Cetirizine

Cetirizine, sold under the brand name Zyrtec and others, is an antihistamine medication used to treat allergies. It is taken by mouth. It starts working within an hour and it lasts for about a day. It works as well as other antihistamines like diphenhydramine.
Common side effects are sleepiness, dry mouth, headache, and a abdominal pain. There is less sleepiness than in first generation antihistamines. Pregnant women can use it, but not women who are breastfeeding. It works by blocking histamine at H1 receptors, mostly outside of the brain.
It was patented in 1981, and was first used in 1987. It is on the World Health Organization's List of Essential Medicines. It is available as a generic medication. It was the 67th most commonly prescribed medication in the United States in 2019, with over 11million prescriptions.