Zyrtec Tab | The most well-balanced over-the-counter second-generation antihistamine

Zyrtec is a medication containing 10mg of cetirizine, a second-generation antihistamine.^1,2 Due to its many practical advantages, I personally consider it the most well-balanced medication across the first, second, and third generations.

Interestingly, it is almost the only antihistamine with a lineage that has been refined twice: from the first generation to the second, and then to the third.^3,4,5

I will explain the advantages of this drug and the differences between its predecessor, Adipam, and its successor, Xyzal.

Reduced first-generation drowsiness and anticholinergic side effects

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While urticaria, allergic rhinitis, and allergic conjunctivitis have different symptoms, they all share the commonality that histamine released from immune cells triggers the symptoms.

When histamine binds to histamine receptors (H1 receptors) on vascular cells, gaps form in the blood vessel walls, and fluid leaks into the surrounding tissue, causing the skin to swell and redden like a mosquito bite. It also binds to H1 receptors on sensory nerves, causing itching.³ This is the wheal, a symptom of urticaria. Symptoms of allergic rhinitis and allergic conjunctivitis occur through a similar mechanism.^6,7

Antihistamines work by blocking H1 receptors so that histamine cannot bind to them.³

However, first-generation antihistamines easily cross the blood-brain barrier, which normally prevents substances in the blood from entering the brain. Therefore, they can also block H1 receptors in the brain, causing drowsiness and a decrease in concentration and reaction speed. Furthermore, they block not only H1 receptors but also muscarinic receptors, which can lead to anticholinergic side effects such as dry mouth, constipation, difficulty urinating, and blurred vision.^3,8

Second-generation antihistamines were developed to compensate for these drawbacks.⁹ Cetirizine enters the brain less than the first-generation hydroxyzine and does not bind well to muscarinic receptors, resulting in less drowsiness and fewer anticholinergic side effects than its predecessor, Adipam.¹⁰

The lineage from Adipam to Zyrtec to Xyzal

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However, there is also a product that further refined this drug. That is Xyzal.

Cetirizine is the primary substance produced when hydroxyzine in Adipam is metabolized in the body. Cetirizine is a 50:50 mixture of two mirror-image forms, and levocetirizine in Xyzal is the purified R-enantiomer that primarily acts on H1 receptors.⁴ This is why Zyrtec is 10mg, while Xyzal is half that at 5mg.

The ingredients, dosage, classification, and prescription requirements for Adipam, Zyrtec, and Xyzal are as follows.^1,2,11,12

Product	Ingredient and Dosage	Classification	Prescription
Adipam Tab	Hydroxyzine HCl 10mg	1st Gen Antihistamine	Required
Zyrtec Tab	Cetirizine HCl 10mg	Modern 2nd Gen Antihistamine	Not Required
Xyzal Tab	Levocetirizine HCl 5mg	Modern 2nd Gen Antihistamine	Required

However, this does not mean that Xyzal has been proven superior to Zyrtec. In comparative studies for allergic rhinitis, there was either no difference in symptoms or, in some results, cetirizine actually performed better, so the findings were not consistent.^13,14

Studies comparing drowsiness and chronic urticaria also failed to confirm a clear superiority for Xyzal.^15,16 While these results do not necessarily mean the two drugs are identical, based on current evidence, it is difficult to say that Xyzal is a better medication than Zyrtec.

On the other hand, Zyrtec has a very practical competitive advantage. Unlike Xyzal, which strictly requires a prescription, Zyrtec can be purchased at a pharmacy without one.

Xyzal does have the advantage of being available in a syrup form (Xyzal Oral Solution), making it favorable for use in children. In fact, because the prices of the two drugs are similar, I do prescribe Xyzal more frequently than Zyrtec. Detailed information about Xyzal can be found in the Xyzal Tab article.

Simpler dosing requirements than Allegra

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Although not an official classification, Xyzal is often called a third-generation antihistamine. Another representative antihistamine referred to as third-generation is Allegra.

Allegra is known to be one of the least drowsy antihistamines, and it is available in a dosage (120mg) that can be purchased without a prescription. However, the administration requirements are strict.^17,18 To achieve proper efficacy, it must be taken with sufficient time apart from fruit juices, antacids, and meals.

Dosing conditions	Zyrtec Tab	Allegra Tab
Approved Age	Adults and children 6 years and older	Adults and adolescents 12 years and older
Timing of Dose	Once daily before bedtime	Once daily before meals
Accompanying Beverage	No specific fruit juice restrictions	Take with water; avoid grapefruit, orange, and apple juice
Antacids	No specific interval required	Interval of about 2 hours with antacids containing aluminum or magnesium hydroxide

In contrast, taking Zyrtec with food only slows the rate of absorption and does not reduce the total amount absorbed.¹⁷ Therefore, I personally find it more practical even when compared to Allegra.

Detailed information about Allegra is organized separately in the Allegra Tab article.

Drowsiness and kidney function must be monitored

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Although it is a well-balanced antihistamine in many respects, this drug is not entirely free of drowsiness. Therefore, you should avoid driving or operating machinery when first taking it.¹⁷

Furthermore, because it is excreted through the kidneys, caution is needed if kidney function is impaired, as the effects may be stronger than intended. If the creatinine clearance, a value indicating kidney function, is less than 50 mL/min, 5 mg should be taken once daily; if it is less than 30 mL/min, 5 mg should be taken once every 2 days. It should not be taken if it is less than 10 mL/min or if the patient is on dialysis.¹⁷

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References

1. MFDS Zyrtec Tab Approval Information, accessed 2026-08-16.
2. Zuberbier T, Abdul Hameed Ansari Z, Abdul Latiff AH, et al. The International Guideline for the Definition, Classification, Diagnosis and Management of Urticaria. Allergy. 2026;81(8):2582-2632. PMID 41649409.
3. Simons FE, Simons KJ. H1 antihistamines: current status and future directions. World Allergy Organ J. 2008;1(9):145-155. PMID 23282578.
4. Chen C. Physicochemical, pharmacological and pharmacokinetic properties of the zwitterionic antihistamines cetirizine and levocetirizine. Curr Med Chem. 2008;15(21):2173-2191. PMID 18781943.
5. Villani FJ, Magatti CV, Vashi DB, Wong J, Popper TL. Arzneimittelforschung. 1986;36(9):1311-1314. PMID 2947582.
6. Naclerio RM, et al. J Allergy Clin Immunol. 1989;83(4):755-761. PMID 2574550.
7. Hingorani M, et al. J Allergy Clin Immunol. 2001;108(3):449-454. PMID 11544467.
8. 2023 American Geriatrics Society Beers Criteria Update Expert Panel. American Geriatrics Society 2023 updated AGS Beers Criteria for potentially inappropriate medication use in older adults. J Am Geriatr Soc. 2023;71(7):2052-2081. DOI 10.1111/jgs.18372.
9. Slater JW, Zechnich AD, Haxby DG. Second-generation antihistamines: a comparative review. Drugs. 1999;57(1):31-47. PMID 9951950. DOI 10.2165/00003495-199957010-00004.
10. Snyder SH, Snowman AM. Receptor effects of cetirizine. Ann Allergy. 1987;59(6 Pt 2):4-8. PMID 2892448.
11. Adipam Tab Product Information, KPA · MFDS Approval Data. Accessed 2026-08-25.
12. MFDS Xyzal Tab 5mg Approval Information, accessed 2026-08-24.
13. UCB Clinical Study Summary A00379. NCT00544388. Report date 2007-07-20. Accessed 2026-08-25.
14. Lee CF, Sun HL, Lu KH, Ku MS, Lue KH. Pediatr Allergy Immunol. 2009;20(5):493-499. PMID 19175892. DOI 10.1111/j.1399-3038.2008.00816.x.
15. Tzanetos DB, Fahrenholz JM, Scott T, Buchholz K. Ann Allergy Asthma Immunol. 2011;107(6):517-522. PMID 22123381. DOI 10.1016/j.anai.2011.08.012.
16. Garg G, Thami GP. J Dermatolog Treat. 2007;18(1):23-24. PMID 17365263. DOI 10.1080/09546630601028745.
17. UCB Korea. Zyrtec Tab Product Insert. Revised 2023-04-20. Accessed 2026-08-25.
18. MFDS Allegra Tab 120mg Approval Information. Accessed 2026-08-25.