The second-generation antihistamine medications Zyrtec® and Reactine® both make use of the drug cetirizine hydrochloride to treat the symptoms of seasonal and chronic allergies. They are also occasionally used in the treatment of chronic idiopathic skin irritation, by reducing the incidence, duration, and severity of pruritis and hives. Unlike first-generation antihistamines like diphenhydramine, the sedative and soporific side effects of cetirizine are significantly reduced in cetirizine. Higher doses of this drug can still cross the blood-brain barrier in significant enough levels to produce sleepiness though, making it worthwhile to determine what the lowest effective cetirizine dose for a patient may be. Factors that can make a dosage adjustment necessary include the patient's age, liver health, and level of kidney function as well as several drug interactions.
When treating the symptoms of seasonal or chronic allergies, the typical initial dose used in a healthy adult is a single daily cetirizine dose of 5 to 10 mg. Children over the age of six may be given an adult dose, while those between the ages of two and five should receive 2.5 mg initially up to a maximum of 5 mg daily in one or two divided doses. Patients between six months and two years of age should receive 2.5 mg cetirizine dose just once daily, although patients one year of age or older may have their cetirizine dose frequency increased to twice daily if necessary. The recommended treatment regimen for chronic or recurrent skin irritation is the same as that for the treatment of allergies in both adult and pediatric populations.
Since cetirizine is metabolized in the liver and removed from the bloodstream in the kidneys, patients with reduced levels of renal or hepatic function may require a lower cetirizine dose than that of a typical adult. Children under six years of age with reduced kidney or liver function should not be given cetirizine at any dosage level due to the lack of research on its safety in this population. In patients with moderate-to-severe renal dysfunction, or CrCl levels of less than 30 ml/min, no more than 5 mg of cetirizine should be given each day, with patients on dialysis following the same instructions. The same cetirizine dose recommendations apply to adult patients with reduced hepatic function.
Although generally considered to be a safe medication, extreme drowsiness is one of the risks of cetirizine. Even though the psychological side effects of cetirizine are not as severe as those produced by first-generation antihistamines, they still pose a significant risk in some circumstances. For this reason, patients under the influence of cetirizine should refrain from operating motor vehicles or heavy machinery.