acetylcholine, serotonin, dopamine, and α-adrenergic receptors, among many others. The drug shows 20,000-fold or greater selectivity for the H1 receptor over the five muscarinic acetylcholine receptors, and hence does not exhibit anticholinergic effects. It shows negligible inhibition of the hERG channel ( > 30 μM) and no cardiotoxicity has been observed with cetirizine at doses of up to 60 mg/day, six times the normal recommended dose and the highest dose of cetirizine that has been studied in healthy subjects. Cetirizine crosses the blood–brain barrier only slightly, and for this reason, produces minimal sedation compared to many other antihistamines. A positron emission tomography (PET) study found that brain occupancy of the H1 receptor was 12.6% for 10 mg cetirizine, 25.2% for 20 mg cetirizine, and 67.6% for 30 mg hydroxyzine. (A 10 mg dose of cetirizine equals about a 30 mg dose of hydroxyzine in terms of peripheral antihistamine effect.) PET studies with antihistamines have found that brain H1 receptor occupancy of more than 50% is associated with a high prevalence of somnolence and cognitive decline, whereas brain H1 receptor occupancy of less than 20% is considered to be non-sedative. In accordance, H1 receptor occupancy correlated well with subjective sleepiness for 30 mg hydroxyzine but there was no correlation for 10 or 20 mg cetirizine. As such, brain penetration and brain H1 receptor occupancy by cetirizine are dose-dependent, and in accordance, while cetirizine at doses of 5 to 10 mg have been reported to be non-sedating or mildly sedating, a higher dose of 20 mg has been found to induce significant drowsiness in other studies. Cetirizine also shows anti-inflammatory properties independent of H1 receptors. The effect is exhibited through suppression of the NF-κB pathway, and by regulating the release of cytokines and chemokines, thereby regulating the recruitment of inflammatory cells. It has 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. Pharmacokinetics Absorption Cetirizine is rapidly and extensively absorbed upon oral administration in tablet or syrup form. The oral bioavailability of cetirizine is at least 70% and of levocetirizine is at least 85%. The Tmax of cetirizine is approximately 1.0 hour regardless of formulation. The pharmacokinetics of cetirizine have been found to increase linearly with dose across a range of 5 to 60 mg. Its Cmax following a single