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How does Zyrtec work?

Zyrtec is a prescription and over-the-counter medication that can relieve symptoms of allergies.

Zyrtec is recommended for people experiencing seasonal allergies or “hay fever”, perennial allergies including allergies to dust mites and mold, and chronic idiopathic hives (hives with no known cause).1 But how does Zyrtec work?

Zyrtec is an antihistamine

The human body releases a chemical called histamine when exposed to an allergen.

Once released, histamine binds to receptors called peripheral H1 receptors and launches an immune system response.

Peripheral H1 receptors are found in the muscles that line the respiratory tract, immune cells, the lining of the digestive tract, and the cells that line the circulatory system.2

The binding of histamine to H1 receptors initiates an allergic response where the respiratory system contracts breathing becomes difficult, blood pressure decreases, and inflammation including skin reactions occurs.

Antihistamines block histamine interactions.

How does Zyrtec work as an antihistamine?

The active ingredient in Zyrtec is an antihistamine called cetirizine hydrochloride.

Cetirizine hydrochloride works by blocking the binding of histamine to peripheral H1 receptors, preventing symptoms of allergic reactions that are normally caused by histamine.2

Zyrtec and allergy symptom relief

In treating both seasonal and perennial allergies, studies have shown that Zyrtec reduces sneezing, runny nose, watery eyes, and eye redness compared to those who do not take Zyrtec.1

One study in particular gave cetirizine, the active ingredient in Zyrtec, to patients with seasonal allergies (seasonal allergic rhinitis) for six weeks.

At the recommended dose of 10 mg daily, researchers observed a significant improvement in sneezing, nasal itching, runny nose, eye itching, and watery eyes after the first week of the study.

This improvement in the severity of symptoms was maintained for the duration of the experiment.3

For chronic idiopathic hives, Zyrtec has been shown to reduce the severity, duration, and frequency of episodes. It has also been shown to reduce itchiness.1

How long does Zyrtec take to work?

Studies have shown that Zyrtec takes approximately 60 minutes to reach peak blood concentration after consumption, although some effects can be felt 20-60 minutes after consumption.

These effects last at least 24 hours.2

What is the best way to take Zyrtec?

Studies have shown that there is no significant difference between consuming the tablet or syrup form of Zyrtec.1

Adverse effects of Zyrtec

Reported side effects have included dry mouth, fatigue, and drowsiness.1

Children have been shown to be more prone to experience side effects than adults.5 How do Zyrtec side effects compare to other allergy medications?

Zyrtec vs. first-generation antihistamines

Zyrtec is considered a “second-generation” antihistamine.

Contrary to first-generation antihistamines, second-generation antihistamines do not cross the blood-brain barrier to a significant extent.2

This reduced access to the brain and spinal cord suggests that Zyrtec does not significantly induce sedation like first-generation antihistamines such as doxylamine and diphenhydramine.6

Sedative effects

The sedative effects induced by first-generation antihistamines include impaired performance and delayed reaction times.

One study showed that the sedation from first-generation antihistamines reduced work efficiency decreased REM sleep, and impaired learning.7

Second-generation antihistamines like Zyrtec may be preferred to avoid these first-generation antihistamine sedative effects.

Zyrtec vs. other second-generation antihistamines

Compared to other second-generation antihistamines, Zyrtec may be more likely to induce sedative effects even though Zyrtec is still considered “non-sedating”.

One study compared the reported instances of drowsiness and sedation experienced by users of four second-generation antihistamines: loratadine (Claritin), fexofenadine (Allegra), acrivastine (Benadryl), and cetirizine (Zyrtec’s active ingredient).

Compared with loratadine, cetirizine was found to be approximately 3.5 times more likely to induce sedative effects.

Acrivastine and fexofenadine were found to be approximately 2.8 times and 0.6 times more likely than loratadine to induce sedation, respectively.8

The study concluded that fexofenadine and loratadine may be the second-generation antihistamines of choice for people working a job requiring alertness.

Zyrtec may limit drug-drug interactions

Zyrtec is absorbed through the gut lining and is excreted largely unchanged in urine.

This excretion makes Zyrtec unique from other second-generation antihistamines because they generally are metabolized by the liver and then excreted in a broken-down form by the kidneys.9 Specifically, one study administered cetirizine to a group of individuals with liver disease.

They found no significant difference between the urinary excretion of cetirizine between healthy and liver-compromised individuals.10

The results suggest that the liver does not play a significant role in cetirizine’s drug metabolism.

Bypassing the liver decreases the chances of drug-drug interaction since the liver is the site where many drugs are metabolized.

Zyrtec may be more effective than its counterparts

One study looked at the effectiveness of cetirizine compared to another second-generation antihistamine fexofenadine (Allegra) in reducing symptoms of a ragweed allergy.

They measured runny nose, watery eyes, sneezing, and itchy nose during a 24-hour period post-dose. They observed that cetirizine showed a 26% larger reduction in these symptoms after 12 hours.11

They also found that cetirizine was more effective at reducing the symptoms of an itchy nose and sneezing at both the 12-hour mark and overall during the observation.

Cetirizine was found to be more effective at 21-24 hours post-dose.

As with prescription drugs, Zyrtec should be kept out of the reach of children.

Due to reports that Zyrtec increased the risk of drowsiness in some users, the FDA recommends using caution when operating machinery and avoiding the consumption of alcohol and other central nervous system depressants while taking Zyrtec.1

Zyrtec may not be right for everyone. Please speak with a doctor or pharmacist to determine if Zyrtec is the right choice for you.


  1. Zyrtec (cetirizine hydrochloride) Tablets and Syrup For Oral Use. Pfizer Labs. Accessed on May 5, 2021. Retrieved from
  2. Naqvi, A. and V. Gerriets. (2021). Cetirizine. StatPearls Publishing; 2021 Jan-.
  3. Grant, J. A. et al. (1995). Cetirizine in patients with seasonal rhinitis and concomitant asthma: prospective, randomized, placebo-controlled trial. The Journal of Allergy and Clinical Immunology; 95(5): P923-932. Doi: 10.1016/S0091-6749(95)70090-0.
  4. DuBuske, L. (1995). Dose-ranging comparative evaluation of cetirizine in patients with seasonal allergic rhinitis. Annals of Allergy, Asthma & Immunology; 74(4): 345-354.
  5. Gehanno. P. et al. (1996). Comparison of ebastine to cetirizine in seasonal allergic rhinitis in adults. Annals of Allergy, Asthma & Immunology; 76(6): 507-512. Doi: 10.1016/S1081-1206(10)63269-3.
  6. Gupta, A. et al. (2006). Brain distribution of cetirizine enantiomers: comparison of three different tissue-to-plasma partition coefficients: K(p), K(p,u), and K(p,uu). Drug Metabolism & Disposition; 34(2): 318-323. Doi: 10.1124/dmd.105.007211.
  7. Church, M. K. et al. (2010). Risk of first-generation H(1)-antihistamines: a GA(2)LEN position paper. Allergy; 65(4): 459-466. Doi: 10.1111/j.1398-9995.2009.02325.x.
  8. Mann, R. D. et al. (2000). Sedation with “non-sedating” antihistamines: four prescription-event monitoring studies in general practice. The British Medical Journal; 320(7243): 1184-1187. Doi: 10.1136/bmj.320.7243.1184.
  9. Corsico. A. G. et al. (2019). Focus on the cetirizine use in clinical practice: a reappraisal 30 years later. Multidisciplinary Respiratory Medicine: 40. Doi: 10.1186/s40248-019-0203-6.
  10. Horsmans, Y. et al. (1993). Single-Dose Pharmacokinetics of Cetirizine in Patients With Chronic Liver Disease. The Journal of Clinical Pharmacology; 33(10): 929-932. Doi: 10.1002/j.1552-4604.1993.tb01924.x.
  11. Day, J. H. et al. (2005). Comparative efficacy of cetirizine and fexofenadine for seasonal allergic rhinitis, 5-12 hours postdose, in the environmental exposure unit. Allergy and Asthma Proceedings; 26(4): 275-282.

Image by Jill Wellington from Pixabay 

Bryn Evans
Bryn Evans
I graduated with a major in biochemistry, a minor in physics, and a certificate in business from Queen’s University. My long-term goal is to become a family physician (MD) and earn a Master’s in Public Health (MPH). I am passionate about public health, mental health, & wellness. I'm currently completing a Certificate in Effective Writing for Healthcare because I recognize how important it is to communicate effectively with the public!


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