Antihistamines are medications that counteract histamine, a substance released during allergic reactions causing symptoms such as sneezing, itching, watery eyes, and a runny nose. Cetirizine (Zyrtec®) and levocetirizine (Xyzal®) are two widely used second-generation antihistamines.
Cetirizine Explained
Cetirizine is a second-generation antihistamine used for various allergic conditions. It effectively treats seasonal allergies, perennial allergies, and chronic hives (urticaria). It works by blocking histamine H1 receptors, which cause allergy symptoms.
It typically begins to relieve symptoms within about 30 minutes to an hour and provides relief for approximately 24 hours. For adults and children 12 years and older, a common dosage is 10 milligrams (mg) once daily. Children aged 6 to 11 years typically take 5 mg twice a day, while children aged 2 to 5 years usually take 2.5 mg twice a day.
Levocetirizine Explained
Levocetirizine is a second-generation antihistamine for similar allergic conditions. It is commonly prescribed for allergic rhinitis, which includes both seasonal and year-round allergies, and chronic idiopathic urticaria (hives). Like cetirizine, levocetirizine works by blocking histamine H1 receptors.
The medication is generally taken once daily, usually in the evening. For adults and children 12 years and older, the typical dosage is 5 mg once daily, though some may find 2.5 mg sufficient. Children aged 6 to 11 years usually take 2.5 mg once daily, and for children 6 months to 5 years, the recommended dose is 1.25 mg once daily, often administered as an oral solution.
The Molecular Distinction and Its Impact
The primary difference between cetirizine and levocetirizine lies in their molecular structure; levocetirizine is the levorotatory enantiomer of cetirizine. Cetirizine is a racemic mixture, containing two enantiomers: levocetirizine (the R-enantiomer) and dextrocetirizine (the S-enantiomer).
Only the levocetirizine component is responsible for the antihistaminic effect. This makes levocetirizine the “active” form, and it is considered more potent per milligram than cetirizine. Because levocetirizine is the isolated active isomer, a lower dose is generally needed to achieve similar antihistamine effects compared to a higher dose of the mixed cetirizine compound.
Key Differences in User Experience
While both medications are effective second-generation antihistamines, subtle differences in user experience can arise. Both cetirizine and levocetirizine are known for causing less sedation compared to older antihistamines. However, some research suggests that levocetirizine might have a slightly lower incidence of drowsiness for some individuals, possibly due to its purified active form.
Both medications typically offer 24-hour relief, with cetirizine starting to work within 30 to 60 minutes and levocetirizine showing effects within about an hour. The dosage of levocetirizine is generally lower in milligrams than cetirizine due to its increased potency, which is a direct result of its molecular composition. Both are widely available over-the-counter, making them accessible choices for allergy relief.
Guidance for Choosing
Selecting between cetirizine and levocetirizine often depends on individual responses and specific needs. Both medications are effective options for managing allergy symptoms and chronic hives. However, responses to antihistamines can vary from person to person.
It is advisable to consider factors such as potential side effects, especially drowsiness, and how symptoms respond to each medication. Consulting a healthcare professional, such as a doctor or pharmacist, is always recommended before starting any new medication. This is particularly important for children, pregnant or nursing individuals, or those with existing health conditions or who are taking other medications, to ensure the most appropriate and safe choice.