant or the combination product Zyrtec-D, which adds pseudoephedrine.
Hives and Skin Itching
Beyond respiratory allergies, Zyrtec is one of the go-to treatments for hives (urticaria), including chronic cases where welts and itching persist for weeks or months without a clear cause. At the standard 10 mg dose, it reduces both the welts and the itching. Research from multiple clinical trials shows that in people with severe chronic hives who don’t respond well to the standard dose, a higher 20 mg dose can produce statistically significant improvements in welts, redness, and itching compared to placebo. That higher dose is something a doctor would supervise, not a self-treatment decision.
How It Compares to Claritin and Allegra
Clinical studies have found that Zyrtec and Allegra (fexofenadine) provide roughly equal symptom relief for moderate-to-severe seasonal allergies, while Zyrtec tends to outperform Claritin (loratadine) in head-to-head comparisons. The tradeoff is drowsiness. In a trial comparing Zyrtec 10 mg to Allegra 180 mg over two weeks, both medications reduced allergy symptoms equally, but patients on Allegra experienced significantly less drowsiness.
So the practical choice often comes down to how sensitive you are to sedation. If drowsiness isn’t an issue for you, Zyrtec’s slightly stronger antihistamine activity may be an advantage. If you need to stay sharp during the day, Allegra or Claritin may be better fits.
Dosing for Adults and Children
Zyrtec is taken once daily, with or without food. Adults and children 12 and older typically take 5 to 10 mg, with most people starting at 10 mg. Children ages 6 to 11 take 5 to 10 mg depending on symptom severity, while children 2 to 5 years old start at 2.5 mg and can go up to 5 mg. For infants 6 months to under 2 years, the dose is 2.5 mg once daily, available as a syrup.
People with kidney or liver problems generally need to stay at the lower end of the dosing range, since Zyrtec is primarily cleared through the kidneys. The same applies to older adults, who are more likely to have reduced kidney function even without knowing it.
Drowsiness and Other Side Effects
Drowsiness is by far the most common side effect. In clinical trials using the 20 mg dose (double the typical OTC dose), about 24% of participants reported feeling sleepy compared to roughly 8% on placebo. At the standard 10 mg dose, the rate is lower but still meaningful enough that you should see how the medication affects you before driving or doing anything that requires full alertness.
Alcohol amplifies these effects. Combining Zyrtec with alcohol can increase dizziness, drowsiness, and difficulty concentrating, and may impair your thinking and judgment beyond what either substance would cause alone. The same goes for other sedating medications.
Rebound Itching After Stopping
In 2023, the FDA added a warning about a rare but potentially severe side effect: intense itching that starts within a few days of stopping Zyrtec after long-term daily use. The agency reviewed 209 reported cases worldwide, and in nearly all of them, the itching appeared within one to five days of discontinuation, with a median onset of two days.
This doesn’t happen to most people, but the risk appears to increase with duration of use. Among cases where usage length was reported, 92% had been taking the medication daily for more than three months. The median duration of use before experiencing rebound itching was 33 months. In some cases the itching was severe enough to be disabling, with people describing it as debilitating to the point of being bedridden.
Restarting the medication resolved the itching in about 90% of people who tried it. Gradually tapering off after restarting worked for about 38% of those who attempted that approach. If you’ve been on Zyrtec daily for several months or longer and want to stop, tapering down gradually rather than quitting abruptly is a reasonable strategy to minimize this risk.
Use During Pregnancy
Zyrtec is considered one of the safer antihistamine options during pregnancy. The Mayo Clinic lists cetirizine alongside loratadine as oral antihistamines that healthcare providers may recommend for managing mild allergy symptoms in pregnant women. That said, the decision involves weighing symptom severity against any potential risk, which varies by trimester and individual health factors.