Antihistamine
The vast majority of marketed H-antihistamines are receptor antagonists and only a minority of marketed compounds are inverse agonists at the receptor. Clinically, H-antihistamines are used to treat allergic reactions and mast cell-related disorders. Sedation is a common side effect of H-antihistamines that readily cross the blood–brain barrier; some of these drugs, such as diphenhydramine and doxylamine, are therefore used to treat insomnia. H-antihistamines can also reduce inflammation, since the expression of NF-κB, the transcription factor the regulates inflammatory processes, is promoted by both the receptor's constitutive activity and agonist (i.e., histamine) binding at the H receptor.
Hydroxyzine
Hydroxyzine, sold under the brand names Atarax among others, is a medication of the antihistamine type. It is used in the treatment of itchiness, anxiety, and nausea, including that due to motion sickness. It is used either by mouth or injection into a muscle.
Common side effects include sleepiness, headache, and a dry mouth. Serious side effects may include QT prolongation. It is unclear if use during pregnancy or breastfeeding is safe. Hydroxyzine works by blocking the effects of histamine. It is in the piperazine family of chemicals.
It was first made by Union Chimique Belge in 1956 and was approved for sale by Pfizer in the United States later that year. In the United Kingdom 28 doses cost less than a pound. In the United States the wholesale cost in 2018 was about 0.05 USD per dose. In the United States about 8 million prescriptions were written for hydroxyzine in 2016.