In accordance, H 1 receptor occupancy correlated well with subjective sleepiness for 30 mg hydroxyzine but there was no correlation for 10 or 20 mg cetirizine. (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 H 1 receptor occupancy of more than 50% is associated with a high prevalence of somnolence and cognitive decline, whereas brain H 1 receptor occupancy of less than 20% is considered to be non-sedative. A positron emission tomography (PET) study found that brain occupancy of the H 1 receptor was 12.6% for 10 mg cetirizine, 25.2% for 20 mg cetirizine, and 67.6% for 30 mg hydroxyzine. Ĭetirizine crosses the blood–brain barrier only slightly, and for this reason, produces minimal sedation compared to many other antihistamines. It shows negligible inhibition of the hERG channel ( IC 50 > 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. The drug shows 20,000-fold or greater selectivity for the H 1 receptor over the five muscarinic acetylcholine receptors, and hence does not exhibit anticholinergic effects. Cetirizine has 600-fold or greater selectivity for the H 1 receptor over a wide variety of other sites, including muscarinic acetylcholine, serotonin, dopamine, and α-adrenergic receptors, among many others. The K i values for the H 1 receptor are approximately 6 nM for cetirizine, 3 nM for levocetirizine, and 100 nM for dextrocetirizine, indicating that the levorotatory enantiomer is the main active form. Pharmacodynamics Ĭetirizine acts as a highly selective antagonist of the histamine H 1 receptor. L-Stereoisomer, levocetirizine (top) and D-stereoisomer of cetirizine. ĭiscontinuing cetirizine after prolonged use (typically, use beyond six months) may result in generalized itching. Adverse effects Ĭommonly reported side effects of cetirizine include headache, dry mouth, drowsiness, and fatigue, while more serious, but rare, adverse effects reported include tachycardia and edema. The drug can be in the form of tablets, capsules or a syrup. In the UK, up to 30 tablets of 10 mg are on the general sales list (of pharmaceuticals) and can be purchased without a prescription and without pharmacist supervision. It is also available as a 1 mg/mL syrup for oral administration by prescription. A 20 mg strength is available by prescription only. Available forms Ĭetirizine is available over-the-counter in the US in the form of 5 and 10 mg tablets. Ĭetirizine is also commonly prescribed to treat acute and (in particular cases) chronic urticaria, more efficiently than any other second-generation antihistamine. Because the symptoms of itching and redness in these conditions are caused by histamine acting on the H 1 receptor, blocking those receptors temporarily relieves those symptoms. Ĭetirizine's primary indication is for hay fever and other allergies. In 2020, it was the 52nd most commonly prescribed medication in the United States, with more than 13 million prescriptions. It is on the World Health Organization's List of Essential Medicines. It was patented in 1983 and came into medical use in 1987. The main side effect to be cautious about is somnolence. Ĭetirizine can be used for paediatric patients. The medication works by blocking histamine H 1 receptors, mostly outside the brain. Use in pregnancy appears safe, but use during breastfeeding is not recommended. Therefore, it is important to avoid driving, and other dangerous scenarios while taking cetirizine. Second-generation antihistamines that do not cause drowsiness are fexofenadine, and loratadine. Compared to other second-generation anti-histamines, cetirizine can cause drowsiness. The degree of sleepiness that occurs is generally less than with first-generation antihistamines because second-generation antihistamines are more selective for the H1 receptor. Ĭommon side effects include sleepiness, dry mouth, headache, and abdominal pain. The degree of benefit is similar to other antihistamines such as diphenhydramine, which is a first-generation antihistamine. Effects generally begin within thirty minutes and last for about a day. Cetirizine, sold under the brand name Zyrtec among others, is a second-generation antihistamine used to treat allergic rhinitis (hay fever), dermatitis, and urticaria (hives).
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