Formerly prescription-only in the US and Canada, cetirizine is now available over the counter in both countries as Zyrtec and Reactine respectively. Zyrtec was the top new non-food product of 2008 in the US, generating sales of $315.9 million. It is also available as a generic. In Australia Zyrtec is available over the counter in pharmacies and in the UK cetirizine can be sold in limited quantities off-the-shelf in any outlet and is often available in supermarkets.

Pharmacology

Cetirizine crosses the blood-brain barrier only slightly, eliminating the sedative side-effect common with older antihistamines; however it still causes mild drowsiness. It has also been shown to inhibit eosinophil chemotaxis and LTB4 release. At a dosage of 20 mg, Boone et al. found that it inhibited the expression of VCAM-1 in patients with atopic dermatitis.

Administration method and metabolisation

Chewable, non-chewable, and syrup forms of cetirizine are similarly absorbed rapidly and effectively, with absorbed food minutely affecting the absorption rate which yields a peak serum level one hour after administration; in a study of healthy volunteers prescribed 10 mg tablets, once daily for 10 days, a mean peak serum level of 311 ng/mL was observed. The metabolic effects of cetirizine are long acting; remaining in the system for a maximum of 21 hours before being excreted, the average elimination half-life is 8 hours. 70% of the drug is excreted or eliminated by kidney function within 72 hours, and 10% is removed through urine or excrement; of which half is observed as unchanged cetirizine compound. Like many other antihistamine medications, cetirizine is commonly prescribed in combination with pseudoephedrine hydrochloride, a decongestant. These combinations are marketed using the same brand name as the cetirizine with a “-D” suffix (Zyrtec-D, Virlix-D, etc.)

Additionally, cetirizine HCl not sold in combination with pseudoephedrine, is commonly known and marketed in the United States under the brand name, “Zyrtec.” Formerly only available by a prescription, both Zyrtec and Zyrtec-D are currently available over the counter in the United States. In the Philippines, a leading cetirizine is Aforvir.

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