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). It is taken by mouth. Effects generally begin within thirty minutes and last for about a day. The degree of benefit is similar to other antihistamines such as diphenhydramine. Common side effects include sleepiness, dry mouth, headache, and abdominal pain. The degree of sleepiness that occurs is generally less than with first-generation antihistamines. Use in pregnancy appears safe, but use during breastfeeding is not recommended. The medication works by blocking histamine H1 receptors, mostly outside the brain. It was patented in 1981 and came into medical use in 1987. It is on the World Health Organization's List of Essential Medicines. It is available as a generic medication. In 2020, it was the 52nd most commonly prescribed medication in the United States, with more than 13million prescriptions. Medical uses Allergies Cetirizine's primary indication is for hay fever and other allergies. Because the symptoms of itching and redness in these conditions are caused by histamine acting on the H1 receptor, blocking those receptors temporarily relieves those symptoms. Cetirizine is also commonly prescribed to treat acute and (in particular cases) chronic urticaria, more efficiently than any other second-generation antihistamine. Available forms Cetirizine is available over-the-counter in the US in the form of 5 and 10 mg tablets. A 20 mg strength is available by prescription only. It is also available as a 1 mg/mL syrup for oral administration by prescription. 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. The drug can be in the form of tablets, capsules or a syrup. Adverse effects Commonly reported side effects of cetirizine include headache, dry mouth, drowsiness, and fatigue, while more serious, but rare, adverse effects reported include tachycardia and edema. Discontinuing cetirizine after prolonged use (typically, use beyond six months) may result in generalized itching. Pharmacology Pharmacodynamics Cetirizine acts as a highly selective antagonist of the histamine H1 receptor. The Ki values for the H1 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. Cetirizine has 600-fold or greater selectivity for the H1 receptor over a wide variety of other sites, including muscarinic